Aarushhh/wikipedia-simple-splitted huggingface dataset:
Ad hominem is a Latin word for a type of argument. It is a word often used in rhetoric. Rhetoric is the science of speaking well, and convincing other people of your ideas.

Translated to English, ad hominem means against the person. In other words, when someone makes an ad hominem, they are attacking the person they are arguing against, instead of what they are saying.

The term comes from the Latin word homo, which means human. Hominem is a gender neutral version of the word homo. In ancient Rome it referred to all free men, or in other words, all free human beings.

Ad hominem can be a way to use reputation, rumors and hearsay to change the minds of other people listening. When a social network has already excluded or exiled one person, or applied a negative label to them, this can work more often.

It is most of the time considered to be a weak and poor argument. In courts and in diplomacy ad hominems are not appreciated.
Ad hominems are not wrong every time. For example, when people think that someone can't be trusted, things that they have said previously can be doubted.

What an ad hominem argument looks like 
In logic, a proof is something that starts with premises, and goes through a few logical arguments, to reach a conclusion.

Normal (valid) proof 
 All humans are mortal.
 Socrates is human.
 Therefore, Socrates is mortal.

Ad hominem example 
 Person A thinks abortion should be illegal.
 Person A is uneducated and poor.
 Therefore, abortion should not be illegal.

In this example it can be seen that the (completely unrelated) fact that person A is uneducated and poor is used to prove that abortion should not be illegal.

Related pages
 Fallacy for a list of other types of (false) rhetorical arguments.

	

Latin words used in English
Native Americans (also called Aboriginal Americans, American Indians, Amerindians or indigenous peoples of the Americas) are the indigenous peoples and their descendants, who were in the Americas before Europeans arrived.

Sometimes these people are called Indians, but this may be confusing, because it is the same word used for people from India. When Christopher Columbus explored, he did not know about the Americas. He was in the Caribbean but thought he was in the East Indies, so he called the people Indians. Today, some think that calling a Native American an Indian is racist.

There are many different tribes of Native American people, with many different languages. Some tribes were hunter-gatherers who moved from place to place. Others lived in one place and built cities and kingdoms.
Many Native Americans died after the Europeans came to the Americas. There were diseases that came with the Europeans but were new to the Native Americans. There were battles with the Europeans. Many native people were hurt, killed, or forced to leave their homes by settlers who took their lands.

Today, there are more than three million Native Americans in Canada and the U.S. combined. About 51 million more Native Americans live in Latin America. Many Native Americans still speak native languages and have their own cultural practices, while others have adopted some parts of Western culture. Many Native Americans face problems with discrimination and racism.

Origins 
The ancestors of Native Americans came to the Americas from Asia. Some of them may have come to America 15,000 years ago when Alaska was connected to Siberia by the Bering land bridge.
The earliest people in the Americas came from Siberia when there was an ice bridge across the Bering Strait. The cold but mainly grassy plain which connected Siberia with Canada is called Beringia.  It is reckoned that a few thousand people arrived in  Beringia from eastern Siberia during the Last Glacial Maximum before moving into the Americas sometime after 16,500 years before the present (BP). This would have occurred as the American glaciers blocking the way southward melted, but before the bridge was covered by the sea about 11,000 years BP.
Before European colonization, Beringia was inhabited by the Yupik peoples on both sides of the straits. This culture remains in the region today, with others. In 2012, the governments of Russia and the United States announced a plan to formally establish "a transboundary area of shared Beringian heritage". Among other things this agreement would establish close ties between the Bering Land Bridge National Preserve and the Cape Krusenstern National Monument in the United States and Beringia National Park in Russia.

Native Americans are divided into many small nations, called First Nations in Canada and tribes elsewhere.

Culture 
Each Native American tribe has their own culture. The cultures can be grouped together depending on region. For example, the tribes living in Mesoamerica have similar cultures.

Food 
Native Americans ate many different things depending on where they lived.

Native Americans from Mesoamerica introduced vanilla, avocados, and chocolate to the world.
Religion 
Before Europeans came, the native peoples of the Americans practiced many different religions. Each tribe had their own different beliefs.

Today, many Native Americans practice Christianity, a religion that was brought to the Americas by Europeans. Meanwhile, others still practice their own religions.

Languages 
Native Americans today speak over a thousand different languages. Some of these languages had writing systems before Europeans came.

Many of these languages are endangered because more people are speaking European languages and not teaching Native American languages to their kids.

Music 
Native Americans make musical instruments using the things around them.

Art 
Native Americans made a lot of different art.

