Chemistry

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**Chemistry** is the science that studies what everything is made of and how it changes. It looks at matter, which is anything that takes up space. Chemistry tries to understand how matter is built, how it behaves, and how it can change. Chemistry explores how tiny particles called atoms and molecules come together, break apart, or rearrange to form new substances. People often call chemistry the “central science” because it connects other sciences like physics (which studies energy and forces) and biology (which studies living things). Chemistry helps explain things all around us, like why iron rusts, how food gives us energy, and how soap cleans our hands.

The history of chemistry is very long. Thousands of years ago, people in places like Mesopotamia, Egypt, India, and China used early forms of chemistry in their daily lives. They practiced metalworking, made dyes for clothes, and learned to ferment food and drinks. These skills were based on watching and experimenting, even though people did not yet understand the science behind them. Later, a practice called alchemy became popular. Alchemists hoped to turn ordinary metals into gold and find a magic liquid that would give eternal life. Even though their ideas were not always scientific, alchemists invented tools and methods that helped future scientists. In the 1600s, chemistry began to change. Scientists like Robert Boyle said that experiments, not just ideas, should be used to understand the world. In the 1700s, Antoine Lavoisier carefully measured chemicals and discovered that matter cannot be created or destroyed, only changed. This became a key idea in modern chemistry. As time went on, scientists developed the atomic theory, created the periodic table, and learned how atoms bond together.

Chemistry is a type of physical science, which means it focuses on understanding matter and how it changes using careful measurements and natural laws. It is closely related to physics, especially in areas like heat and energy (thermodynamics), the behavior of tiny particles (quantum mechanics), and how fast reactions happen (kinetics). These help scientists understand how and why chemical reactions take place. While physics looks for broad, universal rules, chemistry often focuses on how specific molecules and materials behave. Chemists study how atoms come together to form molecules, how energy moves during reactions, and how things like temperature, pressure, and the amount of each substance can affect the speed of a reaction. Using tools and models, chemistry helps us explore both the tiny world of atoms and the larger world of solids, liquids, gases, and plasma. It plays a big role in helping us understand the materials that make up everything around us.

Chemistry is not a subject that stands alone. It is closely connected to many other sciences. Chemistry and physics are closely linked because the movement of atoms, energy, and tiny particles like electrons all follow the laws of physics. In biology, chemistry helps us understand how living things work. It explains how enzymes speed up reactions in the body, how DNA carries genetic information, and how cells turn food into energy. This area of science is called biochemistry. In geology, chemistry helps us learn what rocks and minerals are made of, how volcanoes work, and how elements move through Earth’s surface, oceans, and air. Chemistry is also important in environmental science, astronomy, engineering, and making new materials. Because of these connections, chemistry is one of the most important and wide-reaching sciences.

By around 5000 BCE, people began to discover a new use for fire, making metal from rocks. This early process was called smelting. It marked the beginning of metallurgy, the science of working with metals. Before this, people mostly used native metals, which are found in nature in pure form, like small lumps of gold or copper. But now, they learned how to get copper out of rocks called ores, such as malachite or azurite. To smelt copper, they had to heat the ore in a very hot fire, hot enough to melt the metal and separate it from the rock. This was not easy. It required skill in building and controlling fire. Sometimes they used tools like bellows to blow air and make the fire hotter. They also had to choose the right rocks and other materials to help the process work better. The result was soft copper, which could be shaped into tools, weapons, and jewelry.

One of the earliest examples of prehistoric chemistry was using natural pigments for cave paintings. People ground up minerals like ochre (which is made of iron oxide), charcoal (which is made of carbon), and manganese dioxide into powders. They mixed these powders with animal fat or water to make paint. People used these paints for rock art and decorating their bodies. Another important skill was tanning hides. This meant soaking animal skins in plants with tannins, like tree bark. This process made the leather last longer, stay flexible, and be useful for making clothes or tools. Fermentation also started during this time. At first, it probably happened by accident when fruit or grains went sour. As early as 7000 BCE, people in China were mixing rice, honey, and fruit to make a kind of early alcoholic drink. At the same time, people in Mesopotamia and Egypt were learning how to brew beer and make bread rise by fermenting dough. These processes used wild yeasts and bacteria to turn sugars into alcohol or carbon dioxide, which changed the taste and texture of the food. Even though they did not know about microorganisms, early humans figured out how to make fermentation work.

Metals like gold, silver, and copper became very special. Gold was rare, shiny and beautiful, and also did not get dull or rust, so it was used for ceremonies and to show status. Silver was shiny and easy to shape, so it was used for jewelry and everyday items. Copper was valuable because it could be made into tools and weapons. People started mining these metals on purpose and trading them. One of the biggest discoveries was mixing copper with tin to make bronze. Bronze was first made about 3300 BCE, during the time between the late Stone Age and the early Bronze Age. The oldest proof of making bronze comes from Mesopotamia, an area that is now Iraq and parts of Syria and Turkey. People there started mixing copper with tin to make a metal that was stronger and lasted longer. This new way of making metal eventually spread to other places like the ancient Near East, Europe, and the Indus Valley. Bronze was stronger and harder than tin or copper alone. This was one of the first times people made a new material on purpose by combining substances. Bronze tools and weapons helped societies grow, build cities, and create professional armies.

### Ancient Chemistry
 glass amphora from the Hellenistic period.]]
In ancient civilizations, chemistry started to become more organized and intentional. Unlike the earlier times when people learned mostly by guessing and trying things out, ancient cultures began to keep records and follow steps more carefully. Even though they did not know about atoms or molecules, they still learned how to change and use natural materials in useful ways. Civilizations like ancient Egypt, Mesopotamia, India, China, and Greece all made important contributions. They used chemistry for everyday needs like making metal tools, preserving food, and creating medicines. They also used it for spiritual or religious reasons, such as preparing bodies for burial (embalming), making perfumes, or using special materials in ceremonies. These cultures wrote down their ideas and methods on things like clay tablets and scrolls. This helped them improve their techniques and pass knowledge down to future generations.

In ancient Egypt, chemistry was closely connected to religion, medicine, and daily life. The Egyptians were skilled at using natural materials like minerals, dyes, and plant resins, especially when it came to preserving bodies for the afterlife. They used a special salt mixture called natron, made of natural chemicals like baking soda and salt, to dry out dead bodies and prevent decay. They also added oils, herbs, and resins to stop bad smells and protect the body. Egyptians were also talented metalworkers. They knew how to melt down copper ores like malachite in hot furnaces using charcoal to get pure metal. They used metals like gold, copper, and lead to make tools, jewelry, and statues. They also made green eye paint from malachite and black eyeliner (kohl) from a mineral called galena (lead sulfide). These were not just for beauty. They were believed to help protect their eyes from infections and bright sunlight. Egyptians also made perfumes and scented oils by mixing plant parts, resins, and fats. They used early versions of extraction methods, like soaking flowers in oils or using heat. Some even think they may have used very simple distillation to collect scents. One of their greatest chemical achievements was glass-making. As early as 1500 BCE, they were making colorful glass beads and jars using a mix of sand (silica), soda (a type of salt), and lime, all melted together in hot ovens. By adding different metals, they could change the color. Copper made blue, iron made green, and manganese made purple. Chemistry also played a role in Egyptian medicine. Ancient texts like the Ebers Papyrus included hundreds of recipes for medicines made from minerals, herbs, and animal parts. Some treatments were based on beliefs and rituals, but many were surprisingly accurate and showed real knowledge of how the human body reacted to certain substances.

In Mesopotamia, especially in places like Sumer and Babylon, people made important early discoveries in chemistry, mostly through practical work. They wrote down their knowledge on clay tablets using a writing system called cuneiform. These records show  in detail how they used different materials. One of their biggest contributions was in metalworking. The Sumerians and Babylonians learned how to get metals like copper, tin, lead, silver, and gold from rocks called ores. They built furnaces hot enough to melt these ores. By mixing copper and tin, they created bronze, a strong metal used for making tools, weapons, and art. This marked the start of the Bronze Age. Pottery and ceramics were also important. They used kilns (hot ovens) to bake clay and to make glazes, which gave pottery shiny, colorful finishes. These glazes were made by mixing minerals and metal powders like copper or iron, which changed color when heated. Pottery was used in homes, temples, and for special objects. Mesopotamians also made progress in perfume-making. Ancient clay tablets from cities like Mari and Babylon describe how they got scents from plants, spices, and tree resins. They may have used early versions of distillation, heating ingredients and collecting the vapor to concentrate smells. These perfumes were used in religious ceremonies, for personal grooming, and sometimes in medicine. Another interesting substance they used was bitumen, a sticky, black natural material similar to modern asphalt. They used it for building, waterproofing, and even to preserve bodies like in mummification.

In ancient India, people had a deep and detailed knowledge of chemistry, especially in medicine and spiritual practices. This knowledge was recorded in early texts like the Vedas and Ayurvedic writings. One major area of chemical knowledge was Ayurveda, the traditional Indian system of medicine. Ayurvedic healers used herbs, minerals, and even animal products to make medicines that matched each person’s body type, known as a dosha. These healers used careful steps like heating, grinding, and washing to prepare them. For example, they used mercury (called rasa) in special medicines called rasaushadhis, often mixing it with sulfur. Ancient Indian alchemy, called Rasayana Shastra, also played a big role. While it had mystical goals like living longer or turning base metals into gold, it led to real scientific progress. They used early chemical techniques like calcination (marana), distillation (patan), sublimation (sublimna), and fermentation (sandhana). They built and used tools such as yantras (distillers), musha (crucibles), and retorts (curved tubes), often made from clay or metal. Metallurgy, the science of working with metals, was another area where ancient India stood out. Indian blacksmiths were some of the first to smelt iron, and by the first millennium BCE, they had invented a special kind of strong steel called Wootz steel. This steel was famous for its strength and was traded all over the world. A great example of their skill is the Iron pillar of Delhi, built around the 4th century CE, which has not rusted much even after 1,600 years. India was also among the first cultures to extract and purify zinc. They worked with metals like gold, silver, copper, and lead. In daily life, Indian chemistry showed up in dyeing, cosmetics, and perfumery. Artisans made colorful dyes from plants and minerals for clothes and art. They made perfumes and incense by collecting scented oils from flowers and herbs using early forms of distillation. These recipes and tools were written down in books like the Brihat Samhita, which explained not just what to use, but how to prepare each substance step by step.

