Chemical nomenclature

's *Chymical Nomenclature* in English.]]

**Chemical nomenclature** is the system scientists use to name chemicals in a clear and organized way. It helps chemists talk about substances without confusion, no matter where they are in the world or what language they speak. This system includes rules for naming elements (like hydrogen or oxygen), compounds (like water or carbon dioxide), acids and bases, big molecules like plastics and proteins, and special groups like ions or isotopes. Using the same naming rules helps everyone understand exactly which substance is being discussed. It is very important in science classes, labs, medicine, industries, and safety rules.

The rules for naming chemicals are mostly created and managed by a group called IUPAC (International Union of Pure and Applied Chemistry). They make sure the names follow clear rules, show what the chemical is made of, and can be understood all over the world. IUPAC updates these rules when new discoveries are made. Besides IUPAC names, people also use other types of names like common names (like "water" instead of "dihydrogen monoxide"), common names (like "baking soda" for sodium bicarbonate), and trade names (used in business, like brand names for products). These other names are often easier to remember but may not give as much detail about the chemical’s structure or ingredients.

In inorganic chemistry, there are special rules for naming different kinds of compounds. These include things like ionic compounds, covalent compounds, oxides, acids and bases, and coordination complexes. When naming these compounds, scientists look at things like what elements are in the compound, how many atoms of each element there are, the charge of the elements (called oxidation states), and whether special groups like polyatomic ions (e.g., SO₄²⁻ or NO₃⁻) are present. For example, FeCl₃ is called iron(III) chloride because the iron has a +3 charge. Another example, CO₂ is called carbon dioxide because it has two oxygen atoms. Coordination complexes have their own set of rules to make sure the name tells you what the central metal is, what its charge is, and what ligands (attached groups or ions) are around it.

In organic chemistry, naming compounds can be more complex because there are so many different carbon-based molecules. To keep things clear and organized, scientists use a system created by IUPAC (International Union of Pure and Applied Chemistry). This system helps chemists name compounds in a way that describes exactly how the atoms are connected and arranged. One of the first steps in naming an organic compound is to find the longest carbon chain. This chain is called the parent structure, and it gives the base name of the compound. Next, chemists look for functional groups, special groups of atoms like alcohols (–OH), acids (–COOH), or amines (–NH₂), that change the properties of the molecule. These groups are added to the name using prefixes, infixes, and suffixes to show what’s attached and where. For example, the molecule CH₃CH₂OH has two carbon atoms and an –OH group, so it is named ethanol. Another compound, CH₃CH₂COOH, has three carbon atoms and a –COOH group, so it is called propanoic acid. The naming system also includes ways to show the 3D arrangement of atoms, since the same atoms can be arranged differently. Terms like cis/trans, R/S, and E/Z are used to describe these differences, called stereochemistry and isomerism. This helps chemists understand not just what atoms are in a molecule, but also how they are arranged in space, which can affect how the molecule behaves.

Chemical nomenclature also includes rules for naming acids and bases, which are important types of compounds in chemistry. For example, sulfuric acid has the formula H₂SO₄, and ammonia is NH₃. When acids and bases react, they can form salts, which also have names that follow specific rules. One example is ammonium sulfate, written as (NH₄)₂SO₄, which is a salt made from ammonia and sulfuric acid. Naming rules also apply to more complex compounds. For instance, hydrated compounds are substances that include water molecules in their structure. These are named by adding terms like “monohydrate” or “pentahydrate” to show how many water molecules are present. There are also rules for mixed-valence compounds, which contain the same element in different oxidation states, and organometallic compounds, which have metals bonded to carbon atoms. In biology and biochemistry, some substances like amino acids and nucleotides (building blocks of proteins and DNA) have their own special naming systems because of their complexity and importance in living things. When it comes to polymers (long chains of repeating molecules) and macromolecules, there are two main ways to name them: by the source material (like natural rubber) or by their chemical structure. Many polymers also have short names or abbreviations to make them easier to talk about. For example, polyethylene is often called PE, and polytetrafluoroethylene is known as PTFE, the material used in non-stick cookware (like Teflon).

The IUPAC's rules for naming chemical compounds are written in a series of books. The system for naming organic compounds is written in the *Blue Book*. The system for naming inorganic compounds is written in the *Red Book*. A third book, called the *Green Book*, gives recommendations about the use of symbols for physical quantities. A fourth book, the *Gold Book*, contains the definitions of many of the technical terms used in chemistry. Similar books exist for biochemistry, analytical chemistry, macromolecular chemistry, and clinical chemistry. The books do not cover everything, however. Shorter recommendations for specific circumstances are published sometimes in the journal *Pure and Applied Chemistry*.

In today’s digital world, scientists do not just use regular chemical names. They also use special codes that computers can understand. Two examples are InChI (International Chemical Identifier) and SMILES (Simplified Molecular Input Line Entry System). These are ways to describe chemical structures using text that can be read by computer programs. Even though these codes may look strange and are not easy for people to read, they are very helpful. They let computers store, search, and organize millions of chemical compounds quickly and accurately. This is especially useful in big databases, online searches, and software used for chemistry research. These systems are important tools in fields like cheminformatics (the use of computers in chemistry), data mining, and keeping digital records of chemical information. So while traditional names are still used by people, InChI and SMILES help computers keep track of all the complex chemical information in the modern age.