Today

North America
North America

United States
According to the 2010 United States census, 0.9% of Americans say they are Native American, 2.9 million people, and 0.8% of Americans say they are both Native American and something else.  They are not evenly spread out through the United States.  About a third of the people in Alaska are Native Alaskan and about a sixth of the people in Oklahoma are Native American.

In the United States, most Native Americans live in cities.  About 28% of Native Americans live on Indian reservations.  Many Native Americans are poor, and 24% are extremely poor.  The history of violence against Native Americans persists today in higher rates of violence against Native American people than white people.

Mexico
Many Mexicans are of Native American or mestizo ancestry. Mexico has the largest and most diverse Native American population in Latin America.

Canada
In the 2016 census, More than 1.67 million people in Canada identified as Indigenous, making them 4.9 per cent of Canada’s population.
Central America

Guatemala 
About 40% of the people of Guatemala identify as Native American. Many indigenous groups in the country are descendants of the Maya.

Many Native Americans in Guatemala are poor. Many of them have left the country to find better jobs elsewhere.

South America

Bolivia 
The majority of Bolivians belong to indigenous groups. Many are Aymara and Quechua.

Peru 
Peru has a large indigenous population, around 80% of Peru's population identify as indigenous or mestizo.

Indigenous activism 
In the later half of the 20th century, many Native Americans started to protest the unfair treatment they experienced from the societies they lived in.

Some Native Americans have become famous in politics. For example, an Aymara man named Evo Morales was elected as president of Bolivia in 2005. He was the first indigenous presidential candidate in Bolivia and South America.

Related pages 
First Nations
Plains Indians 
Native Americans in the United States

References
An apple is the edible fruit of a number of trees, known for its juicy, green, or red fruits. The tree (Malus spp.) is grown worldwide. Its fruit is low-cost, popular, and common all over the earth. 

Applewood is a type of wood that comes from this tree.

The apple tree comes from southern Kazakhstan, Kyrgyzstan, Uzbekistan, and northwestern part of China. Apples have been grown for thousands of years in Asia and Europe. They were brought to North America by European settlers. Apples have religious and mythological significance in many cultures.

Apples are generally grown by grafting, although wild apples grow readily from seed. Apple trees are large if grown from seed, but small if grafted onto roots (rootstock). There are more than 10000 known variants of apples, with a range of desired characteristics. Different variants are bred for various tastes and uses: cooking, eating raw and cider production are the most common uses.
Trees and fruit are attacked by fungi, bacteria and pests. In 2010, the fruit's genome was sequenced as part of research on disease control and selective breeding in apple production.

Worldwide production of apples in 2013 was 90.8 million tonnes. China grew 49% of the total.

Botanical information 
The apple has a small, leaf-shedding tree that grows up to  tall. The apple tree has a broad crown with thick twigs.
The leaves are alternately arranged simple ovals. They are 5 to 12 centimetres long and 3–6 centimetres (1.2–2.4 in) wide. It has a sharp top with a soft underside. Blossoms come out in spring at the same time that the leaves begin to bud. The flowers are white. They also have a slightly pink color. They are five petaled, and 2.5 to 3.5 centimetres (0.98 to 1.4 in) in diameter. The fruit matures in autumn. It is usually 5 to 9 centimetres (2.0 to 3.5 in) in diameter. There are five carpels arranged in a star in the middle of the fruit. Every carpel has one to three seeds.
Wild ancestors 
The wild ancestor of apple trees is Malus sieversii. They grow wild in the mountains of Central Asia in the north of Kazakhstan, Kyrgyzstan, Tajikistan, and Xinjiang, China, and possibly also Malus sylvestris. Unlike domesticated apples, their leaves become red in autumn. They are being used recently to develop Malus domestica to grow in colder climates.

History

The apple tree was possibly the earliest tree to be cultivated. Its fruits have become better over thousands of years. It is said that Alexander the Great discovered dwarf apples in Asia Minor in 300 BC. Asia and Europe have used winter apples as an important food for thousands of years. From when Europeans arrived, Argentina and the United States have used apples as food as well. Apples were brought to North America in the 1600s. The first apple orchard on the North American continent was said to be near Boston in 1625. In the 1900s, costly fruit industries, where the apple was a very important species, began developing.
In culture

Paganism 

In Norse mythology, the goddess Iðunn gives apples to the gods in Prose Edda (written in the 13th century by Snorri Sturluson) that makes them young forever. English scholar H. R. Ellis Davidson suggests that apples were related to religious practices in Germanic paganism. It was from there, she claims, that Norse paganism developed. She points out that buckets of apples were discovered in the place of burial for the Oseberg ship in Norway. She also remarks that fruit and nuts (Iðunn having been described as changing into a nut in Skáldskaparmál) have been discovered in the early graves of the Germanic peoples in England. They have also been discovered somewhere else on the continent of Europe. She suggests that this may have had a symbolic meaning. Nuts are still a symbol of fertility in Southwest England.
Cooking 
Sometimes apples are eaten after they are cooked. Often, apples are eaten uncooked. Apples can also be made into drinks. Apple juice and apple cider are drinks made with apples. 