In ancient China, chemistry was closely connected to Taoist beliefs and natural ideas, especially the concepts of yin and yang and the Five Elements: wood, fire, earth, metal, and water. These ideas helped guide how people thought substances changed and interacted with each other. One of the most important parts of Chinese chemistry was alchemy, “the way of the elixir”. Alchemists tried to create special elixirs, magical mixtures that they believed could bring immortality or spiritual powers. They used materials like mercury, sulfur, arsenic, and other minerals and metals. While some of these elixirs were dangerous or poisonous, the work led to better knowledge of how different substances behaved and reacted. For example, Chinese alchemists learned how to heat cinnabar (a mercury mineral) to get liquid mercury. These experiments accidentally led to one of the greatest discoveries in chemistry: gunpowder. Around the time of the Tang Dynasty (7th–10th century CE), Taoist alchemists trying to make an elixir of life mixed sulfur, saltpeter, and charcoal, and instead discovered an explosive powder, gunpowder. This changed warfare forever and eventually spread around the world. The recipe was written down in military books like the Wujing Zongyao (written in 1044 CE). Besides alchemy and gunpowder, the Chinese were also masters of metallurgy. Long before Europe, they had created blast furnaces to make cast iron. They developed strong metal mixtures like bronze (copper and tin) and even steel. These metal tools and weapons helped China grow and succeed. Ancient China also developed medicine. Books like the Shennong Bencao Jing (written around the 1st century CE) listed hundreds of plants, minerals, and animal parts used to treat illness. Medicines were made using chemical techniques like grinding, boiling, fermenting, and roasting. Doctors noticed how different amounts, preparation styles, and combinations of ingredients affected the body, even though they explained it using traditional ideas like qi (life energy) and balance between organs. Finally, ancient Chinese people were experts in ceramics, dyeing, and glass-making. The art of making porcelain used special clay (kaolinite) and very high firing temperatures in kilns. They carefully controlled the heat and mixed minerals to make beautiful glazes. Chinese artists also used metal powders to make colorful finishes, like cobalt for blue, copper for green, and iron for brown or black.

### Classical Theories of Matter
In the history of chemistry, classical theories of matter were the early ideas people had about what everything is made of and how it changes. These ideas came from ancient civilizations long before scientists understood atoms like we do today. Even though these early theories did not use experiments like modern chemistry, they helped people start thinking about the nature and structure of matter. These ideas set the stage for many important discoveries later in chemistry.

In ancient Greece, people had different ideas about what matter was made of. One early idea came from Empedocles in the 5th century BCE. He said all matter was made of four basic elements: earth, water, air, and fire. These elements could mix and change because of two forces: love (which brings things together) and strife (which pulls things apart). Later, Aristotle added a fifth element called aether, which he believed made up the sky and stars. He also linked the four elements to qualities like hot, cold, wet, and dry, which helped explain how matter could change. Another important idea came from Democritus and his teacher Leucippus. They said that everything was made of tiny, unbreakable particles called atoms. These atoms were different in shape and size, and how they combined created all the different materials around us. Even though they did not have experiments to prove it, this idea was very similar to what scientists believe today and was very different from Aristotle’s ideas.

In ancient India, there was an idea similar to atoms in a school of thought called Vaisheshika, started by a thinker named Kanada around the 2nd century BCE. They believed that everything is made of tiny, unbreakable particles called “anu” or “paramanu.” These particles combine in different ways to make all kinds of materials. They also created a way to group things based on qualities like taste, color, and how they feel. Even though their ideas were mixed with philosophy and spirituality, they were trying to explain how things change and why there are so many different materials.

### Medieval Chemistry
'' by Joseph Wright of Derby, 1771]]
During the medieval period (about the 5th to 15th century AD), chemistry was not yet a formal science like it is today. Instead, it was mixed with hands-on crafts, spiritual beliefs, and ideas passed down from ancient times. This period was like a bridge between ancient alchemy and the modern science of chemistry that would come later. People during this time used chemical knowledge in many parts of daily life. For example, medicine used herbal mixtures, metal-based compounds, and special liquids called elixirs to treat sickness. These were made through experiments and trial-and-error methods. Metalworking, or metallurgy, was also important. Craftsmen learned how to extract metals like iron, copper, tin, and lead from ores. They used fire and special tools to melt and combine metals, which involved real chemical changes like smelting and making alloys. Other industries like glassmaking and dyeing cloth also depended on chemistry. Workers learned how heat, minerals, and plants could change color or texture. Even though they did not fully understand why it worked, they gained skills and passed them on through guilds and workshops. This time in history helped keep old knowledge alive and slowly improved it, setting the stage for the science of chemistry to grow in the future.

During the medieval period, the Islamic world became one of the most advanced places for chemical knowledge and learning. After the fall of the Western Roman Empire, the Islamic Golden Age (from the 8th to the 14th century) saw a great effort to collect and build upon the science of earlier civilizations. Scholars in cities like Baghdad, Cairo, and Cordoba translated important books from Greek, Persian, and Indian languages into Arabic. But they did not just copy old ideas, they improved them. One famous scientist, Jabir ibn Hayyan (known in Europe as Geber), is often called the “father of Arab chemistry.” He wrote hundreds of books describing different substances and how they reacted with each other. He also created new ways to study materials using experiments. Jabir worked with processes like distillation, crystallization, and calcination. He even helped design tools like alembics and retorts, which were used in early chemistry labs. Jabir tried to understand matter by observing its properties, like how easily it burns or changes. Even though his ideas were influenced by alchemy and religious beliefs, he took important steps toward using experiments to test ideas. His work also introduced many Arabic words for chemicals and tools that later became part of European chemistry. Other Islamic scholars, like Abu Bakr al-Razi (Rhazes) and Al-Tughra'i, continued to build on Jabir’s work. Al-Razi was especially important in medicine. He made antiseptics, acids, and cures for different illnesses. He was also one of the first people to tell the difference between trying to turn metals into gold (a common alchemical goal) and doing useful science to understand how materials behave. These Islamic chemists helped create important tools and methods that were later shared with European scientists. Their books were translated into Latin in places like Sicily and Spain, allowing European scholars to learn from their experiments. This knowledge helped Europe begin its own journey toward modern chemistry.

In medieval Europe, alchemy was shaped by a mix of ancient ideas, Christian beliefs, and mystical thinking. European alchemists believed that changing metals was not just about science, it also had a spiritual meaning. For them, turning ordinary metals into gold was a symbol of purifying the soul and reaching a higher, more perfect state. One of their main goals was to create the Philosopher’s stone, a magical substance they thought could change metals into gold and even give eternal life. Even though alchemy was full of spiritual ideas, it also led to real scientific progress. Important European alchemists like Roger Bacon, Albertus Magnus, and Raymond Lull helped connect mystical ideas with careful observation and early experiments. Roger Bacon, a monk, believed that learning through experiments was important and studied things like light and materials. Albertus Magnus wrote a lot about minerals and metals and helped organize what people knew about them. Raymond Lull used symbols to explain complex ideas, and some of his systems were later used to describe chemical reactions.

During the Middle Ages, the translation of books played a big role in helping chemistry grow in Europe. As Europeans came into more contact with the Islamic world, through trade, the Crusades, and the reconquest of Spain, they discovered many scientific texts written in Arabic. These included both translations of ancient Greek writings and original works by Muslim scientists. Cities like Toledo in Spain and Palermo in Italy became important places where scholars translated Arabic texts into Latin. Thanks to this, the ideas of famous Islamic chemists like Jabir ibn Hayyan and Abu Bakr al-Razi became known across Europe. Their work helped inspire European scientists and planted the seeds for future experiments. At the same time, monasteries in Europe were important for keeping and sharing knowledge. Even though monks mainly focused on religion, they also worked on tasks that used chemical skills. For example, making fancy books called illuminated manuscripts required knowledge about how to make colorful inks from minerals and plants. Monks also brewed beer, made wine, preserved food, and prepared herbal medicines. These activities were based on chemical knowledge, even if the monks did not call it “chemistry.” Craft workers in cities also helped chemistry grow through hands-on work. People like blacksmiths, potters, glassmakers, tanners, dyers, and brewers learned how to change raw materials into useful things. They melted metals, made shiny glazes for pottery, created dyes from plants and bugs, and controlled how things fermented. These workers learned by doing, often as apprentices in guilds. Even though they were not scientists, they used chemistry in their everyday jobs and passed their knowledge down through generations.

By the late Middle Ages, as Europe moved into the early modern period, alchemy started to change. People were becoming less interested in trying to make gold or find ways to live forever, because those goals rarely worked. Instead, more people began to focus on real results. Things they could see, test, and use in everyday life. One important thinker during this time was Paracelsus, a doctor in the 1500s. He believed that alchemy should be used to help people, especially in medicine. He used minerals like mercury, arsenic, and antimony in treatments, substances that were usually feared. Paracelsus is famous for saying, "The dose makes the poison," which means that even dangerous substances can be helpful if used in the right amount. This idea became an important rule in medicine and toxicology. Paracelsus also believed that people should learn about nature by observing and experimenting, not just by reading old books. He studied how chemicals reacted and how they affected the human body. This helped move chemistry away from mystical ideas and closer to real science. Another key figure was Andreas Libavius, a German doctor and chemist. He believed that knowledge should be shared clearly and openly. In 1597, he wrote Alchemia, one of the first chemistry textbooks. In it, he explained tools, techniques, and substances in a simple and organized way. This was very different from the secret codes and symbols used by older alchemists.

### The Birth of Chemistry
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The birth of classical chemistry in the 1500s and 1600s was a big turning point. During this time, chemistry began to move away from the old mystical ideas of alchemy and become a more careful, scientific way of understanding the world. This change happened during the Scientific Revolution, a time when people started to focus more on observing nature, doing experiments, and thinking for themselves instead of just trusting old books and beliefs. Classical chemistry did not appear overnight. It grew slowly as scientists developed better ways to test ideas, improved their lab tools, and began to think differently about matter and how it changes. One important change was that scientists started to question the old idea that everything was made of four or five elements, like earth, water, fire, and air. Instead, they looked more closely at how substances were made and how they reacted in real-life experiments. This new way of thinking helped turn chemistry into a true science, based on facts, testing, and clear thinking.

Robert Boyle was an important figure in the history of chemistry. His book The Sceptical Chymist, published in 1661, is often seen as the moment when chemistry started to move away from old alchemical ideas and become a true science. He challenged long-held beliefs about what matter is made of and how it changes. At the time, many people still believed in the old idea that everything was made of four elements: earth, air, fire, and water. Boyle disagreed. Instead, he suggested that all matter is made of tiny particles, which he called "corpuscles." These particles could combine in different ways to form all the materials in the world. This idea was an early step toward what we now call atomic theory. One of Boyle’s biggest contributions was his new definition of a chemical element. He said an element is a substance that cannot be broken down into anything simpler by chemical methods. Even though he did not know about all the elements we recognize today, his definition helped future scientists, like Antoine Lavoisier, build a better system for understanding them. Boyle was also a strong supporter of careful, repeatable experiments. He believed scientists should write down exactly what they did and what happened, so others could try the same experiments and check the results. This idea of clear, honest reporting is a key part of how science works today. Unlike many alchemists who kept their findings secret, Boyle wanted to share knowledge. He helped start the Royal Society of London, a group that encouraged scientists to share ideas and discoveries openly.