## History
### Pre-systematic Naming
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Before scientists created a standard way to name chemicals, naming was messy and confusing. People in different parts of the world used their own words to describe the same substance, or they gave the same name to completely different things. This was because names were based on things like how a substance looked, what it was used for, or how it was made. For example, a substance might be called “blue stone” in one place because of its color, and “healer’s powder” in another place because of its use in medicine, even though both names referred to the same thing. Or two totally different substances might both be called “salt” just because they looked similar. Back then, scientists did not fully understand what chemicals were made of. Chemistry was still developing, and people often worked alone or in small groups without sharing knowledge across countries. As a result, there was no universal system for naming things, which made it hard to communicate or build on each other’s work.

During the Middle Ages and Renaissance, people called alchemists studied early forms of chemistry. But their naming system was very different from what we use today. Alchemists did not just describe what a substance looked like or what it did, they gave names that were full of symbols and hidden meanings. For example, gold was called the “Sun” because it was shiny and powerful. Silver was called the “Moon” because of its white glow. Mercury was called the “Spirit” because it could change form easily, moving between liquid and vapor. These names were not just about the materials themselves, they were also tied to deep beliefs about nature, the stars, and even the soul. Alchemists often used secret symbols and strange words to keep their discoveries hidden from others. They believed that turning one metal into another, like lead into gold, was also a kind of spiritual transformation. So their names and writings were filled with mystery and magic. This made it hard for others to understand their work and slowed the development of a clear chemical language. Another example is "aqua regia," which means “royal water” in Latin. This name was used for a strong acid mixture that could dissolve gold, something most other chemicals could not do. Since gold was seen as the “king” of metals, the acid that could destroy it was called royal. While these names gave useful hints about how a substance behaved, they were not part of a clear or universal system. People in different countries or regions might use the same name for different things, or use different names for the same substance. This often caused confusion, especially as science became more global and people needed a common way to talk about chemicals. The word “alkali” originally came from Arabic (al-qaly), but scholars changed it to sound more like Latin when writing it in their texts. Even newer scientific names, like “hydrogen,” follow this pattern. “Hydro” comes from the Greek word for water, and “-gen” means “to produce” or “form.” So, hydrogen literally means “water-forming,” since it combines with oxygen to make water. These classical roots helped give scientific names meaning, even before an official naming system existed.

The Méthode de nomenclature chimique introduced a new, logical way to name chemical substances. Instead of using old-fashioned or random names, the new system focused on what each substance was made of and how it behaved in chemical reactions. This meant that a substance’s name could tell you which elements it contained and how much of each was present. For example, if two elements joined together to form a compound, the name would show both elements and their proportions. This helped scientists understand what they were working with, just by looking at the name. It also made it easier to guess the names of new compounds by following the same set of rules. One of the smartest parts of this system was how flexible it was. It did not need to be changed every time a new substance was discovered. Instead, it could grow and include new chemicals just by applying the same naming rules. This clear and expandable system helped chemistry become more organized and scientific, making it easier for scientists to share and build on each other’s work. They named hydrogen “water former” because it creates water when it burns in air.

To deal with the growing number of chemical compounds, scientists started working on ways to organize and name them in a more logical way. They tried to create early systems for naming chemicals, but progress was slow and not the same everywhere. In many cases, older names, called trivial names, continued to be used. These names often came from where the substance was found or how it was used in the past. For example, acetic acid, which is what gives vinegar its sour taste, got its name from the Latin word acetum, meaning vinegar. Names like these were simple and familiar, but they did not tell you anything about the structure of the molecule or how it related to other chemicals. Because these traditional names were so common and people were used to them, they did not go away easily. So, scientists ended up using two kinds of names at the same time, trivial names for everyday use and more systematic names for scientific accuracy.

The effort to bring more order to chemical names got a big push from the work of Jöns Jacob Berzelius, a Swedish chemist in the early 1800s. He was one of the first scientists to try to create a universal chemical language that could be used by chemists all over the world. One of his most important contributions was creating the system of chemical symbols that we still use today. For example, H stands for hydrogen, O for oxygen, and C for carbon. Before this, chemical names were long, confusing, or even symbolic, and they varied between countries and scientists. Berzelius also worked hard to figure out the atomic weights of elements, basically, how heavy each type of atom is compared to others. This helped chemists better understand how elements combine to form compounds, because it allowed them to calculate the ratios in which different atoms joined together. His system gave chemistry a more mathematical and logical foundation. Even though Berzelius focused mostly on elements rather than large molecules, his ideas helped scientists write down chemical formulas in a clear and organized way. For example, instead of writing out "water is made of two parts hydrogen and one part oxygen," they could simply write H₂O. This was a huge step forward because it made it much easier to communicate and share chemical knowledge.