The flesh of the fruit is firm with a taste anywhere from sour to sweet. Apples used for cooking are sour, and need to be cooked with sugar, while other apples are sweet, and do not need cooking. There are some seeds at the core, that can be removed with a tool that removes the core, or by carefully using a knife.

The scientific name of the apple tree genus in the Latin language is Malus. Most apples that people grow are of the Malus domestica species.

Most apples are good to eat raw (not cooked), and are also used in many kinds of baked foods, such as apple pie. Apples are cooked until they are soft to make apple sauce.
Apples are also made into the drinks apple juice and cider. Usually, cider contains a little alcohol, about as much as beer. The regions of Brittany in France and Cornwall in England are known for their apple ciders.

Apple variants 
If one wants to grow a certain type of apple, it is not possible to do this by planting a seed from the wanted type. The seed will have DNA from the apple that the seeds came from, but it will also  have DNA from the apple flower that pollinated the seeds, which might be a different variant of apple. This means that the tree which would grow from planting would be a mixture of two, or a hybrid. In order to grow a certain type of apple, a small twig, or 'scion', is cut from the tree that grows the type of apple desired, and then added on to a specially grown stump called a rootstock. The tree that grows will create apples of the type needed.
There are more than 7,500 known variants of apples. Different variants are available for temperate and subtropical climates. One large collection of over 2,100 apple variants is at the National Fruit Collection in England. Most of these variants are grown for eating fresh (dessert apples). However, some are grown simply for cooking or making cider. Cider apples are usually too tart to eat immediately. However, they give cider a rich flavor that dessert apples cannot.

Most popular apple cultivars are soft but crisp. Colorful skin, easy shipping, disease resistance, 'Red Delicious' apple shape, and popular flavor are also needed. Modern apples are usually sweeter than older cultivars. This is because popular tastes in apples have become different. Most North Americans and Europeans enjoy sweet apples. Extremely sweet apples with hardly any acid taste are popular in Asia and India.

World production
World production

Apples are grown around the world.  China produces more than half of all commercially grown apples.  In 2020/2021, China produced 44,066,000 metric tons. Other important producers were the European Union (EU) (11,719,000 metric tons, the United States (4,490,000 metric tons), and Turkey (4,300,000 metric tons).    Total world production was 80,522,000 metric tons.

In the United Kingdom 
In the United Kingdom there are about 3000 different types of apples. The most common apple type grown in England is the 'Bramley seedling', which is a popular cooking apple.

Apple orchards are not as common as they were in the early 1900s, when apples were rarely brought in from other countries. Organizations such as Common Ground teach people about the importance of rare and local varieties of fruit.
In North America 
Many apples are grown in temperate parts of the United States and Canada. "Washington State currently produces over half the Nation's domestically grown apples and has been the leading apple-growing State since the early 1920s."  New York and Michigan are the next two leading states in apple production.  "The total reported area dedicated to the crop in the United States is 336,940 acres or 526.47 square miles." 

In many areas where apple growing is important, people have huge celebrations:

 Annapolis Valley Apple Blossom Festival - held five days every spring (May-June) in Nova Scotia
 Shenandoah Apple Blossom Festival - held six days every spring in Winchester, Virginia.
 Washington State Apple Blossom Festival - held two weeks every spring (April-May) in Wenatchee, Washington.

Varieties of apples 

There are many different varieties of apples, including:
Aport
 Cox's Orange Pippin
 Fuji (apple)
 Gala
 Golden Delicious (sometimes called a Green Delicious Apple)
 Granny Smith
 Jonathan
 Jonagold
 McIntosh
 Pink Lady
 Red Delicious
 Winesap
 Yellow Sweeting, the first variety of the U.S.

Family 
Apples are in the group Maloideae. This is a subfamily of the family Rosaceae. They are in the same subfamily as pears.

References

Further reading 
 Potter D. et al 2007. Phylogeny and classification of Rosaceae. Plant Systematics and Evolution. 266 (1–2): 5–43.

Other websites 

 
 
 

Basic English 850 words
 
National symbols of Poland
An Abrahamic Religion is a religion whose followers believe in the prophet Abraham. They believe Abraham and his sons/grandsons hold an important role in human spiritual development. The best known Abrahamic religions are Judaism, Christianity and Islam. Smaller religious traditions sometimes included as Abrahamic religions are Samaritanism, Druze, Rastafari, Babism and Bahá'í Faith. Mandaeism (a religion that holds many Abrahamic beliefs) is not called Abrahamic because its followers think Abraham was a false prophet

True Abrahamic religions are monotheistic (the belief that there is only one God). They also all believe that people should pray to God and worship God often. Among monotheistic religions, the Abrahamic religions have the world's largest number of followers.