During the 1500s and 1600s, as chemistry was changing, the tools and techniques used in the laboratory improved a lot. Old alchemical tools like retorts, alembics, crucibles, and furnaces were still used, but they were made better. A retort was often made of glass and metal. It was used to heat liquids and collect their vapors, and helped scientists separate parts of a mixture by heating it. An alembic was another tool that helped separate and cool down gases, turning them back into liquids. Over time, alembics were made with better seals and more consistent shapes so they worked better and gave more reliable results. Crucibles were small containers used to heat materials to very high temperatures. They were often made from strong ceramics that could handle the heat and be used many times, which was especially important when working with metals and minerals. Many chemical processes were also becoming clearer and more organized. For example, distillation was used to purify substances like alcohol or perfumes. Filtration was used to separate solid bits from liquids using cloth, paper, or fine mesh. Precipitation meant making a solid appear out of a liquid by mixing certain chemicals, useful for finding out how different substances react. Crystallization helped scientists purify salts and study the shapes of crystals. Glassmaking became very important too. In places like Venice and other parts of Europe, people learned to make clear, heat-resistant glass. This allowed scientists to see what was happening inside their experiments. Tools like flasks, beakers, funnels, test tubes, and condensers became more uniform, meaning they looked and worked the same in different labs. This made it easier for scientists to copy each other’s work and learn together. New measuring tools also made a big difference. Scientists started using balances (scales) that were more sensitive, which helped them measure materials more accurately. This led to the early idea that the total amount of matter stays the same during a reaction, a key idea in modern chemistry. Thermometers, improved by people like Galileo, let scientists track temperature changes during experiments. With better tools and more exact methods, chemistry became less about guessing and more about testing, recording, and sharing results.

In the world of medicine, a new idea called iatrochemistry began to change how people thought about health and healing. Iatrochemistry combined chemistry with medicine and was based on the bold ideas of a scientist named Paracelsus. At the time, most doctors believed in humoral theory, which said sickness came from imbalances in body fluids. But Paracelsus disagreed. He believed that diseases came from outside the body or from chemical problems inside it, and that they should be treated with chemical medicines, not just herbs or bloodletting. Paracelsus introduced the idea of using minerals and man-made substances to treat illnesses. This opened the door to pharmacology, the science of making and testing medicines. By the 1600s, doctors and chemists started working together more closely. They tested materials like antimony, mercury, arsenic, and different salts and tinctures to see if they could cure diseases. Some of these early treatments were harmful or did not work well, but the process of testing, observing, and improving them helped medicine become more scientific. Apothecaries and early pharmacy labs became important places where these treatments were studied and made. At the same time, growing industries in Europe helped chemistry move forward in other ways. As people needed more metals, dyes, ceramics, textiles, and glass, workers had to learn more about how materials changed and reacted. For example, mining and metalwork required people to understand how to find and melt metal ores, how to mix metals into alloys, and how to deal with waste. These jobs needed a good understanding of heat and chemical reactions, so chemists began studying materials more carefully. In the textile and dye industries, creating bright, long-lasting colors for fabrics took a lot of experimenting. Workers used substances like alum, indigo, madder, and cochineal (a red dye made from insects) and had to figure out how to make the colors stick to the cloth. Glassmakers and potters also used chemistry to improve their crafts. They learned to mix silica, soda, and lime in just the right amounts to make clear or colorful glass. By adding small amounts of metals like cobalt for blue, copper for green, or gold for red, they could make beautiful glassware. In trying to copy Chinese porcelain, European makers studied how clays and glazes behaved when heated in kilns, leading to more discoveries in chemistry.

One of the most important changes in science during the 1600s was the creation of scientific societies. These were groups where scientists could meet, share their work, and learn from each other. One of the first and most famous of these groups was the Royal Society of London, founded in 1660. It brought together people like doctors, inventors, and scientists who were all interested in studying nature in a careful, organized way. The Royal Society had a special motto: "Nullius in verba," which means "take nobody’s word for it." This meant they believed in evidence and experiments, not just trusting old books or famous people. Scientists had to prove their ideas through testing and clear results. This approach was a big step forward in turning chemistry into a serious science. Similar groups started in other countries too. In France, the Académie des Sciences was created in 1666. These societies gave chemistry a new home in public places where experiments could be watched, repeated, and discussed by others. Scientists now had places to show their work, get feedback, and improve their ideas based on what others had done. Another big change was the rise of scientific journals. These were like magazines for scientists to publish their discoveries. The Royal Society started one of the first, called Philosophical Transactions of the Royal Society, in 1665. It helped chemists share their methods, tools, and results with other scientists across Europe. This helped science move faster because others could repeat experiments, test new ideas, or improve old ones. Books like Physica Subterranea by Johann Joachim Becher and writings by Georg Ernst Stahl (who supported the phlogiston theory) showed how print helped scientists share and debate their ideas, even when they did not agree. In addition to books and journals, letters were very important. Scientists like Robert Boyle, Antoine Lavoisier, Isaac Newton, and Robert Hooke often wrote to each other, sharing drawings, experiments, and new ideas. These letters helped build an international community of scientists, even before the internet or telephones existed. At first, chemistry was not seen as important as math or physics. Some people thought it was too close to the strange ideas of alchemy. But chemistry began to earn respect because it was useful and connected to the Age of Enlightenment, a time when people believed in reason, progress, and discovery. Slowly, universities began teaching chemistry, and students got to learn in real labs, not just from books. This helped turn chemistry into a respected professional science, just like it is today.

### The Chemical Revolution
At the start of the 18th century, chemistry still relied on old ideas from alchemy and natural philosophy. One of the most famous ideas was the phlogiston theory, first created by Johann Joachim Becher and later improved by Georg Ernst Stahl. This theory said that all things that could burn had a special substance called phlogiston inside them. Phlogiston was thought to be invisible and weightless, and was released when something burned. According to the phlogiston theory, when a material burned or when metal rusted, it was losing phlogiston into the air. For example, when wood turned into ash or iron became rust, people believed the phlogiston left the material, and what was left behind was called “dephlogisticated.” This idea also tried to explain other things, like how animals breathe, thinking that breathing was the slow release of phlogiston from the body. The phlogiston theory seemed to explain many observations about fire and burning, so it became very popular in European chemistry for many years. However, no one could ever find or measure phlogiston directly. Scientists could not isolate it or prove it was real. Sometimes they had to change the idea to fit what they saw. For example, when metals actually gained weight after burning, they said phlogiston had "negative weight," which did not make sense.

One of the biggest changes in 18th-century chemistry was the rise of pneumatic chemistry, which is the study of gases and their properties. Chemists like Stephen Hales helped start pneumatic chemistry by inventing special tools to catch and measure gases. He used a method to collect gases over water, which let scientists isolate different kinds of “airs” (what people called gases back then). This was a big deal because it gave chemists a way to study gases carefully and learn how they acted. Building on this work, Joseph Black made an important discovery with a gas called carbon dioxide, which he called “fixed air.” Black showed that fixed air was different from normal air. He found that this gas could put out flames and that it reacted with limewater to make a solid. These tests proved that fixed air had its own special chemical properties. Black’s discovery showed that air was not just one thing but a mix of different gases.

The study of gases led to many important discoveries. Henry Cavendish found hydrogen, which he called “inflammable air” because it burned easily with a pale blue flame. Cavendish also showed that when hydrogen burns, it combines with oxygen to make water. This was a big surprise because people used to think water was a basic element. Cavendish proved that water is actually made of two gases, which changed how scientists understood chemistry. At the same time, two chemists, Joseph Priestley and Carl Wilhelm Scheele, both discovered oxygen. They found this gas independently, but Priestley was the first to publish his results. He called it “dephlogisticated air” because he believed in the phlogiston theory, thinking oxygen was air that had lost something called phlogiston. Scheele probably found oxygen first but published later. He described oxygen’s strong ability to help things burn and support breathing, though he also still believed in the old phlogiston ideas. Even though their ideas about phlogiston were wrong, these discoveries of oxygen, hydrogen, and carbon dioxide changed chemistry forever. These findings helped lead to the end of the phlogiston theory and the start of modern chemistry.

Even though scientists had made big discoveries about gases and started to question the phlogiston theory, it was a French chemist named Antoine-Laurent de Lavoisier who finally proved the theory wrong and helped create modern chemistry. Lavoisier showed that burning and breathing were not about releasing an invisible substance called phlogiston. Instead, these were chemical reactions where things combined with oxygen, which is a part of air. Lavoisier did careful experiments where he weighed things before and after chemical reactions. He found out that the total weight stayed the same, even though the substances changed. This idea is called the conservation of mass, meaning matter cannot be made or destroyed, only changed into something new. This was a brand-new way of thinking that went against the old phlogiston ideas.

Besides his important experiments, Lavoisier knew it was necessary to have a clear and simple way to name chemicals. Before his work, chemical names were confusing because the same substance could have different names in different places. In 1787, Lavoisier and his helpers, like Berthollet and Guyton de Morveau, created a system to name chemicals based on what they were made of and their properties. This made it easier for scientists to talk and work together. In 1789, Lavoisier published a famous book called Traité Élémentaire de Chimie. This book is considered the first modern chemistry textbook. In it, he clearly explained what chemical elements are, substances that cannot be broken down into simpler parts by chemical means. He also listed many elements known at the time, such as oxygen, hydrogen, nitrogen, and some metals. This helped create the idea of elements that chemists still use today. Lavoisier’s work also led to a new way of studying chemistry called stoichiometry. This means understanding the exact amounts of substances used and produced in chemical reactions. He introduced balanced chemical equations that showed these reactions in symbols and numbers. Because of this, chemistry became a science based on measurements and math, making it easier to predict and repeat experiments.

### The Rise of Modern Chemistry
s as depicted in John Dalton's *A New System of Chemical Philosophy* (1808)]]
In the 19th century, a big change in chemistry was the development of atomic theory by John Dalton, an English teacher and chemist. In 1803, Dalton said that everything is made of tiny, invisible particles called atoms. Each element is made of its own kind of atom, and each kind has a specific weight. Dalton’s idea was based on careful observations and experiments. He also explained that atoms combine in simple, whole-number amounts to form compounds. This helped explain why compounds always have the same kinds of atoms in the same amounts. For example, carbon and oxygen make two different compounds: carbon monoxide (CO) and carbon dioxide (CO₂). In these compounds, the amount of oxygen compared to carbon is always in simple ratios like 1 to 2. Dalton’s work supported the idea that matter is made of small, separate particles. He even made one of the first lists of atomic weights, comparing everything to the weight of hydrogen atoms. Dalton also used special symbols to stand for atoms and molecules, which later became the chemical symbols we use today.