The formal naming of organic compounds began to take shape in the 1830s, when chemists like Jöns Jacob Berzelius, Justus von Liebig, and Friedrich Wöhler started introducing names for newly discovered substances. For example, they came up with names like benzoyl chloride and ethyl iodide, which reflected the compounds’ chemical makeup more clearly than older, traditional names. This period marked the early steps toward a more systematic way of naming carbon-based compounds, laying the foundation for modern organic nomenclature. The first serious international effort to create a consistent naming system happened in 1889, when an International Commission for the Reform of Chemical Nomenclature was formed. The goal of this group was to create a clear and logical method for naming organic chemicals so scientists from different countries could communicate more effectively. In 1892, this effort led to a historic meeting where 34 leading chemists from nine European countries gathered and agreed on a set of rules now known as the Geneva Rules. These rules were specifically focused on aliphatic compounds, organic molecules that consist of straight or branched chains of carbon atoms (as opposed to ring structures). The Geneva Rules introduced two major principles that are still used today:
* The longest continuous carbon chain in the molecule is identified and used as the base name or parent name.
* Functional groups, specific groups of atoms that affect the compound’s chemical behavior are represented using suffixes added to the base name.

For example, if a molecule contains a carboxylic acid group (–COOH) attached to a three-carbon chain, the name would be propanoic acid: "prop-" for the three-carbon chain, and "-oic acid" to show the presence of the acid group. The Geneva group planned to extend these rules to cyclic (ring-shaped) compounds, but this part of the project was never completed at the time. Still, the rules they established became a cornerstone for future naming systems and are considered a major milestone in the development of systematic chemical nomenclature.

One of the most important jobs of IUPAC has been to create and update clear, systematic rules for naming chemical compounds. As chemistry progressed through the 20th century, scientists discovered and created more and more types of substances, from simple materials like table salt to complex molecules such as plastics, medicines, and biological molecules like proteins and DNA. With so many new compounds being studied, a consistent and logical naming system became more necessary than ever. To meet this need, IUPAC developed a set of formal naming rules that scientists around the world could follow. These rules helped chemists name both inorganic compounds (like salts and acids) and organic compounds (which contain carbon and include things like alcohols, acids, sugars, and drugs). The system used a compound’s structure, functional groups (the reactive parts of the molecule), and how its atoms are connected to generate names. These systematic names provided clues about the molecule’s makeup and shape. For example, a name could tell a chemist how many carbon atoms the molecule had, what kind of bonds were present (single, double, or triple), and where certain important parts of the molecule were located. This made it possible for chemists to understand and visualize a compound just from its name, without needing a picture. To help scientists name and describe chemical substances clearly and consistently, IUPAC created a set of important reference books, each focused on a different part of chemistry. These books are often known by their color-coded names and serve as guides for chemists around the world.

The Red Book is focused on inorganic chemistry. It explains how to name substances like salts, acids, bases, and coordination compounds, which are often made from metals and nonmetals. It gives rules for writing names that reflect the chemical structure and the oxidation states of the elements involved. For example, it helps explain why we call FeCl₃ "iron(III) chloride." The Blue Book is for organic chemistry, which deals with carbon-based compounds. This book goes into detail about how to name molecules made of carbon chains, rings, and other groups like alcohols or acids. It gives rules for using prefixes, suffixes, and numbers to describe exactly what atoms are in the compound and where they are located. For instance, it explains how to name CH₃CH₂OH as "ethanol." The Green Book is a bit different. Instead of focusing on names of compounds, it helps scientists standardize units, quantities, and symbols used in physical chemistry. This ensures that when a scientist measures temperature, energy, or pressure, they use consistent units like Kelvin for temperature or Joules for energy, so results are easy to understand and compare globally. IUPAC also created other helpful books. The Gold Book collects and defines scientific terms used in chemistry. It helps make sure that when a chemist uses a word like “enthalpy” or “electronegativity,” everyone understands exactly what it means. The Orange Book focuses on analytical chemistry, which is the science of measuring and detecting substances. It includes recommended procedures and definitions to support accurate chemical analysis. Together, these color-coded books form the backbone of chemical communication. They make it easier for scientists to share ideas, publish papers, and teach chemistry in a way that is clear, consistent, and internationally recognized.

As the field of chemistry continued to grow and branch into new areas, IUPAC expanded its naming systems to cover these newer, more specialized topics. This was important because each subfield introduced new types of molecules that could not be easily named using the older rules. In biochemistry, for example, scientists study complex molecules found in living things, such as proteins, enzymes, DNA, and carbohydrates. These molecules often have large and complicated structures. To create a clear and standard way of naming them, IUPAC teamed up with another organization, the International Union of Biochemistry and Molecular Biology (IUBMB). Together, they made rules for naming amino acids, nucleotides, enzymes, and other important biomolecules. For example, enzymes are often named based on the type of reaction they perform, like "lactase" which breaks down lactose.