Religions 

 
Mythology
Algebra (from Arabic: الجبر‎, transliterated "al-jabr", meaning "reunion of broken parts") is a part of mathematics. It uses variables to represent a value that is not yet known or can be replaced with any value. When an equals sign (=) is used, this is called an equation. A very simple equation using a variable is: . In this example, , or it could also be said that " equals five". This is called solving for .

Besides equations, there are inequalities (less than and greater than). A special type of equation is called the function. This is often used in making graphs because it always turns one input into one output.

Algebra can be used to solve real problems because the rules of algebra work in real life and numbers can be used to represent the values of real things. Physics, engineering and computer programming are areas that use algebra all the time. It is also useful to know in surveying, construction and business, especially accounting.
People who do algebra use the rules of numbers and mathematical operations used on numbers. The simplest are adding, subtracting, multiplying, and dividing. More advanced operations involve exponents, starting with squares and square roots.

Algebra was first used to solve equations and inequalities. Two examples are linear equations (the equation of a straight line,  or ) and quadratic equations, which has variables that are squared (multiplied by itself, for example: , , or ).
History 
Early forms of algebra were developed by the Babylonians and Greek geometers such as Hero of Alexandria. However the word "algebra" is a Latin form of the Arabic word Al-Jabr ("casting") and comes from a mathematics book Al-Maqala fi Hisab-al Jabr wa-al-Muqabilah, ("Essay on the Computation of Casting and Equation") written in the 9th century by a Persian mathematician, Muhammad ibn Mūsā al-Khwārizmī, who was a Muslim born in Khwarizm in Uzbekistan. He flourished under Al-Ma'moun in Baghdad, Iraq through 813-833 CE, and died around 840 CE. The book was brought into Europe and translated into Latin in the 12th century. The book was then given the name "Algebra". (The ending of the mathematician's name, al-Khwarizmi, was changed into a word easier to say in Latin, and became the English word algorithm).

Examples

Here is a simple example of an algebra problem:
Sue has 12 candies, and Ann has 24 candies. They decide to share so that they have the same number of candies. How many candies will each have?

These are the steps you can use to solve the problem:
To have the same number of candies, Ann has to give some to Sue. Let  represent the number of candies Ann gives to Sue.
 Sue's candies, plus , must be the same as Ann's candies minus . This is written as: 
 Subtract 12 from both sides of the equation. This gives: . (What happens on one side of the equal sign must happen on the other side too, for the equation to still be true. So in this case when 12 was subtracted from both sides, there was a middle step of . After a person is comfortable with this, the middle step is not written down.)
 Add  to both sides of the equation. This gives: 
 Divide both sides of the equation by 2. This gives . The answer is six. This mean that if Ann gives Sue 6 candies, they will have the same number of candies.
 To check this, put 6 back into the original equation wherever  was: 
 This gives , which is true. They each now have 18 candies.
With practice, algebra can be used when faced with a problem that is too hard to solve any other way. Problems such as building a freeway, designing a cell phone, or finding the cure for a disease all require algebra.

Writing algebra 
As in most parts of mathematics, adding  to  (or  plus ) is written as ;

subtracting  from  (or  minus ) is written as ;

and dividing  by  (or  over ) is written as  or .

In algebra, multiplying  by  (or  times ) can be written in 3 different ways: ,  or just . All of these notations mean the same thing:  times . The symbol "" used in arithmetic is not used in algebra, because it looks too much like the letter , which is often used as a variable.

When we multiply a number and a variable in algebra, we can simply write the number in front of the letter: . When the number is 1, then it is not written because 1 times any number is that number () and so it is not needed. And when it is 0, we can completely remove the terms, because 0 times any number is zero ().
As a side note, you do not have to use the letters  or  in algebra. Variables are just symbols that mean some unknown number or value, so you can use any letter for a variable (except  (Euler's number) and  (Imaginary unit), because these are mathematical constants).  and  are the most common, though.