In the 19th century, there was a big change in how scientists understood organic chemistry. For a long time, people believed in something called vitalism. This was the idea that organic compounds, chemicals found in living things, could only be made by living organisms because of a special "vital force." They thought these compounds could not be made in a lab. But in 1828, a German chemist named Friedrich Wöhler did an important experiment that changed everything. He heated a simple chemical called ammonium cyanate, which is inorganic (not made by living things), and it turned into urea, a compound found in the urine of living organisms. This showed that organic compounds could be made in the lab without any living organism involved. Wöhler’s discovery opened the door for scientists to study and create many more organic compounds. Chemists started working with a wide range of carbon-based substances like alcohols, acids, dyes, and medicines. One important result of Wöhler’s work was a new focus on chemical structure. Scientists discovered that certain groups of atoms, called functional groups, determine how molecules behave. Chemists like Justus von Liebig and August Kekulé helped develop ideas about how atoms bond together in molecules. Kekulé, for example, suggested the famous ring shape of the benzene molecule in 1865.

In the 1800s, one of the biggest breakthroughs in chemistry came from a Russian scientist named Dmitri Mendeleev. In 1869, Mendeleev created the periodic table which completely changed how scientists understood the building blocks of matter. At that time, about 63 elements were known, but they seemed like a random list with no clear pattern. Mendeleev had a smart idea. He arranged the elements in rows and columns based on their atomic weights and how they reacted chemically. When he did this, he noticed a repeating pattern in their properties. He called this the “periodic law”. What made his table special was that he left blank spaces where he thought unknown elements would one day fit. He even predicted what these missing elements would be like, what their atomic weights would be and how they would behave. Later, scientists discovered elements like germanium, scandium, and gallium, and they turned out to be almost exactly as Mendeleev had predicted.

In the 1800s, chemistry made big leaps forward, especially in the areas of analysis and energy. One of the most exciting new tools was spectroscopy, which was developed in the 1850s by two scientists named Robert Bunsen and Gustav Kirchhoff. They discovered that when elements are heated or excited, they give off light in unique patterns, like fingerprints. These patterns, called spectral lines, helped scientists identify which elements were present in a substance just by looking at the light it gave off. Using this new technique, Bunsen and Kirchhoff found two new elements, cesium in 1860 and rubidium in 1861, by their special spectral lines. Spectroscopy did not just help on Earth; it also helped scientists study stars. By looking at the light from stars, they could figure out what elements the stars were made of, even from millions of miles away. Spectroscopy started helping astronomers in the early 1800s. Scientists noticed that when sunlight passed through a special glass called a prism, it created a rainbow of colors, but with some dark lines missing. In 1802, W. H. Wollaston saw these dark lines, and in 1815, Joseph von Fraunhofer studied them more closely. These dark lines were called absorption lines, and they were very important. They showed that certain colors of light were being absorbed by gases in the Sun’s outer layers. This gave scientists their first clues about what the Sun was made of. Later, the same method helped astronomers figure out the chemical makeup of other stars, too. At the same time, chemists were also learning more about how energy works in chemical reactions. A scientist named Germain Hess came up with Hess’s Law, which says that the total energy change in a chemical reaction is the same, no matter how many steps it takes. Later, James Prescott Joule made another big discovery. He showed that heat and mechanical energy (like moving parts) are really just different forms of the same thing. His famous experiment with a paddle wheel helped prove this. These ideas became part of the first law of thermodynamics, which says that energy cannot be created or destroyed, it can only change form.

The Industrial Revolution in the 1800s brought big changes to the world, and chemistry played a major role in making that happen. One of the biggest breakthroughs was in steelmaking. In the 1850s, Henry Bessemer invented a process to turn melted iron into strong steel by blowing air through it to remove unwanted materials. This Bessemer process made steel much cheaper and easier to produce. As a result, steel was used to build railroads, ships, bridges, and buildings all over the world. At the same time, chemistry helped create synthetic dyes. In 1856, a young chemist named William Henry Perkin was trying to make medicine but accidentally created a bright purple dye called mauveine. This dye was the first of its kind and made colorful clothing affordable for many people. Perkin’s discovery started a new industry, and soon many companies were making different kinds of synthetic dyes for clothes and fabrics. Explosives were another area where chemistry made a big impact. Chemists like Ascanio Sobrero and Alfred Nobel developed powerful materials such as nitroglycerin and dynamite. These explosives were used in mining, building tunnels, and even in war. Nobel’s inventions made blasting safer and more effective. He later created the Nobel Prizes to honor great achievements in science and other fields. Chemistry also helped farmers by improving fertilizers. Scientists like Justus von Liebig discovered that plants need certain minerals to grow well. This led to the creation of artificial fertilizers like superphosphates and ammonium salts, which helped crops grow faster and better. These fertilizers supported a growing population and helped feed more people. Finally, chemistry changed medicine. Scientists learned how to take useful chemicals from plants and make them in labs. Important medicines like aspirin, morphine, and quinine were produced in large amounts, making them more available to people around the world. This helped save lives and treat diseases more effectively.

During the 1800s, chemistry became a more organized and respected science. One of the biggest changes was that chemistry started to be taught seriously at universities. Before this time, most science was done by individuals working on their own. But now, schools in Europe and North America began to understand that experimental science, like chemistry, could help improve society and make countries stronger. Chemistry got its own professors (called university chairs) and special laboratories where students could learn both ideas and hands-on experiments. One of the most famous schools for this was the University of Giessen in Germany, where Justus von Liebig started a chemistry lab in the 1820s. His lab became a model for others around the world. Liebig believed students should learn by doing research and experiments, not just by listening to lectures. Many important chemists were trained there, and the idea of the "chemical laboratory" became common in schools. Outside of schools, scientific societies also helped chemistry grow. In 1841, the Chemical Society of London was founded. It gave chemists a place to share their work, honor great discoveries, and influence government decisions. Similar groups formed in other countries, like the Deutsche Chemische Gesellschaft in Germany. These groups helped chemists stay connected and work together. Another important part of this time was the rise of science journals. Chemists needed a way to share their discoveries with others. In 1832, Liebig started a journal called Liebigs Annalen der Chemie, where chemists could publish their findings. It was carefully reviewed by other scientists, which helped make the information trustworthy. As more journals were created, new discoveries spread faster and helped science move forward. As chemistry became more organized, it also became more of a professional career. Chemists were no longer just curious hobbyists or workers in factories, they were seen as trained professionals with special skills. Companies started building their own industrial labs and hiring chemists to help make products like medicine, fabric dyes, and metals. This helped turn chemistry into an important part of both science and industry.

### 20th Century Chemistry
In the 20th century, chemistry changed a lot because scientists made important discoveries about atoms and even smaller parts inside them. Before, people thought atoms were the smallest pieces of matter and could not be broken down. But in 1897, J.J. Thomson discovered the electron, a tiny negatively charged particle inside the atom. This showed that atoms were made of smaller parts. Thomson created the “plum pudding” model, imagining electrons like little raisins stuck inside a positively charged pudding. Not long after, in 1911, Ernest Rutherford did the famous gold foil experiment. He found out that the atom is not like a pudding but has a tiny, dense, positively charged center called the nucleus, with electrons moving around it mostly through empty space. In 1913, Niels Bohr improved this idea. He said electrons move in specific paths or shells around the nucleus. They can jump between these shells by gaining or losing energy. This idea helped explain why atoms give off certain colors of light, called spectral lines. However, Bohr’s model did not work well for bigger atoms, so scientists kept exploring. In the 1920s and 1930s, a new field called quantum mechanics was created. Erwin Schrödinger suggested that electrons behave like waves, and their positions can only be described by probabilities, not exact locations. Werner Heisenberg added that we cannot know both an electron’s exact position and speed at the same time. This is called the Uncertainty Principle. These ideas came together into the modern quantum mechanical model of the atom, which is the foundation of chemistry today. It helps explain not only how atoms look and behave but also why elements are arranged in the periodic table, how atoms stick together in chemical bonds, and the shapes of molecules.

In 1913, Henry Moseley used X-rays to study atoms and discovered that an element’s place in the periodic table should be based on its atomic number (the number of protons in its nucleus), not its atomic weight. This helped fix problems in the old version of the table and made it more accurate. Moseley’s work also helped scientists find missing elements by spotting gaps in the sequence of atomic numbers. At the same time, scientists were learning about radioactivity. In 1896, Henri Becquerel discovered that uranium could give off invisible rays without any outside energy. Later, Marie and Pierre Curie studied this further and discovered new radioactive elements like polonium and radium. These elements gave off energy because their atoms were unstable and could break down on their own. This showed that atoms were not unchangeable after all. These discoveries led to a new field called nuclear chemistry, which focuses on the reactions that happen in an atom’s nucleus. In the 1930s, scientists like Otto Hahn, Fritz Strassmann, Lise Meitner, and Otto Frisch discovered nuclear fission, a process where atoms like uranium split into smaller parts and release a huge amount of energy. This discovery had a huge impact. During World War II, the U.S. used nuclear fission to build the atomic bomb in a secret project called the Manhattan Project. These bombs caused massive destruction and changed the world forever. After the war, people also began using nuclear fission to make electricity in nuclear power plants. Later, scientists studied nuclear fusion, the reaction that powers the sun. Fusion could give us cleaner and safer energy, but we are still working on how to control it on Earth.

Another important breakthrough in 20th-century chemistry was the rise of physical chemistry. Physical chemistry used the principles of physics to understand matter. In the United States, an early supporter of this field was Ira Remsen, who helped improve science education by promoting hands-on experiments in chemistry classes. This helped train a new generation of scientists who were both skilled and curious. One of the most important figures in physical chemistry was Gilbert N. Lewis. He came up with the idea that atoms bond by sharing pairs of electrons. He also created the Lewis dot structure, a simple way to show how atoms are connected. His work helped people understand things like covalent bonds (where atoms share electrons), acids and bases, and chemical energy. Later, in the 1930s, Linus Pauling took these ideas even further by using quantum mechanics to explain chemical bonds. He introduced the concept of hybridization, which helped explain the shapes of molecules, like the four-cornered tetrahedral shape of methane. Pauling also created the electronegativity scale, which shows how strongly atoms attract electrons when they bond. Pauling’s impact went beyond theory. He was also a leader in using tools like X-ray crystallography and spectroscopy to figure out the exact 3D shapes of molecules. These tools became especially important in biochemistry. Pauling used them to study proteins and discovered their basic structures, such as the alpha helix and beta sheet. His work helped start the field of structural biology, which looks at how the shape of a molecule affects what it does in the body.