### Modernization and Digital Standardization of Chemical Nomenclature
Since the early 20th century, the International Union of Pure and Applied Chemistry (IUPAC) has played a key role in updating and expanding the rules for naming chemical substances. As scientists discovered new elements and created new types of compounds, IUPAC worked to make sure the naming system could handle this growing complexity while staying clear and consistent. For example, when new elements like nihonium (Nh), moscovium (Mc), tennessine (Ts), and oganesson (Og) were officially recognized in the 21st century, IUPAC helped assign names and symbols for them based on a mix of international input, tradition, and clear rules. These names followed a pattern that made them recognizable and easy to use in all scientific languages. In organic chemistry, new classes of molecules like fullerenes, hollow carbon cages that look like soccer balls, required entirely new naming methods. These were very different from straight or branched carbon chains, so IUPAC developed special rules to describe their shape and the positions of atoms in the structure. Another example is organometallic compounds, which contain metal-carbon bonds and are used in many modern chemical reactions, including in drug development and material science. Naming these required a blend of inorganic and organic rules, so IUPAC had to create new guidelines that could handle the mix of elements and bonding types.

Some of the chemical names we hear all the time are actually common or traditional names, not the long, official ones used in science. These names are popular because they are much easier to say and remember. Take ammonia, for example. It is a common cleaning product, and its chemical formula is NH₃. The official IUPAC name is nitrogen trihydride, but hardly anyone uses that. “Ammonia” is just easier. Another example is acetic acid, the ingredient that gives vinegar its sour smell and taste. Scientists call it ethanoic acid, but “acetic acid” is much more widely known. Then there is glucose, a sugar your body uses for energy. Its official name is something long and complicated like D-glucopyranose, but even doctors and scientists just say “glucose” because it is simpler and everyone understands it. Sometimes, a name works both ways. For instance, methane, a gas used as fuel, is short, easy to say, and also follows the IUPAC naming rules, so it is both a common and official name. But when a molecule is more complex, the scientific name can get really long. One example is acetaminophen, the common name for a pain reliever. Its full IUPAC name is N-(4-hydroxyphenyl)acetamide, but that is mostly used in science papers or on legal documents.

Common and traditional chemical names often came from how the substance looked, where it was found, or what it was used for. For example, acetic acid, the sour part of vinegar, gets its name from the Latin word acetum, which means vinegar. And baking soda, which we use in baking and cleaning, is the common name for sodium bicarbonate. Many of these names also come from alchemy, an early form of chemistry that mixed science with mystical ideas. Alchemists gave cool or dramatic names to chemicals based on how they acted or looked. One example is “aqua regia”, which means “royal water” in Latin. It got that name because it could dissolve gold. Another is “oil of vitriol”, an old name for sulfuric acid, which looked oily and was very strong and corrosive. Some names were based on everyday use and trade. People working in construction or farming used “lime” to refer to calcium oxide, and “potash” was a name for potassium-rich substances made from wood ashes. These names became popular because they were easy to say and helped people understand what the substances were used for. Nature also inspired many chemical names. Citric acid, found in lemons and oranges, was named after citrus fruits. Salicylic acid, used to make aspirin, got its name from the willow tree, whose bark naturally contains this compound. The Latin name for willow is Salix. Over time, these traditional names were used so much that they showed up in textbooks, science papers, and product labels.

Even though common chemical names do not always follow strict scientific rules, people still use them a lot today because they are simple, familiar, and easy to understand. You will see these names used in classrooms, medicine, industry, and even in everyday life. In many cases, being clear and quick is more important than giving the full scientific name. For example, when a teacher is teaching students about acids, they will probably say “acetic acid” instead of “ethanoic acid”, because it is easier to remember and say. In cooking, it is much simpler to call something “baking soda” than to say “sodium hydrogen carbonate” on a recipe or food label. These names are so useful that even the group that makes the official rules for chemical names, the International Union of Pure and Applied Chemistry (IUPAC), knows they are important. IUPAC has officially accepted some common names, like “ammonia” for NH₃ and “water” for H₂O, even though they do not follow the usual naming rules. To help everyone understand both types of names, IUPAC publishes glossaries and guides that list common names along with their scientific (systematic) versions. These resources explain when it is okay to use a common name and how it connects to the correct IUPAC name. This approach helps make chemistry more flexible. Scientists can still be accurate when they need to be, but they can also be clear and practical when talking to students or the general public.

### IUPAC Nomenclature
 tells us the molecule has two carbon atoms and an alcohol group (which is a -OH).]]
Chemists all over the world need a way to name chemical substances so that everyone understands exactly what they are talking about. That’s why they use something called IUPAC nomenclature. This is a special naming system created by a group called the International Union of Pure and Applied Chemistry, or IUPAC for short. IUPAC was formed in 1919 to make sure that scientists in different countries could all speak the same "chemical language." Before IUPAC, chemists in different places used different names for the same chemicals. That caused a lot of confusion. IUPAC helped fix this by making a set of clear rules for naming chemicals. These rules tell scientists how to name substances based on what atoms are in them and how those atoms are connected. The main idea is that every chemical should have one name that clearly shows its structure. For example, the name ethanol tells us the molecule has two carbon atoms and an alcohol group (which is a -OH). Another example is ethanoic acid, which is another name for vinegar’s main ingredient (also called acetic acid). This name tells us the molecule has two carbon atoms and a special group called a carboxylic acid. The IUPAC system works for all kinds of chemicals, simple ones, complicated ones, big ones like plastics, and even metal-containing ones. To create these names, IUPAC uses a mix of prefixes (like “di-” or “tri-”), root words (like “meth-” for one carbon), suffixes (like “-ol” for alcohol), and numbers to show where each part of the molecule is. This helps scientists write and understand chemical names without guessing.