Functions and Graphs 

An important part of algebra is the study of functions, since they often appear in equations that we are trying to solve. A function is like a machine you can put a number (or numbers) into and get a certain number (or numbers) out. When using functions, graphs can be powerful tools in helping us to study the solutions to equations.
A graph is a picture that shows all the values of the variables that make the equation or inequality true. Usually this is easy to make when there are only one or two variables. The graph is often a line, and if the line does not bend or go straight up-and-down it can be described by the basic formula . The variable  is the y-intercept of the graph (where the line crosses the vertical axis) and  is the slope or steepness of the line. This formula applies to the coordinates of a graph, where each point on the line is written .
In some math problems like the equation for a line, there can be more than one variable ( and  in this case). To find points on the line, one variable is changed. The variable that is changed is called the "independent" variable. Then the math is done to make a number. The number that is made is called the "dependent" variable. Most of the time the independent variable is written as  and the dependent variable is written as , for example, in . This is often put on a graph, using an  axis (going left and right) and a  axis (going up and down). It can also be written in function form: . So in this example, we could put in 5 for  and get . Put in 2 for  would get . And 0 for  would get . So there would be a line going through the points , , and  as seen in the graph to the right.

If  has a power of 1, it is a straight line. If it is squared or some other power, it will be curved. If it uses an inequality ( or ), then usually part of the graph is shaded, either above or below the line.
Rules 
In algebra, there are a few rules that can be used for further understanding of equations. These are called the rules of algebra. While these rules may seem senseless or obvious, it is wise to understand that these properties do not hold throughout all branches of mathematics. Therefore, it will be useful to know how these axiomatic rules are declared, before taking them for granted. Before going on to the rules, reflect on two definitions that will be given.
 Opposite: the opposite of  is .
 Reciprocal: the reciprocal of  is .

Commutative property of addition 
Commutative means that a function has the same result if the numbers are swapped around. In other words, the order of the terms in an equation does not matter. When two terms (addends) are being added, the commutative property of addition is applicable. In algebraic terms, this gives .

Note that this does not apply for subtraction (i.e.  except if ).
Commutative property of multiplication 
When two terms (factors) are being multiplied, the commutative property of multiplication is applicable. In algebraic terms, this gives .

Note that this does not apply for division (i.e. , when  and , except if ).

Associative property of addition 
Associative refers to the grouping of numbers. The associative property of addition implies that, when adding three or more terms, it doesn't matter how these terms are grouped. Algebraically, this gives . Note that this does not hold for subtraction, e.g.  (see distributive property).

Associative property of multiplication 
The associative property of multiplication implies that, when multiplying three or more terms, it doesn't matter how these terms are grouped. Algebraically, this gives . Note that this does not hold for division, e.g. .
Distributive property 
The distributive property states that the multiplication of a term by another term can be distributed. For instance: . (Do not confuse this with the associative properties! For instance: .)

Additive identity 
Identity refers to the property of a number that it is equal to itself. In other words, there exists an operation of two numbers so that it equals the variable of the sum. The additive identity property states that any number plus 0 is that number: . This also holds for subtraction: .

Multiplicative identity 
The multiplicative identity property states that any number times 1 is that number: . This also holds for division: .

Additive inverse property 
The additive inverse property is somewhat like the inverse of the additive identity. When we add a number and its opposite, the result is 0. Algebraically, it states the following: , which is the same as . For example, the additive inverse (or opposite) of 1 is -1.
Multiplicative inverse property 
The multiplicative inverse property means that when we multiply a number and its reciprocal, the result is 1. Algebraically, it states the following: , which is the same as . For example, the multiplicative inverse (or reciprocal) of 2 is 1/2. To get the reciprocal of a fraction, switch the numerator and the denominator: the reciprocal of  is .

Advanced Algebra
In addition to "elementary algebra", or basic algebra, there are advanced forms of algebra, taught in colleges and universities, such as abstract algebra, linear algebra, and universal algebra. This includes how to use a matrix to solve many linear equations at once. Abstract algebra is the study of things that are found in equations, going beyond numbers to the more abstract with groups of numbers.
Many math problems are about physics and engineering. In many of these physics problems time is a variable. The letter used for time is . Using the basic ideas in algebra can help reduce a math problem to its simplest form making it easier to solve difficult problems. Energy is , force is , mass is , acceleration is  and speed of light is sometimes . This is used in some famous equations, like  and  (although more complex math beyond algebra was needed to come up with that last equation).

Related pages
 List of mathematics topics
 Order of operations
 Parabola
 Computer Algebra System

References

Other websites 

 Khan Academy: Algebra theory and practice
 algebrarules.com: A free place to learn the basics of Algebra
 Khan Academy: Origins of Algebra, free online micro lectures

Algebra
An atom is the basic unit of matter. All normal matter – everything that has mass – is made of atoms. This includes solids, liquids, and gases. The atom cannot be broken to parts by chemistry, so people once thought it was the smallest and simplest particle of matter. There are over 100 different kinds of atoms, called chemical elements. Each kind has the same basic structure, but a different number of parts.