In the 20th century, organic chemistry, grew very quickly and made a huge impact on the world. One of the biggest changes was that chemists invented new ways to build complicated molecules with great care and precision. These new methods, like the Grignard reaction or Wittig reaction, gave scientists powerful tools to create all kinds of useful chemicals, including medicines and materials. One important part of this progress was the rise of polymer chemistry, which led to the invention of new materials called plastics. In 1935, a chemist named Wallace Carothers at DuPont created nylon, the first completely man-made fiber. This showed that large, chain-like molecules (called polymers) could be designed to have special properties, like strength or flexibility. After nylon, scientists quickly made other important materials, such as polyethylene, polystyrene, PVC (polyvinyl chloride), Teflon, and polyester. These plastics changed everyday life. They were used in packaging, clothing, cars, buildings, and much more. At the same time, pharmaceutical chemistry, the science of making medicines, grew rapidly. In the early 1900s, chemists had already made useful drugs like aspirin and barbiturates, but the discovery of penicillin in 1928 by Alexander Fleming changed medicine forever. Penicillin could fight deadly bacterial infections, and its mass production helped save countless lives, especially during World War II. After that came sulfa drugs, the first widely used antibiotics, which helped treat many infections even before penicillin was common. Later in the century, scientists made even more important medicines, including antiviral drugs for diseases like HIV/AIDS, chemotherapy drugs to treat cancer, and medications for mental health and brain disorders. New advancements in stereochemistry and chiral synthesis made medicines safer and more effective. In the final decades of the century, techniques like combinatorial chemistry, high-speed drug testing, and computer-aided drug design helped scientists discover new medicines much faster.

In the middle of the 20th century, chemistry began to focus more on understanding the molecules that make up living things. One of the most important discoveries during this time was the structure of DNA, made by James Watson and Francis Crick in 1953. They used X-ray images taken by Rosalind Franklin and base-pairing rules discovered by Erwin Chargaff to figure out that DNA is shaped like a double helix, two strands twisted around each other. These strands are held together by base pairs. Adenine pairs with thymine, and guanine pairs with cytosine. This discovery helped connect chemistry with biology and gave rise to a new field called biochemistry, which studies how chemicals work inside living things. Scientists began to study important molecules like proteins, carbohydrates, lipids, and nucleic acids in more detail. They used special tools like X-ray crystallography, NMR (nuclear magnetic resonance), and mass spectrometry to see the shapes and structures of these molecules. Researchers also started to understand how enzymes speed up chemical reactions in the body, how cells communicate, and how metabolism works to give us energy. This new knowledge helped create another field called molecular biology, which focuses on how genes work and how cells use DNA to make proteins. In the 1970s, scientists developed ways to cut and paste DNA, creating a powerful technique called recombinant DNA technology. This let them move genes from one organism to another, helping launch the field of genetic engineering. Later, in the 21st century, scientists created an even more precise gene-editing tool called CRISPR-Cas9, which allows for direct changes to DNA. These discoveries changed many areas of life. In medicine, they led to new drugs, gene therapies, and personalized treatments based on a person’s DNA. In farming, they helped create genetically modified organisms (GMOs) that grow better and resist pests. In science, they gave researchers powerful tools to explore how life works, including how we inherit traits and how species evolve.

In the 20th century, analytical chemistry changed a lot because of new and powerful tools that helped scientists study chemicals more carefully and accurately. Before this time, scientists mostly used older methods called "wet chemistry," which involved mixing chemicals, making them change color, or forming solids to find out what was in a sample. These methods worked okay but were not precise enough for the harder problems chemists faced in fields like organic chemistry, biochemistry, and materials science. One big step forward was the invention of the mass spectrometer. This tool measures the mass and charge of tiny particles called ions. Scientists like Francis Aston and Arthur Jeffrey Dempster helped improve mass spectrometry, which became super useful for finding out the weights of molecules, discovering different forms of atoms (isotopes), and identifying unknown chemicals. Mass spectrometry is so sensitive it can detect even tiny amounts of substances, down to billionths or trillionths of a gram. Another important tool was nuclear magnetic resonance (NMR) spectroscopy, first shown in the 1940s and widely used by the 1950s and 1960s. NMR lets chemists study the magnetic properties of atoms like hydrogen and carbon in molecules. This helps them understand the exact structure of molecules, how atoms bond, and how molecules move around in liquids. Later, more advanced versions of NMR allowed even deeper study of complex molecules, especially in drug research and organic chemistry. X-ray crystallography also became very important during the 20th century. This technique uses X-rays to find the three-dimensional shapes of molecules with amazing detail. At first, it was used on simple crystals, but later scientists could use it to study big molecules like vitamins, hormones, and proteins. For example, Rosalind Franklin’s X-ray images helped explain the famous double-helix structure of DNA. Over time, X-ray crystallography helped scientists learn about many complicated biological molecules, which is very important for understanding how our bodies work and for making new medicines. Other helpful tools include infrared spectroscopy, which helps identify parts of molecules by measuring vibrations, and ultraviolet-visible (UV-Vis) spectrophotometry, which helps chemists watch chemical reactions and study certain types of molecules. Together, these instruments made it possible for chemists to study chemicals in ways they never could before, speeding up discoveries and helping create new materials, medicines, and technologies.

By the late 20th century, the chemical industry got much bigger, including not only old areas like medicine and dyes but also new ones like petrochemicals (chemicals made from oil), agrochemicals (chemicals for farming), and advanced materials. One big change came from petrochemistry. Scientists learned how to turn crude oil into many useful chemicals. These chemicals were then used to make plastics, synthetic rubber, and solvents. These new materials changed everyday things like packaging, buildings, cars, and many other products. Another very important invention was the Haber process (also called the Haber-Bosch process). It was developed in the early 1900s by Fritz Haber and later made into a big industrial method by Carl Bosch. This process uses high pressure, high temperature, and a special iron catalyst to turn nitrogen gas from the air and hydrogen gas into ammonia (NH₃). Ammonia is a key ingredient in nitrogen fertilizers. The Haber process had a huge impact on farming. It allowed factories to make nitrogen fertilizers in large amounts, which helped farmers grow much more food. This helped feed the rapidly growing world population and played a big role in the Green Revolution, a time when farming became much more productive. Today, more than half of the people on Earth depend on food grown with fertilizers made using the Haber process. Without it, natural soil would not have enough nitrogen to grow enough food for everyone.

In the field of energy, chemists made big progress in creating new ways to produce cleaner and better power, so people would not have to rely so much on fossil fuels like oil and coal. One important invention was the fuel cell, which changes chemical energy directly into electricity using reactions between hydrogen and oxygen. Although the idea of fuel cells started way back in 1839 with Sir William Grove, it was not until the mid-1900s that fuel cells became practical. In the 1960s, companies like General Electric built hydrogen-oxygen fuel cells to power NASA’s spacecraft because they were reliable and clean. Later, in the 1990s and 2000s, fuel cells became important for clean energy cars and power stations. Chemists worked on making better catalysts, materials like platinum, that helped fuel cells work faster and last longer. At the same time, batteries got much better thanks to advances in chemistry and materials science. Early batteries, such as the lead-acid battery invented in 1859, were heavy and did not store much energy. Nickel-cadmium batteries, made in the early 1900s, could be recharged but had problems like toxicity and losing capacity over time. A big breakthrough happened in the 1980s when John B. Goodenough and his team created the lithium cobalt oxide cathode, which led to the lithium-ion battery. Sony started selling these batteries in 1991. Lithium-ion batteries are lightweight and hold a lot of energy, which changed portable devices like laptops and smartphones. They also helped make electric cars better, allowing them to travel longer distances and pushing more people toward cleaner transportation. Chemists also played a big role in solar energy by improving solar panels that turn sunlight directly into electricity. The first practical silicon solar cell was made in 1954 by Bell Labs scientists Daryl Chapin, Calvin Fuller, and Gerald Pearson. This showed that solar energy could work well, although it was expensive at first. Over the years, chemists and materials scientists worked on making solar cells cheaper and more efficient by improving the materials inside them.

Beyond energy, materials science helped create many important inventions in fields like electronics, space travel, and computers. One big discovery was semiconductors, like silicon and germanium. These materials made it possible to invent the transistor in 1947 at Bell Labs by scientists John Bardeen, Walter Brattain, and William Shockley. Transistors replaced bulky vacuum tubes and made electronic devices smaller, faster, and more reliable. This invention became the building block for modern computers, phones, and other technology. In the 1980s, scientists Georg Bednorz and K. Alex Müller discovered high-temperature superconductors. These materials are used in powerful magnets for MRI machines and particle accelerators and could someday help build more efficient power systems. Chemists also worked on composite materials, which are made by combining two or more substances to make something stronger or better. For example, carbon fiber reinforced plastics and advanced ceramics are light but very strong and can resist heat and corrosion. These materials are used to build airplanes that use less fuel, safer and lighter cars, and even parts for rockets and spacecraft.

Even though the 20th century brought amazing discoveries in chemistry, it also caused serious problems for people and the planet. One major wake-up call came in 1962, when scientist and writer Rachel Carson published a book called Silent Spring. In it, she warned about the dangers of using pesticides like DDT without understanding the harm they could do. These chemicals were killing birds, harming wildlife, and even making people sick. Her book helped spark the environmental movement, leading to the banning of DDT in many countries and the creation of environmental protection laws and agencies. But that was not the only concern. Chemistry was also used in ways that hurt people, especially during wars. In World War I, mustard gas was used as a weapon. In the Vietnam War, chemicals like Agent Orange were sprayed to destroy forests, but they also harmed soldiers and civilians. These tragedies led to international agreements like the Chemical Weapons Convention in 1993, which aimed to stop countries from using chemistry to make weapons. Factories also caused pollution by releasing toxic chemicals into the air, water, and soil. This included things like heavy metals, greenhouse gases, and chlorinated hydrocarbons. Problems like acid rain, holes in the ozone layer from CFCs, and huge amounts of plastic waste showed that chemical innovation could have long-term effects on the Earth. These issues taught scientists and leaders that chemistry should be used more responsibly to protect both people and the environment.

### 21st Century Chemistry
Chemistry in the 21st century has continued to grow and improve in exciting ways. One of the biggest changes is the use of computers to model and predict chemical reactions. This area is called computational chemistry. It lets scientists create virtual experiments and watch how atoms and molecules behave without needing to do everything in a real lab. This saves time, money, and materials. A special method called Density Functional Theory (DFT) helps chemists understand how molecules are built, how they react, and how much energy they need. In the past, these tools could only handle small, simple molecules, but now they can work with much larger and more realistic ones. This is very useful in designing new medicines, materials, and catalysts. In 2013, three scientists, Martin Karplus, Michael Levitt, and Arieh Warshel, won the Nobel Prize in Chemistry for their work in combining different types of modeling. They found a way to blend quantum mechanics with classical physics to study big, complex systems like enzymes. Another exciting development is called ultrafast laser spectroscopy. This technique uses femtosecond laser pulses (that is one quadrillionth of a second) to take snapshots of chemical reactions as they happen. It is like watching atoms and molecules moving, bonding, and changing in real time. This helps scientists better understand what is happening during reactions, including how bonds break, electrons move, and energy flows.