The IUPAC naming system is split into different sections because there are many kinds of chemical substances, and each type needs its own set of rules. These rules are collected in special books, and each book focuses on a different area of chemistry. One big area is inorganic chemistry. This includes things like salts, acids, bases, oxides, and some more complicated compounds called coordination complexes. To help scientists name these properly, IUPAC made a guide called the Red Book. For example, the name sodium carbonate (Na₂CO₃) tells us the compound is made of sodium and carbonate ions, while dinitrogen tetroxide (N₂O₄) tells us the molecule has two nitrogen atoms and four oxygen atoms. The Red Book also explains how to name tricky molecules like hexaaquacobalt(III) chloride, which has a metal ion (cobalt) surrounded by six water molecules, plus some chloride ions. Another important area is organic chemistry, which is all about carbon-based molecules. These are the kinds of molecules found in living things, fuels, plastics, and medicines. The guide for naming them is called the Blue Book. It shows how to name compounds based on how many carbon atoms they have, what special parts (called functional groups) are attached, like alcohols or acids, and where those parts are located. For example, butane (C₄H₁₀) is a simple fuel, and ethanoic acid is a small acid also known as acetic acid (the main ingredient in vinegar). The Blue Book also teaches how to name molecules with double or triple bonds, or with special shapes, like cis-trans isomers (where parts of the molecule are on the same or opposite sides) or chiral centers (which affect how the molecule behaves in 3D). It even covers advanced molecules like rings or heterocycles (rings that include atoms other than carbon). This is very useful when naming medicines, natural chemicals, and other important biological molecules.

Besides the well-known Red Book (for naming inorganic compounds) and Blue Book (for naming organic compounds), IUPAC has created other important books to help name chemicals in more specific areas. These books are color-coded too, and each one focuses on a different part of chemistry. The Purple Book is all about macromolecules and polymers. These are very large molecules made from smaller, repeating parts called monomers. Examples include plastics and synthetic fibers. Naming these materials can be tricky because the building blocks can be arranged in many different ways, straight chains, loops, or even branching shapes. The Purple Book gives scientists a clear way to name and describe these complex materials so they can talk about them without confusion. The White Book is used for biochemistry, the chemistry of living things. It was made by IUPAC along with another group called the International Union of Biochemistry and Molecular Biology (IUBMB). This book helps name important substances in the body, like enzymes, amino acids, nucleotides, and proteins. For example, enzymes (which help speed up chemical reactions in the body) are named based on the job they do. Having a clear naming system is very important in medicine and biology, where it is necessary to know exactly what each molecule does. The Green Book is a little different. Instead of focusing on chemical names, it deals with quantities, units, and symbols used in measurements. So if scientists are talking about energy, pressure, or concentration, the Green Book helps them all use the same words and units, like joules, pascals, or moles per liter. This is especially important when scientists from different countries work together or share their results in scientific papers.

The IUPAC naming system is a big part of how chemists talk about chemicals. These names are not random. They are carefully made to show what a molecule looks like and how it is built. That way, a chemist anywhere in the world can look at the name and understand the exact structure of the compound. In organic chemistry, for example, the name hexane tells us a lot. The part "hex-" means there are six carbon atoms, and the "-ane" ending means they are connected with single bonds only. So just from the name, a chemist knows it is a six-carbon chain with hydrogen atoms filling in the rest. Another example is ethanoic acid, which you might know better as acetic acid, the sour chemical in vinegar. “Ethanoic acid” tells a scientist it has two carbon atoms ("eth-") and a special group called a carboxylic acid ("-oic acid"). In inorganic chemistry, the names can describe even more complex things. For instance, the compound [Fe(CN)₆]⁴⁻ is called hexacyanoferrate(II). That may sound tricky, but it actually gives a lot of helpful information. It tells us the metal in the center is iron, there are six cyanide ions around it ("hexacyano"), and the iron has a +2 charge (that’s what the Roman numeral II means). If someone just called it “iron cyanide,” it would not give nearly as much detail. IUPAC names also help describe molecules in 3D, which is important because some molecules act differently depending on how their atoms are arranged in space. For example, a molecule called (R)-2-butanol is a type of alcohol with four carbon atoms, and the “2-” tells us the –OH group is on the second carbon. The “R” means the atoms are arranged in a specific 3D shape. If they were arranged the other way, it would be called (S)-2-butanol,same atoms, but different shape and behavior.

One important thing to know about the IUPAC naming system is that it is not set in stone. It changes and improves over time, just like science does. Chemistry is always moving forward. Scientists keep discovering new substances, creating new materials, and finding better ways to study molecules. Because of this, the rules for naming chemicals also have to keep up. For example, when scientists create a new element in the lab, IUPAC gives it an official name and symbol. That’s how elements like oganesson (Og) and livermorium (Lv) were added to the periodic table. These names were chosen and approved by IUPAC after careful checking. Also, when researchers find new kinds of chemical structures or bonding patterns, IUPAC updates its rules so those can be named properly too. To keep everyone on the same page, IUPAC publishes updated versions of its important reference books. These include the Red Book for inorganic chemistry, the Blue Book for organic chemistry, the Green Book for units and symbols, and the Gold Book, which explains key terms in chemistry. Every new edition includes the latest scientific knowledge and helps chemists use the best and most accurate names. These updates are especially important now that so much chemistry is done using computers and digital tools. IUPAC works to make sure its naming systems also work with software, using formats like InChI and SMILES, which are ways to write chemical structures that computers can understand. Because of these changes, chemists all over the world, whether working in labs or on computers, can communicate clearly and accurately.