Atoms are very small, but their exact size depends on the type. Atoms are from 0.1 to 0.5 nanometers across. One nanometer is about 100,000 times smaller than the width of a human hair. This makes one atom impossible to see without special tools. Scientists learn how they work by doing experiments.

Atoms are made of three kinds of subatomic particles. These are protons, neutrons, and electrons. Protons and neutrons have much more mass. They are in the middle of the atom, the nucleus. Lightweight electrons move quickly around them. The electromagnetic force holds the nucleus and electrons together.
Atoms with the same number of protons belong to the same chemical element. Examples of elements are carbon and gold. Atoms with the same number of protons, but different numbers of neutrons, are called isotopes. Usually an atom has the same number of electrons as protons. If an atom has more or less electrons than protons, it is called an ion, and has an electric charge.

Atoms can join by chemical bonds. Many things are made of more than one kind of atom. These are chemical compounds or mixtures. A group of atoms connected by chemical bonds is called a molecule. For example, a water molecule is made of two hydrogen atoms and one oxygen atom. The forming or breaking of bonds is a chemical reaction.
Atoms split if the forces inside are too weak to hold them together. This is what causes radioactivity. Atoms can also join to make larger atoms at very high temperatures, such as inside a star. These changes are studied in nuclear physics. Most atoms on Earth are not radioactive. They are rarely made, destroyed, or changed into another kind of atom.

History 
The word "atom" comes from the Greek (ἀτόμος) "atomos", which means indivisible or uncuttable. One of the first people to use the word "atom" is the Greek philosopher Democritus, around 400 BC. He thought that everything was made of particles called atoms, which could not be divided into smaller pieces. Some Hindu, Jain, and Buddhist philosophers also had ideas like this. Atomic theory was a mostly philosophical subject, with not much scientific investigation or study, until the early 1800s.
In 1777 French chemist Antoine Lavoisier defined the term element as we now use it. He said that an element was any substance that could not be broken down into other substances by the methods of chemistry. Any substance which could be broken down was a compound.

In 1803, English philosopher John Dalton suggested that elements were made of tiny, solid balls called atoms. Dalton believed that all atoms of the same element have the same mass. He said that compounds are formed when atoms of more than one element combine. In any one compound, the atoms would always combine in the same numbers.
In 1827, British scientist Robert Brown looked at pollen grains in water under his microscope. The pollen grains appeared to be shaking. Brown used Dalton's atomic theory to describe patterns in how they moved. This was called Brownian motion. In 1905 Albert Einstein used mathematics to prove that the pollen particles were being moved by the motion, or heat, of individual water molecules. By doing this, he proved that atoms are real without question.

In 1869, Russian scientist Dmitri Mendeleev published the first periodic table. The periodic table groups elements by their atomic number (how many protons they have; this is usually the same as the number of electrons). Elements in the same column, or group, usually have similar qualities. For example, helium, neon, argon, krypton, and xenon are all in the same column and are very similar. All these elements are gases that have no color or smell. Also, they cannot combine with other atoms to form compounds. Together they are known as noble gases.
The physicist J.J. Thomson was the first person to discover electrons. This happened while he was working with cathode rays in 1897. He learned they had a negative charge, and the rest of the atom had a positive charge. Thomson made the plum pudding model, which said that an atom was like plum pudding: the dried fruit (electrons) were stuck in a mass of pudding (having a positive charge). 

In 1909, Ernest Rutherford used the Geiger–Marsden experiment to prove that most of an atom is in a very small space, the atomic nucleus. Rutherford took a photo plate and covered it with gold foil. He then shot alpha particles (made of two protons and two neutrons stuck together) at it. Many of the particles went through the gold foil, which proved that atoms are mostly empty space. Electrons are so small and fast-moving that they did not block the particles from going through. Rutherford later discovered protons in the nucleus.
In 1913, Niels Bohr created the Bohr model. This model showed that electrons travel around the nucleus in fixed circular orbits. This was better than the Rutherford model, but it was still not completely true.

In 1925, chemist Frederick Soddy discovered that some elements had more than one kind of atom, called isotopes. Soddy believed that each different isotope of an element has a different mass. To prove this, chemist Francis William Aston built the mass spectrometer, which measures the mass of single atoms. Aston proved that Soddy was right. He also found that the mass of each atom is a whole number times the mass of the proton. This meant that there must be some particles in the nucleus other than protons. In 1932, physicist James Chadwick shot alpha particles at beryllium atoms. He saw that a particle shot out of the beryllium atoms. This particle had no charge, but about the same mass as a proton. He named this particle the neutron.
The best model so far comes from the Schrödinger equation. Schrödinger learned that the electrons exist in a cloud around the nucleus, called the electron cloud. In the electron cloud, it is impossible to know exactly where electrons are. The Schrödinger equation says where an electron is likely to be. This area is called the electron's orbital.