Scientists now use amazing tools like atomic force microscopes (AFM), scanning tunneling microscopes (STM), and X-ray photoelectron spectroscopy (XPS) to look at atoms and molecules on surfaces. These tools are very important for creating things like better catalysts, tiny sensors, and nanomaterials that help clean up pollution. Scientists can also use special tools like optical tweezers to measure the forces on just one molecule at a time. Another field is nanochemistry. Chemists work with materials that are only a few nanometers wide. These materials, like quantum dots and nanoparticles, have special properties that can be used in solar panels, tiny computer chips, and targeted drug delivery that helps treat diseases more precisely. Scientists are designing better batteries, fuel cells, and machines that split water into hydrogen and oxygen, which could help us use clean energy. In biophysical chemistry, scientists combine chemistry with biology and computer science. They study how proteins fold, how enzymes work, and how cell membranes behave. Tools like cryogenic electron microscopes (cryo-EM) and NMR machines help them see the shapes of molecules, which is very helpful in fighting diseases and creating new medicines.

In the 21st century, analytical chemistry has become much more advanced and important in many areas of science and everyday life. One big improvement has been in mass spectrometry (MS), a tool that helps scientists figure out what different chemicals are and how much of each is present. In the past, MS was mostly used for small molecules. Today, it can study large molecules, complicated mixtures, and even single cells. Modern versions, like tandem mass spectrometry (MS/MS) and high-resolution machines such as time-of-flight (TOF) and orbitrap analyzers, are incredibly sensitive. They can detect tiny amounts of a substance, as little as one part in a trillion. These tools are used in areas like medicine, drug testing, studying proteins and cells, and checking for pollution in the environment. Another big part of analytical chemistry is chromatography, which helps separate mixtures. The most common types are gas chromatography (GC), high-performance liquid chromatography (HPLC), and the newer ultra-performance liquid chromatography (UPLC). UPLC, developed in the early 2000s, works faster and gives clearer results than older methods. Often, chromatography is combined with mass spectrometry to make hyphenated techniques like LC-MS/MS or GC-MS. These powerful tools can separate and identify thousands of substances in just one experiment, which is very useful in things like medical testing, making new medicines, and checking food safety. A big trend in the 21st century is making scientific tools smaller and more portable. This is called miniaturization. One exciting result of this is the creation of lab-on-a-chip and microfluidic devices. These tiny tools can do complex chemical tests on a small chip, using only tiny drops of liquid. Because they are small and easy to carry, these devices can be used outside of a regular lab, like at a patient’s bedside, in the field, or in places with few resources. They can give fast results, which is very helpful in emergencies or remote areas. These tools are used for many things, such as checking blood sugar for people with diabetes, testing for diseases, like COVID-19 or the flu, finding toxins in water or the environment, and analyzing drugs in crime investigations.

Today, analytical chemistry works closely with data science. Modern science tools create huge amounts of data, and chemists need better computer programs to help understand it all. They use things like statistics, machine learning, and pattern recognition to find useful information in the data. This helps chemists in detecting pollution in the air, water, or soil, even at tiny amounts (less than one part per billion). It also helps them find illegal drugs or chemicals in crime scenes. It also helps in tracking new pollutants like microplastics and PFAS, which used to be hard to measure. Using special tools like ICP-MS, chemists can find heavy metals (like lead or mercury) in people or the environment In medicine, analytical chemistry helps create and test new medicines. Chemists make sure drugs are pure, safe, and work the way they should. They also check that medicines stay good over time. Groups like the FDA (Food and Drug Administration) and EMA (European Medicines Agency) use these tests to decide if a medicine can be sold.

Biochemistry in the 21st century has grown really fast. One of the biggest achievements was the Human Genome Project, finished in 2003. Scientists were able to read the entire DNA sequence of humans for the first time. This helped researchers learn more about how genes work. How they are turned on and off, and how they make proteins. Scientists have also sequenced the DNA of many other living things, like tiny bacteria, helpful plants like Arabidopsis, and fruit flies. This helped them compare genes across species and better understand how life evolved and functions. One of the most powerful tools in today’s biochemistry is called CRISPR-Cas9. Discovered in 2012 by Emmanuelle Charpentier and Jennifer Doudna, CRISPR lets scientists edit DNA quickly and accurately. It has changed the way we do genetic engineering. With CRISPR, we could fix genetic diseases, improve crops, and study what different genes do in living things Another important area is proteomics, the study of proteins. Using mass spectrometry, scientists can now identify thousands of proteins in a sample and study how they are changed or interact with each other. This helps us understand how cells work on a very detailed level. Tools like cryogenic electron microscopes (cryo-EM) allow scientists to actually see large molecules and protein machines in 3D, almost down to the atom. This technology was so important that it won the Nobel Prize in Chemistry in 2017.

One important area is called metabolomics, which is the study of tiny molecules (called metabolites) in cells and tissues. These molecules help us see what's really happening inside living things. Scientists now combine information from genomics (DNA), transcriptomics (RNA), proteomics (proteins), and metabolomics to get a complete picture of how cells work. This big-picture approach is called systems biology. It helps scientists understand complex diseases like cancer, diabetes, and Alzheimer’s by looking at how everything in a cell connects and works together. Biochemistry also helps in drug discovery and personalized medicine. Using computer modeling and new tools, scientists can design medicines that match a person’s specific needs. A field called pharmacogenomics studies how people’s genes affect the way they respond to medicine. This helps doctors choose the right drug and the right dose for each person. Another interesting area is synthetic biology. This is when scientists engineer living cells to do new jobs, like making biofuels, biodegradable plastics, or even new medicines. They do this by reprogramming cells with carefully chosen DNA instructions. In agriculture, biochemists are helping to create GMO (genetically modified organisms) and gene-edited crops that are healthier, more nutritious, and more resistant to pests, diseases, and climate change. By studying plant biochemistry, scientists are finding ways to help plants grow better and use water and nutrients more efficiently. Biochemistry is also key in solving global health problems. One example was the creation of mRNA vaccines for COVID-19. These vaccines use molecules that tell our cells to make a harmless part of the virus, helping the immune system learn how to fight it. Scientists are now working on similar vaccines for other diseases like flu, HIV, and even cancer.

In the 21st century, chemistry has also become very important in helping us create better and cleaner ways to store and use energy. One big area of progress is rechargeable batteries. Lithium-ion batteries, which first came out in the 1990s, have gotten much better over the years. Today, they are used in many things like smartphones, laptops, and electric cars. Chemists have made these batteries safer and more powerful by creating better materials for the battery parts. Some of these materials include lithium iron phosphate (LiFePO₄) and nickel manganese cobalt (NMC). Scientists are also working on new types of batteries, like lithium-sulfur, solid-state, and sodium-ion batteries. These may be cheaper, safer, and last longer, which could help even more people use clean energy. Another exciting technology is the fuel cell. Fuel cells make electricity by combining hydrogen and oxygen, and the only thing they give off is water, so they do not pollute the air. This makes them a great option for clean cars. One type, called a PEM fuel cell, is especially useful. Chemists are working on better and cheaper catalysts, which are materials that help the fuel cell work faster. One challenge with fuel cells is how to store hydrogen safely and easily. Scientists are exploring special materials, like metal–organic frameworks (MOFs) and solid–state hydrides, that can hold hydrogen in a compact and safe way.

Chemists are also leading the way in improving solar energy. The most common solar panels today are made from silicon, but scientists are working on new materials that could make solar power cheaper, lighter, and more flexible. One interesting material is called a perovskite. In 2009, perovskite solar cells could only turn about 4% of sunlight into electricity. By the early 2020s, that number had jumped to over 25%. Perovskites are cheap to make and can be used on bendable surfaces, but they have some problems. They do not last very long, and they contain lead, which can be harmful. Chemists are working to fix these issues. Other types of solar panels include dye-sensitized solar cells (DSSCs) and organic solar cells, which can be see-through or flexible. These new types of solar cells could be used in windows or on clothing. Another interesting area is called photocatalysis. This means using sunlight to make chemical reactions happen. Scientists are developing materials like titanium dioxide mixed with tiny amounts of certain metals, that can use sunlight to split water into hydrogen and oxygen or turn carbon dioxide into useful fuels like methanol or methane. This is similar to how plants do photosynthesis, and scientists call their version artificial photosynthesis. Chemists are also helping to make green hydrogen, a clean fuel, using a method called electrolysis. This process splits water using electricity, and it works even better when chemists add catalysts that help speed up the reaction. Chemists are also working on ways to capture carbon dioxide (CO₂) from the air or from factories. They are designing new materials like amine-based liquids, metal-organic frameworks (MOFs), and porous carbons that can trap CO₂. Once captured, the CO₂ can be turned into fuels, plastics, or even medicines, helping to reduce pollution and fight climate change.

Chemistry is also helping to create new materials. One example is nanomaterials. These are materials made at an incredibly small scale. Chemists can now make nanoparticles, nanotubes, and nanowires with exact shapes and sizes. This has led to discoveries like carbon nanotubes and graphene, which are super strong and great at conducting electricity and heat. These materials are now being used in batteries, tiny sensors, and advanced electronics. Another new type of material are called smart materials. These materials can change when something around them changes like temperature, light, or electric signals. For example, some materials can “remember” a shape and return to it after being bent. Others can heal themselves after getting damaged. Smart materials are used in things like robotics, medical devices, and spacecraft. In medicine, special smart gels can deliver medicine exactly when and where it is needed or help new tissue grow. Polymers, which are long chains of molecules, have also improved. Chemists can now make biodegradable polymers that break down safely or design polymers for 3D printing, medicine, and even to act like proteins or DNA. Chemists can now use machine learning and computer simulations to test and find the best materials faster than ever before. Programs like the Materials Genome Initiative help scientists go from new idea to real product more quickly. Chemists are also designing electronic and photonic materials for things like flexible phone screens, glowing displays (OLEDs), and even quantum computers. Tiny particles called quantum dots are used in colorful TVs and medical imaging. And in the race to build quantum computers, special materials are helping create more stable and powerful qubits.