### Numerical and Structural Identifiers 
#### CAS Registry Numbers 
 (58-08-2).]]
The Chemical Abstracts Service (CAS) is part of the American Chemical Society, and it was created to help scientists keep track of all the new chemicals being discovered. Chemistry is a huge field, and new substances are found or created all the time. CAS helps organize all this information so scientists do not get overwhelmed or confused. Since it started in the early 1900s, CAS has become one of the most important organizations for keeping records about chemicals. It runs a giant, constantly updated database called the CAS Registry. This database has information on millions of different substances, not just everyday chemicals, but also rare compounds, mixtures, plastics, isotopes, metals, and even biological molecules like DNA and proteins. One of the most useful things in the CAS Registry is something called the CAS Registry Number. This is a special number given to every chemical in the database, kind of like an ID number. Unlike chemical names, which can be long or might change over time, a CAS number is simple and never changes. It is made up of numbers with hyphens, like 58-08-2, which is the CAS number for caffeine. These numbers do not tell you what the chemical is made of, they are just a unique code for each substance, like a fingerprint or license plate. What makes them really useful is that they work the same way in every country and every language. So even if people call a chemical by different names in different places, they can all understand each other by using the CAS number.

Even though CAS Registry Numbers are not part of the official naming rules for chemicals, they are still very important. These numbers act like permanent ID tags for each chemical substance. If the name of a chemical changes because of new discoveries or updates to naming rules, its CAS number stays the same. That’s what makes them so useful for keeping track of chemicals. CAS numbers are especially helpful in places where accuracy and safety really matter, like in databases, legal papers, safety documents, and international rules. They make sure that everyone is talking about the same exact substance, even if the name is different. One big reason CAS numbers are so helpful is that they prevent confusion. A single chemical can have many names: a long scientific name, a common name, and even a brand name. But it will always have only one CAS number. That means scientists, companies, and governments can all be sure they are referring to the right substance, no matter what language they speak or what name they use.

CAS numbers are used in many important and practical ways. One place you will often see them is on Safety Data Sheets (SDS). These are official documents that explain how to safely handle a chemical, what dangers it might have, and what to do in an emergency. Many countries require these sheets by law, and the CAS number makes sure the safety information is matched to the exact right chemical. Government groups like the U.S. Environmental Protection Agency (EPA), the European Chemicals Agency (ECHA), and the United Nations also use CAS numbers. They help these organizations track and regulate chemicals to keep people and the environment safe. In the chemical industry, companies use CAS numbers in catalogs, shipping labels, and inventory systems. This helps avoid mistakes when dealing with thousands of different substances. CAS numbers make it clear exactly which chemical is being used or shipped. CAS numbers are also used in online research tools like SciFinder, PubChem, and Reaxys. If you type in a CAS number, you can quickly find details about a chemical’s structure, uses, properties, and even scientific studies about it.

#### International Chemical Identifier (InChI)
 has the structure shown above. The standard InChI for morphine is `InChI=1S/C17H19NO3/c1-18-7-6-17-10-3-5-13(20)16(17)21-15-12(19)4-2-9(14(15)17)8-11(10)18/h2-5,10-11,13,16,19-20H,6-8H2,1H3/t10-,11+,13-,16-,17-/m0/s1`.]]

The International Chemical Identifier (InChI) is a special kind of code that describes what a chemical molecule looks like, using just text. It was created by IUPAC and the National Institute of Standards and Technology (NIST) to help scientists and even computers, share and search for chemical information more easily online. Instead of using traditional chemical names, which can change depending on language or naming style, InChI gives each substance a unique string of characters that is based on its actual molecular structure. This means every chemical has one clear identity that is the same everywhere in the world. InChI is really smart because it breaks down a molecule into layers of information. It shows how the atoms are connected, where the hydrogen atoms are, whether the molecule can change forms (called tautomers), what its 3D shape is (stereochemistry), and if there are any isotopes (atoms of the same element with different weights). All this is packed into a line of text that looks like this for ethanol: `InChI=1S/C2H6O/c1-2-3/h3H,2H2,1H3`. Even though this might look like random letters and numbers, it is a standardized code. That means no matter who or where you are in the world, if you describe the same chemical structure, you will get the same InChI. This helps computers, databases, and search engines quickly recognize and organize chemical information without any confusion.