In 1937, German chemist Otto Hahn became the first person to make nuclear fission in a laboratory. He discovered this by chance when shooting neutrons at a uranium atom, hoping to make a new isotope. However, instead of a new isotope, the uranium changed into a barium atom, a smaller atom than uranium. Hahn had "broken" the uranium atom. This was the world's first recorded nuclear fission reaction. This discovery led to the creation of the atomic bomb and nuclear power, where fission happens over and over again, creating a chain reaction.
Later in the 20th century, physicists went deeper into the mysteries of the atom. Using particle accelerators, they discovered that protons and neutrons were made of other particles, called quarks.

Structure and parts

Parts 

An atom is made of three main particles: the proton, the neutron, and the electron. Protons and neutrons have nearly the same size and mass (about  grams). The mass of an electron is about 1800 times smaller (about  grams).  Protons have a positive charge, electrons have a negative charge, and neutrons have no charge. Most atoms have no charge. The number of protons (positive) and electrons (negative) are the same, so the charges balance out to zero. However, ions have a different number of electrons than protons, so they have a positive or negative charge.
Scientists believe that electrons are elementary particles: they are not made of any smaller pieces. Protons and neutrons are made of quarks of two kinds: up quarks and down quarks. A proton is made of two up quarks and one down quark, and a neutron is made of two down quarks and one up quark.

Nucleus 

The nucleus is in the middle of the atom. It is made of protons and neutrons. The nucleus makes up more than 99.9% of the mass of the atom. However, it is very small: about 1 femtometer (10−15 m) across, which is around 100,000 times smaller than the width of an atom, so it has a very high density.
Usually in nature, two things with the same charge repel or shoot away from each other. So for a long time, scientists did not know how the positively charged protons in the nucleus stayed together. We now believe that the attraction between protons and neutrons comes from the strong nuclear force. This force also holds together the quarks that make up the protons and neutrons. Particles called mesons travel back and forth between protons and neutrons, and carry the force.
The number of neutrons in relation to protons defines whether the nucleus stays together or goes through radioactive decay. When there are too many neutrons or protons, the atom tries to make the numbers smaller or more equal by removing the extra particles. It sends out radiation in the form of alpha, beta, or gamma decay. Nuclei can also change in other ways. Nuclear fission is when the nucleus breaks into two smaller nuclei, releasing a lot of energy. This release of energy makes nuclear fission useful for making bombs, and electricity in the form of nuclear power.
The other way nuclei can change is through nuclear fusion, when two nuclei join or fuse to make a larger nucleus. This process requires very high amounts of energy to overcome the electric repulsion between the protons, as they have the same charge. Such high energies are most common in stars like our Sun, which fuses hydrogen for fuel. However, once fusion happens, far more energy is released, because some of the mass becomes energy.
The energy needed to break a nucleus into protons and neutrons is called its nuclear binding energy. This energy can be converted to mass, as stated by Einstein's famous formula E = mc2. Medium-sized nuclei, such as iron-56 and nickel-62, have the highest binding energy per proton or neutron. They will probably not go through fission or fusion, because they cannot release energy in this way. Very small and very large atoms have low binding energy, so they are most willing to go through fission or fusion.

Electrons 

Electrons orbit, or travel around, the nucleus. They are called the atom's electron cloud. They are attracted to the nucleus because of the electromagnetic force. Electrons have a negative charge, and the nucleus always has a positive charge, so they attract each other.
The Bohr model shows that some electrons are farther from the nucleus than others in different levels. These are called electron shells. Only the electrons in the outer shell can make chemical bonds. The number of electrons in the outer shell determines whether the atom is stable or which atoms it will bond with in a chemical reaction. If an atom has only one shell, it needs two electrons to be complete. Otherwise, the outer shell needs eight electrons to be complete.
The Bohr model is important because it has the idea of energy levels. The electrons in each shell have a certain amount of energy. Shells that are farther from the nucleus have more energy. When a small burst of energy called a photon hits an electron, the electron can jump into a higher-energy shell. This photon must carry exactly the right amount of energy to bring the electron to the new energy level. A photon is a burst of light, and the amount of energy determines the color of light. So each kind of atom will absorb certain colors of light, called the absorption spectrum.  An electron can also send out, or emit, a photon, and fall into a lower energy shell. For the same reason, the atom will only send out certain colors of light, called the emission spectrum.
The complete picture is more complicated. Unlike the Earth moving around the Sun, electrons do not move in a circle. We cannot know the exact place of an electron. We only know the probability, or chance, that it will be in any place. Each electron is part of an orbital, which describes where it is likely to be. No more than two electrons can be in one orbital; these two electrons have different spin.
For each shell, numbered 1, 2, 3, and so on, there may be a number of different orbitals. These have different shapes, or point in different directions. Each orbital can be described by its three quantum numbers. The principal quantum number is the electron shell number. The azimuthal quantum number is represented by a letter: s, p, d, or f. Depending on the principal and azimuthal quantum numbers, the electron can have more or less energy. There is also a magnetic quantum number, but it does not usually affect the energy level. As more electrons are added, they join orbitals in order from lowest to highest energy. This order starts as follows: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d. For example, a chlorine atom has 17 electrons. So, it will have:
2 electrons in the 1s orbital
2 electrons in the 2s orbital
6 electrons in the 2p orbitals
2 electrons in the 3s orbital
5 electrons in the 3p orbitals
In other words, it has 2 electrons in the first shell, 8 in the second shell, and 7 in the third shell.
Properties