In the 21st century, chemistry faces many big and complicated challenges. One of the most serious is climate change. To help stop global warming, chemists are working to create cleaner energy sources. These include better solar panels, fuel cells, and batteries that do not rely on fossil fuels. But making these technologies affordable, safe, and easy to use on a large scale is still a tough problem. Another goal is to find ways to capture carbon dioxide, a major greenhouse gas, and turn it into useful fuels or chemicals. Right now, that process uses a lot of energy and is not very efficient. Green chemistry is another important focus. Many chemical industries today use toxic substances and create a lot of waste. Chemists are now trying to design cleaner ways to make products using renewable resources, safer materials, and less energy. This shift is difficult, especially when it comes to changing how big factories operate, but it is necessary for a more sustainable future. Plastic pollution is also a major concern. Plastics are everywhere, from packaging to clothes, but they do not break down easily and often end up in oceans and soil. Chemists are working to invent biodegradable and recyclable plastics that still work well but do not harm the planet. They're also trying to find better ways to remove microplastics and other plastic waste from the environment. In the world of medicine, chemists are helping to fight antibiotic resistance and new diseases. Bacteria are becoming harder to kill. There have not been many new antibiotics in recent years. Chemists must design new drugs and ways to deliver them more effectively. There is also growing interest in personalized medicine, where treatments are made to match a person’s genes. Water shortages and polluted water are big problems in many parts of the world. Chemists are developing new ways to clean water using less energy. They are also making better sensors to detect pollutants, like pesticides and heavy metals, in the water, soil, and air. Many high-tech devices, like smartphones and electric cars, need special materials such as lithium, cobalt, and rare earth elements. But mining these materials can harm the environment, and supplies are limited. Chemists are trying to find safer ways to extract these elements, reuse them, or even replace them with better alternatives.

## Types of chemistry
There are several types of chemistry. Analytical chemistry looks at which chemicals are in things. For example, looking at how much arsenic is in food. Organic chemistry looks at things that have carbon in them. For example, making acetylene. Inorganic chemistry looks at things that do not have carbon in them. One example is making an integrated circuit. Theoretical chemistry tries to explain chemical data with mathematics and computers.

A large area of chemistry is polymer chemistry. This looks at plastics. One example is making nylon. Because plastics are made of carbon, polymer chemistry is part of organic chemistry. Another area is biochemistry. This looks at the chemistry of living things. An example would be seeing how arsenic poisons people. Biochemistry is also part of organic chemistry. There are many other small branches of chemistry.

## Concepts of chemistry
### Matter
 is an example of matter]]
Matter is anything that has mass and takes up space. This means it fills up some amount of room, even if we cannot always see it. Everything around us, from tiny grains of sand to huge planets and stars, is made of matter. Even things like air or steam, which seem invisible or light, are still matter because they are made of tiny particles that have mass and take up space. We can notice matter in many ways. Some things, like a rock or a cup of water, are easy to see and touch. Other things, like the air in a room or the gas in a balloon, are harder to notice, but they are still there. You can feel them when the wind blows or when a balloon gets bigger. Matter is really important in science, especially in chemistry and physics, because it helps us understand what everything is made of and how it behaves. All matter is made up of atoms and molecules, which are tiny building blocks.

Matter can exist in different states, or forms, depending on how its tiny particles are arranged and how much energy they have. The three most common states are solid, liquid, and gas. In a solid, the particles are packed closely together in a fixed pattern. They do not move around much, which is why solids have a definite shape and volume. For example, a rock or a pencil keeps its shape unless something forces it to change. In a liquid, the particles are still close, but they are not in a set pattern. They can slide past each other, which lets liquids flow and take the shape of their container. However, liquids still keep the same volume. A cup of water stays the same amount, whether it is in a glass or a bowl. In a gas, the particles are spread far apart and move very fast. Gases do not have a fixed shape or volume. They will spread out to fill any space they are in, like air in a balloon. Gases can also be compressed or squished into a smaller space. Besides these three, there are also other, more unusual states of matter. One is plasma, which is made of super-energized particles with electric charges. Plasma is found in things like stars and lightning. Another rare state is called a Bose–Einstein condensate (BEC). It happens only at very, very cold temperatures close to absolute zero. In this state, particles move so slowly they start to act like one single particle instead of many.

Matter can be grouped into two main types based on what it's made of: pure substances and mixtures. Pure substances are made of only one kind of material and have the same properties throughout. They always have the same makeup, no matter where you find them. Pure substances can be either elements or compounds. Elements are the simplest type of pure substance. They cannot be broken down into anything simpler using normal chemical methods. Each element is made of just one kind of atom. Examples include oxygen (O₂), hydrogen (H₂), and iron (Fe). There are over 100 known elements, and they are all listed in the periodic table. Compounds are also pure substances, but they are made of two or more different elements that are chemically bonded together in a fixed ratio. This means the elements combine in a specific way to form a new substance. Some examples are water (H₂O), carbon dioxide (CO₂), and salt (NaCl). In a compound, the elements lose their individual properties and form something completely new.

Mixtures are made when two or more substances are physically combined, not chemically joined. This means each part of the mixture keeps its own properties, and you can often separate them using simple methods like filtering, boiling, or picking them apart. Mixtures come in two main types: homogeneous and heterogeneous. A homogeneous mixture (also called a solution) looks the same all the way through. The different parts are evenly mixed, and you cannot see or easily separate them. For example, when salt dissolves in water, it becomes a solution. You cannot see the salt anymore, but it is still there. Another example is air, which is a mixture of gases. A heterogeneous mixture looks different in different parts. The substances are not evenly mixed, and you can often see and separate the different parts. Examples include a salad, sand and iron filings, or oil and water. In these mixtures, the parts stay separate and can be picked out or separated by hand or simple tools.

The properties of matter help us understand how different substances act, react, and change in various situations. These properties are usually divided into two main types: physical properties and chemical properties. Physical properties are things we can see, measure, or feel without changing what the substance is made of. These include things like color, smell, taste, melting point, boiling point, density, hardness, electrical conductivity, and solubility (how well something dissolves). For example, water boils at 100°C and freezes at 0°C. One special thing about physical properties is that they do not change the substance. You can measure them many times, and the substance stays the same. For instance, if you melt ice, it turns into liquid water, but it is still H₂O. That means it’s a physical change, not a chemical one, and it can be reversed by freezing the water again. Some physical properties depend on how much of the substance you have. These are called extensive properties, like mass and volume. Others stay the same no matter how much you have. These are called intensive properties, like density and boiling point.

Chemical properties describe how a substance can change into something completely new. Unlike physical properties, which can be seen or measured without changing the substance, chemical properties can only be observed when a chemical reaction happens. These reactions change the substance's molecular or atomic structure, meaning it becomes a different substance. Some common chemical properties include flammability (how easily something burns), reactivity with acids or bases, rusting of iron, tarnishing of silver, and the ability to decompose or oxidize. For example, when wood burns, it reacts with oxygen in the air and turns into ash, carbon dioxide, water vapor, and heat. This is a chemical change, and the fact that wood can burn is a chemical property. Another example is iron rusting. When iron is exposed to air and moisture, it reacts to form rust, which is a new substance with different properties.

### Atoms
 atom, depicting the nucleus (pink) and the electron cloud distribution (black). The black bar is one angstrom ( or ).]]
The idea of atoms goes all the way back to around 400 BCE, when ancient Greek thinkers like Democritus and Leucippus came up with the idea that all matter is made of tiny, invisible particles. They called these particles "atomos," which means "uncuttable" or "indivisible." Democritus believed that atoms had different shapes and sizes and moved through empty space, combining in different ways to make everything we see. However, this idea was just a guess. It was not based on experiments or evidence. Another famous philosopher, Aristotle, had a different idea. He believed that everything was made of just four elements: earth, water, air, and fire. Because Aristotle was very popular and influential, people accepted his ideas for almost 2,000 years, and the atom theory was mostly forgotten. That changed in the early 1800s, when an English scientist named John Dalton brought the idea of atoms back. In 1803, Dalton proposed the first modern atomic theory. He said that all matter is made of atoms, and that atoms cannot be created or destroyed. He also said that all atoms of the same element are exactly the same, and that chemical reactions happen when atoms are rearranged. Dalton’s ideas were based on experiments, especially from studying how elements combine in fixed amounts. But, Dalton thought atoms were just solid spheres like a ball with no parts inside.

As scientists built better tools and did more experiments, they began to learn much more about what was inside an atom. In 1897, a scientist named J.J. Thomson discovered the electron by using a special tube called a cathode ray tube. This showed that atoms are not solid and indivisible after all. Thomson came up with the "plum pudding model," where he imagined the atom as a ball of positive charge with negative electrons scattered inside it, like raisins in a pudding. But in 1911, Ernest Rutherford did a famous experiment with gold foil that changed this idea. He found that most of the atom is empty space, and that the positive charge is packed into a tiny, dense center called the nucleus. This led to the nuclear model of the atom, where electrons orbit around a central nucleus. However, this model still did not explain why the electrons did not just crash into the nucleus. Then in 1913, Niels Bohr improved Rutherford’s model by using ideas from quantum theory. Bohr said that electrons move in fixed energy levels or shells around the nucleus. He also said that electrons can jump from one level to another by gaining or losing energy. Bohr’s model worked well for explaining the behavior of hydrogen, the simplest atom, but it did not work as well for bigger atoms. In the 1920s, scientists created an even better model using quantum mechanics. Scientists like Erwin Schrödinger, Werner Heisenberg, and Max Born showed that electrons do not move in perfect orbits. Instead, they exist in areas called electron clouds or orbitals, which are regions where electrons are likely to be found. This modern model of the atom is the most accurate one we have today and helps explain how atoms behave in chemistry and physics.

An atom is the smallest unit of matter. It is the basic building block that makes up everything around us. At the center of every atom is the nucleus, a tiny, dense part that contains two types of particles: protons and neutrons. Protons have a positive charge, while neutrons have no charge, they are neutral. The number of protons in an atom’s nucleus is called the atomic number, and it tells us what element the atom is. For example, all hydrogen atoms have one proton, and all oxygen atoms have eight protons. The mass number of an atom is the total number of protons and neutrons in the nucleus. Because protons and neutrons are both heavy (compared to electrons), almost all of the atom’s mass is in the nucleus, even though it is very small. Outside the nucleus is where the electrons are found. Electrons are much smaller and have a negative charge. In older models, scientists thought electrons moved in set paths around the nucleus, like planets orbiting the sun. But today, we know from quantum mechanics that electrons move in regions called orbitals or electron clouds. These are areas where electrons are most likely to be found, but their exact position cannot be known for sure. Electrons are also arranged in energy levels or shells around the nucleus. Each level can hold only a certain number of electrons. Atoms are electrically neutral when they have the same number of protons and electrons. But sometimes, atoms can gain or lose electrons, which changes their overall charge. When an atom loses electrons, it becomes a positively charged ion, called a cation. When an atom gains electrons, it becomes a negatively charged ion, called an anion. Also, atoms of the same element can have different numbers of neutrons. These are called isotopes. Even though isotopes have the same number of protons and act the same in chemical reactions, their different numbers of neutrons make them have slightly different masses. Some isotopes are stable, while others can break down over time, releasing energy.