The InChI system is so detailed that it can tell the difference between molecules that have the same atoms but are arranged in different ways. These are called isomers or tautomers, and even small changes in their structure can make a big difference in how they behave. Because InChI captures these tiny differences, it is really useful in computational chemistry, chemical databases, and online science journals, where being exact really matters. But sometimes, the full InChI string, especially for large or complex molecules, can get very long and hard to use. To make things easier, scientists created a shorter version called the InChIKey. This is a 27-character code made of letters and numbers. Think of it like a digital fingerprint for the full InChI, short, fast, and easy to search. The InChIKey of ethanol is: `LFQSCWFLJHTTHZ-UHFFFAOYSA-N`. One thing to know is that the InChIKey is a “hash,” which means it is a one-way code. You cannot turn it back into the full chemical structure, but that’s okay. Its main job is to quickly and uniquely identify a molecule, kind of like a barcode at the store.

InChI and InChIKey are now very important tools that help chemists organize and share chemical information online. These special codes act like a universal language for chemicals, making it easy to connect information across different websites and databases, like PubChem, ChemSpider, Reaxys, and CAS. If a chemist wants to look up a substance, they can search using its InChI or InChIKey and quickly find everything about it, like its structure, uses, properties, and safety info, even if each website uses different naming styles. That’s because the InChI and InChIKey do not change, no matter what country or system is being used. These tools are also very useful for chemical search engines and AI (artificial intelligence) programs. For example, AI systems that help discover new medicines or materials use InChI and InChIKey to make sure they’re looking at the right molecules and comparing them correctly. They even help with something called the semantic web, which is a way for computers to better understand and connect information on the internet. Because of InChI and InChIKey, different software programs can “talk” to each other more clearly and avoid mistakes, like mixing up two chemicals with similar names.

#### SMILES (Simplified Molecular Input Line Entry System)
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SMILES stands for Simplified Molecular Input Line Entry System. It is a way for chemists to write the structure of a molecule using just letters, numbers, and symbols, instead of drawing it out with lines and shapes. This makes it much easier to type, store, and share chemical information using computers. SMILES was invented in the 1980s by a scientist named David Weininger. He wanted to make it easier for chemists to work with digital tools and databases. With SMILES, chemists can describe how atoms are connected in a molecule using a single line of text. This system is very useful in a field called cheminformatics, which is the use of computers to study chemistry. SMILES makes it possible for software to search, compare, and analyze chemical structures quickly. Because SMILES codes are short and easy for computers to read, they are used in things like chemical databases, AI programs for drug design, and virtual modeling of molecules.

In SMILES, scientists write the structure of a molecule using simple text characters that follow special rules. Each atom is shown by its chemical symbol, like C for carbon, O for oxygen, and N for nitrogen. Bonds between atoms are shown in easy ways too. If two atoms are joined by a single bond, you just write them next to each other, like CC for two carbon atoms. A double bond is shown with an equals sign (=), and a triple bond is shown with a hash symbol (#). For example, the molecule ethanol (which is found in alcoholic drinks) has the formula CH₃CH₂OH, and in SMILES it is written as CCO. The first C is the first carbon, the second C is the second carbon connected to it, and the O is the oxygen connected to the second carbon. Another example, is the molecule ethene (C₂H₄), which has two carbon atoms connected by a double bond. In SMILES, it is written as C=C, with the equal sign (=) showing the double bond between the two carbon atoms. SMILES can also handle more complicated molecules, like ones with branches or rings. If a molecule has branches (atoms that stick off the main chain), SMILES uses parentheses to show them. For example, isopropanol, a type of alcohol, is written as CC(C)O. This means the second carbon has another carbon (the branch) attached to it, and then there's an oxygen (–OH group). For ring-shaped molecules, SMILES uses numbers to show where the ring starts and ends. For example, cyclohexane, which is a ring of six carbon atoms, is written as C1CCCCC1. The number “1” tells you that the first and last carbon atoms are connected to form a ring. These clever rules let chemists write very complex molecules as a simple line of text, which is great for using with computers, databases, and software.

One of the best things about SMILES is that it is helpful for both people and computers. Chemists can often look at a simple SMILES string and understand what molecule it represents. For example, CCO shows two carbon atoms and one oxygen atom, this is the SMILES code for ethanol. At the same time, computers can take a SMILES string and turn it into a full 2D or 3D picture of the molecule. They can also use the SMILES code to search for similar molecules, or even predict properties like boiling point, how well the substance dissolves, or how it might behave in the body. This makes SMILES very powerful in science and research. It is especially useful in things like drug discovery, chemical modeling, and virtual screening, where scientists work with huge collections of molecules stored in computers. Because SMILES codes are short and simple, it is easy to store, search, and compare thousands or even millions of compounds quickly. There are also special versions of SMILES, like Canonical SMILES, which always write the same molecule in just one specific way. This helps avoid confusion in chemical databases, so the same compound is not entered more than once by mistake. SMILES can also include extra details, like the 3D shape of the molecule (called stereochemistry) using the @ symbol, or the exact kind of atom, like an isotope, using extra numbers and symbols. Even though SMILES has its own rules and takes some time to learn, many people find it easier and shorter to use than long IUPAC chemical names, especially when working with computers.