The electron configuration of an atom is the way its electrons are arranged in different energy levels and areas around the nucleus. This arrangement is very important because it helps determine how the atom will react with other atoms and form chemical bonds. In today’s model of the atom, called the quantum mechanical model, electrons do not travel in neat circles around the nucleus like planets around the sun. Instead, they move around in areas called orbitals, which are regions where there is a high chance of finding an electron. These orbitals are grouped into energy levels, also called shells, which are labeled with numbers like 1, 2, 3, and so on. Each energy level has one or more subshells, which are labeled as s, p, d, and f. The first energy level (1) has only an s subshell. The second level (2) has s and p subshells. The third level (3) has s, p, and d, and so on. Each orbital can hold up to two electrons. The Aufbau principle helps scientists understand how electrons fill the space around an atom’s nucleus. According to this rule, electrons always fill the lowest energy orbitals first before moving to higher ones. The order in which electrons fill these orbitals goes like this: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, and so on. This order is based on the energy levels of the orbitals, and sometimes it can seem a bit out of order. For example, 4s fills before 3d because 4s has a lower energy level than 3d. There are also other important rules to remember when talking about electron configuration. Pauli exclusion principle says that only two electrons can fit into the same orbital, and they must spin in opposite directions. Hund’s rules says that when electrons go into orbitals that have the same energy (like three 2p orbitals), they will first go in one at a time with the same spin. Only after each orbital has one electron will they start to pair up. This helps reduce repulsion between electrons. We use a special way to write electron configurations. For example, the configuration for a carbon atom, which has 6 electrons, is: 1s² 2s² 2p². This means carbon has 2 electrons in the 1s orbital, 2 in the 2s, and 2 in the 2p. For bigger elements, we often use noble gas shorthand to save time. This means we start with the symbol of the nearest noble gas (like neon or argon) in brackets, and then continue from there. For example, sodium has 11 electrons. Instead of writing out everything, we can write: [Ne] 3s¹. Here, [Ne] stands for the first 10 electrons (1s² 2s² 2p⁶), and the 11th electron is in the 3s orbital. The valence electrons, or the electrons in the outermost shell, are very important. They decide how an atom behaves in chemical reactions, how it bonds with other atoms and what kind of compounds it can form.

### The Periodic Table
 (in periodic tables), and a traditional dividing line between metals and nonmetals. The f-block actually fits between groups 2 and 3; it is usually shown at the bottom of the table to save horizontal space.]]
A long time ago, people only knew about a few natural elements like gold, silver, copper, and iron. As science progressed, especially in the 1600s and 1700s, scientists started discovering more elements and learning about chemical reactions. In the late 1700s, a scientist named Antoine Lavoisier made one of the first lists of elements. He also suggested that elements were basic substances that could not be broken down into anything simpler. By the early 1800s, more elements had been discovered, and scientists began to see patterns in their properties. The big breakthrough came in 1869 when a Russian chemist named Dmitri Mendeleev created a version of the periodic table. He arranged the 63 known elements by increasing atomic mass and grouped elements with similar properties into columns. What was really amazing about Mendeleev’s table was that he left spaces for elements that had not been discovered yet and predicted what their properties would be. When scientists later discovered elements like gallium, scandium, and germanium, their properties matched Mendeleev’s predictions, which made his table even more accepted. Although there were some problems with his system, like elements that did not fit perfectly by mass, Mendeleev’s table helped scientists understand the relationships between different elements. In the early 1900s, Henry Moseley improved the table by arranging elements by atomic number, which solved many of the problems. He showed that elements’ properties depend on the number of protons in their nucleus. This led to the modern periodic law, which says that elements’ properties change in a regular pattern based on their atomic number. Since then, the periodic table has grown to include over 100 elements, including many that are made in labs. It has also become more complex with the addition of new groups of elements like noble gases, transition metals, and lanthanides and actinides (the f-block elements).

The periodic table is a special chart that shows all the known chemical elements in a very organized way. It is arranged based on each element’s atomic number (how many protons it has), how its electrons are set up, and how it behaves in chemical reactions. The table is built to match how atoms work, especially how electrons move around the nucleus. This setup helps scientists understand how each element will act during chemical changes. The table has rows called periods and columns called groups or families. Periods go from left to right and are numbered from 1 to 7. Each period shows how many energy levels, or electron shells, the atoms have. As you move across a period, each element gets one more proton and one more electron, and they go into the same energy level. This changes things like the size of the atom and how strongly it attracts electrons. Groups go up and down and are numbered from 1 to 18. Elements in the same group have the same number of valence electrons. These are the electrons in the outer shell of the atom. Because of this, elements in the same group act in similar ways. For example, Group 1 is called the alkali metals (like lithium and sodium). They all have one valence electron and are very reactive, especially with water. Group 17 is the halogens (like fluorine and chlorine). They have seven valence electrons and are also very reactive. Group 18 is the noble gases (like helium and neon). They have full outer shells of electrons and do not react much with other elements.

The periodic table is also split into four big sections called blocks. The s, p, d, and f blocks. These blocks are based on how electrons fill up different parts of an atom, called orbitals. The s-block includes Groups 1 and 2, plus hydrogen and helium. In these elements, the s orbitals are being filled with electrons. The p-block includes Groups 13 to 18. These elements are filling up their p orbitals. The s-block and p-blocks together make up most of the main group elements, which include both metals and nonmetals. The d-block is in the middle of the table and includes Groups 3 to 12. These are the transition metals. They are special because they can form different types of charged ions and complex compounds. The f-block is at the bottom of the table and includes the lanthanides and actinides. These elements are filling their f orbitals. Even though they are placed separately to keep the table neat, they actually belong in the 6th and 7th rows (periods). The periodic table also shows trends, or patterns, in the properties of elements. Atomic radius (the size of an atom) gets smaller across a period from left to right because the nucleus pulls electrons in tighter. But it gets larger going down a group because more electron shells are added. Electronegativity (how strongly an atom attracts electrons) usually increases across a period and decreases down a group. Ionization energy (the energy needed to remove an electron) follows a similar pattern. It increases across a period and decreases down a group.

### Basic concepts
The basic unit of an element is called an atom. An atom is the smallest building block that you can cut an element into without the element breaking down (turning into a lighter element, for example through nuclear fission or radioactive decay). A chemical compound is a substance made up of two or more elements. In a compound, two or more atoms are joined to form a molecule. The tiniest speck of dust or drop of liquid, that one can see is made up of many millions or billions of these molecules. Mixtures are substances where chemicals are mixed but not reacted. An example would be mixing sand and salt. This can be undone again to produce salt and sand separately. Chemical compounds are changed by a chemical reaction. An example would be heating sodium bicarbonate, common baking soda. It will make water, carbon dioxide, and sodium carbonate. This reaction cannot be undone.

One very important concept in chemistry is that different atoms interact with one another in very specific proportions. For example, two hydrogen atoms interacting with one oxygen atom lead to the water molecule, H2O. This relationship is known as the "Law of constant proportions" and leads to the idea of "stoichiometry", a term that refers to the ratios of different atoms in chemical compounds. For example, in water, there are always exactly 2 hydrogen atoms to 1 oxygen atom. In carbon dioxide, there are exactly 2 oxygen atoms for 1 carbon atom. These relationships are described using chemical formulas such as H2O (two hydrogen atoms and one oxygen atom) and CO2 (one carbon atom and two oxygen atoms).

### Mole
Because atoms of different elements react with one another in very specific proportions but atoms of different elements have different weights, chemists often describe the number of different elements and compounds in terms of the number of "moles". A "mole" of any element contains the same number of atoms: 602,214,150,000,000,000,000,000 atoms. The atomic mass of an element can be used to see how much of the element makes a mole. For example, the atomic mass of copper is about 63.55. That means about 63.55 grams of copper metal has a mole of atoms. The atomic mass of chlorine is about 35.45. That means 35.45 grams of chlorine has a mole of atoms in it.

Moles can be used to see how many molecules are in chemical compounds, too. Copper(II) chloride is an example. CuCl2 is its chemical formula. There is one copper atom (63.55) and two chlorine atoms (35.45 · 2 = 70.90). Add all the molar masses of the elements together to get the molar mass of the chemical compound (63.55 + 70.90 = 134.45). That means in 134.45 grams of copper(II) chloride, there is one mole of copper(II) chloride molecules. This concept is used to calculate how much chemicals are needed in a chemical reaction if no reactants (chemicals that are reacted) should be left. If too much reactant is used, there will be some reactants left in the chemical reaction.

### Acids and bases
Acids and bases are a common type of chemical. Using the simplest definitions, acids add  ions when in water, and bases add  ions when in water. Acids can react with bases: the  takes the extra hydrogen from  to make an extra water molecule, . The other parts of the acid and base make a salt.

An example would be reacting hydrochloric acid (HCl) and sodium hydroxide (NaOH). Hydrochloric acid releases H+ and Cl- ions in water. The base releases Na+ and OH- ions. The H+ and the OH- react to make water. There is a solution of sodium chloride (NaCl) left. Sodium chloride is a salt.

This definition of acids and bases, called the Arrhenius acid-base theory, is not used by modern chemists. It is too oversimplified for what really happens in water, and cannot describe anything dissolved in another solvent like ammonia. Instead, chemists use the Brønsted–Lowry acid–base theory and Lewis acid-base theory, which are more complicated but more useful in chemistry. Instead of looking at reactions in water, these theories focus on hydrogen ions and pairs of electrons.

## Usefulness
Chemistry is very useful in everyday life and makes up the foundation of many branches of science. Most objects are made by chemists (people who do chemistry). Chemists are constantly working to find new and useful substances. Chemists make new drugs and materials like paints that we use every day.

## Safety
 give protection to the eyes when pouring chemicals.]]
s. He is looking at a rock from the moon.]]
Many chemicals are harmless, but there are some chemicals that are dangerous. For example, mercury(II) chloride is very . Chromates can cause cancer. Tin(II) chloride pollutes water easily. Hydrochloric acid can cause bad burns. Some chemicals like hydrogen can explode or catch fire. To stay safe, chemists experiment with chemicals in a chemical lab. They use special equipment to keep the chemicals contained when they do reactions. They also wear special clothing to stay safe. The chemicals used in medicines and in products like bleach have been tested to make sure they are safe if used correctly.

## Related pages 
* Periodic table
* List of common elements
* Laboratory techniques
* Aerosol

## References