### Other Specialized Naming Systems
Outside of the IUPAC system, there are other ways to name chemicals, especially when the goal is to make names easier to use and understand. IUPAC names are great for being precise, but they can sometimes be very long and complicated. This can be a problem in areas like medicine, public health, or industry, where people need simple names they can use in daily life, especially if they are not chemists. One important naming system is called the International Nonproprietary Names (INN) system. It is run by the World Health Organization (WHO) and is used to create simple, standard names for medicines. These names are not brand names. They are meant to be generic so anyone around the world can use them. INN names are made using special parts of words called stems, which give clues about what the drug does. For example, drugs that end in “-pril” are usually used to treat high blood pressure, while drugs ending in “-olol” are often used for heart conditions. These endings help doctors and pharmacists quickly recognize what kind of medicine it is. In the United States, there is a similar system called USAN (United States Adopted Names). It is managed by health organizations like the American Medical Association. USAN names often match INN names, but they are also chosen to be safe, clear, and easy to remember. Unlike IUPAC names, these drug-naming systems do not try to explain the full chemical structure. Instead, they focus on what the medicine does, how it is used, and how to communicate clearly with healthcare workers and patients. These simpler names help make important medicines more understandable and accessible to everyone.

In biochemistry, scientists study important molecules like enzymes, proteins, and genes. These molecules are often very large and complicated, so scientists use special naming rules to keep things clear and consistent. These rules are managed by groups like the International Union of Biochemistry and Molecular Biology (IUBMB), which works with IUPAC to make sure scientists around the world all use the same system. Instead of focusing only on what the molecule is made of, biochemistry names often tell us what the molecule does or what role it plays in the body. For example, enzymes are proteins that speed up chemical reactions. They are named based on the kind of reaction they help with. Enzymes also get special ID numbers called EC numbers (short for Enzyme Commission numbers). These look like 1.1.1.1, and each part of the number gives information about what type of reaction the enzyme helps with. This helps scientists keep track of thousands of different enzymes, even if some have similar names. Proteins and genes are named in many different ways. Some are named after the diseases they are related to, others are named for their function in cells, and some are labeled with just letters and numbers. For example, BRCA1 is a gene related to breast cancer risk, and TP53 is a gene that helps stop tumors from forming. These names do not always tell you what the gene looks like, but they are standardized so scientists everywhere use the same names and do not get confused. To make sure everything stays organized, large projects like the Human Genome Project keep official lists of gene and protein names. This is especially helpful when comparing genes in different species, like humans, mice, or fruit flies, because scientists need a clear way to talk about the same gene in different organisms.

Another important part of naming chemicals is figuring out how to name polymers and macromolecules. These are very large molecules made up of smaller parts that repeat, like plastics, rubber, and even biological materials such as proteins and DNA. Because polymers can get really big and complicated, scientists have come up with two main ways to name them. Source-based naming is the simpler method. It names the polymer based on the material it was made from. For example, polyethylene comes from ethylene, and polystyrene comes from styrene. These names are easy to say and remember, which is why we often see them in everyday life, like on plastic containers or packaging. Structure-based naming is more detailed. It describes the exact repeating parts in the polymer chain. For example, [polyoxyethylene] tells us about the repeating part –CH₂CH₂O– inside the molecule. Structure-based names follow special rules made by IUPAC and are useful when scientists need to be very precise. The Purple Book is the official IUPAC guide that explains how to name polymers and macromolecules. But in real-world situations, like in factories, advertising, or product labeling, people usually use the simpler source-based names. That is because polymers can change a lot depending on things like size, branching, or how they were made, and the detailed names can get confusing.

Besides IUPAC and common naming systems, scientists and industries use special types of chemical names for specific purposes. These systems are helpful when regular IUPAC names would be too long, too technical, or just not practical. One example is the naming of coordination compounds. These are molecules where a metal atom is in the center and surrounded by other atoms or groups called ligands. Naming these compounds follows special rules that consider things like what ligands are attached, the oxidation state of the metal, and the shape of the molecule. For instance, the compound [Co(NH₃)₆]Cl₃ is called hexaamminecobalt(III) chloride. This name tells a chemist exactly how the molecule is put together, even if it looks tricky at first. In geology and materials science, scientists use mineral names to talk about natural substances like rocks, crystals, and ores. These names often describe what the mineral is made of, its structure, or where and how it was discovered. For example, quartz is a common mineral made of silicon dioxide (SiO₂), and olivine is a group of minerals with a special formula and crystal pattern. These names are approved by experts like the International Mineralogical Association. In the food industry, chemical additives are sometimes labeled using E-numbers in Europe. These are short codes that make it easy to identify safe ingredients like preservatives, vitamins, or food colorings. For example, E300 is the code for ascorbic acid, which is also known as vitamin C. These numbers help make food labeling clear and standardized. In agriculture, chemicals like pesticides and fertilizers are also given special names by global organizations. These names help track the chemicals, ensure safe use, and support international trade. All of these naming systems are designed to make chemical names more practical and understandable, whether you are a scientist, a farmer, a shopper, or someone reading a food label. Instead of showing the full chemical structure, these names focus on clarity, safety, and usefulness in the real world.

## References 
## Other websites 
* Interactive IUPAC Compendium of Chemical Terminology (interactive "Gold Book")
* IUPAC Nomenclature Books Series (list of all IUPAC nomenclature books, and means of accessing them)
* IUPAC Compendium of Chemical Terminology ("*Gold Book*")