Atomic number 

The number of protons in an atom is called its atomic number. Atoms of the same element have the same atomic number. For example, all carbon atoms have six protons, so the atomic number of carbon is six. Today, 118 elements are known. Depending on how the number is counted, 90 to 94 elements exist naturally on earth. All elements above number 94 have only been made by humans. These elements are organized on the periodic table.

Atomic mass and weight 
Because protons and neutrons have nearly the same mass, and the mass of electrons is very small, we can call the number of protons and neutrons in an atom its mass number.  Most elements have several isotopes with different mass numbers. To name an isotope, we use the name of the element, followed by its mass number. So an atom with six protons and seven neutrons is called carbon-13.
Sometimes, we need a more exact measurement. The exact mass of an atom is called its atomic mass. This is usually measured with the atomic mass unit (amu), also called the dalton. One amu is exactly 1/12 of the mass of a carbon-12 atom, which is  grams. Hydrogen-1 has a mass of about 1 amu. The heaviest atom known, oganesson, has a mass of about 294 amu, or  grams. The average mass of all atoms of a particular element is called its atomic weight.

Size 
The size of an atom depends on the size of its electron cloud. Moving down the periodic table, more electron shells are added. As a result, atoms get bigger. Moving to the right on the periodic table, more protons are added to the nucleus. This more positive nucleus pulls electrons more strongly, so atoms get smaller. The biggest atom is caesium, which is about 0.596 nanometers wide according to one model. The smallest atom is helium, which is about 0.062 nanometers wide.
How atoms interact 
When atoms are far apart, they attract each other. This attraction is stronger for some kinds of atoms than others. At the same time, the heat, or kinetic energy, of atoms makes them always move. If the attraction is strong enough, relative to the amount of heat, atoms will form a solid. If the attraction is weaker, they will form a liquid, and if it is even weaker, they will form a gas. 

Chemical bonds are the strongest kinds of attraction between atoms. The movement of electrons explains all chemical bonds.
Atoms usually bond with each other in a way that fills or empties their outer electron shell. The most reactive elements have an almost full or almost empty outer shell. Atoms with a full outer shell, called noble gases, do not usually form bonds.

There are three main kinds of bonds: ionic bonds, covalent bonds, and metallic bonds.
In an ionic bond, one atom gives electrons to another atom. Each atom becomes an ion: an atom or group of atoms with a positive or negative charge. The positive ion (which has lost electrons) is called a cation; it is usually a metal. The negative ion (which has gained electrons) is called an anion; it is usually a nonmetal. Ionic bonding usually results in a regular network, or crystal, of ions held together.

In a covalent bond, two atoms share electrons. This usually happens when both atoms are nonmetals. Covalent bonds often form molecules, ranging in size from two atoms to many more. They can also form large networks, such as glass or graphite. The number of bonds that an atom makes (its valency) is usually the number of electrons needed to fill its outer electron shell.
In a metallic bond, electrons travel freely between many metal atoms. Any number of atoms can bond this way. Metals conduct electric current because electric charge can easily flow through them. Atoms in metals can move past each other, so it is easy to bend, stretch, and change the shape of metals.

All atoms attract each other by Van der Waals forces. These forces are weaker than chemical bonds. They are caused when electrons move to one side of an atom. This movement gives a negative charge to that side. It also gives a positive charge to the other side. When two atoms line up their sides with negative and positive charges, they will attract.

Although atoms are mostly empty space, they cannot pass through each other. When two atoms are very close, their electron clouds will repel each other by the electromagnetic force.
Magnetism 
To understand how magnets work, we can look at the properties of the atom. Any magnet has a north and south pole, and a certain strength. The direction and strength of a magnet, together, are called its magnetic moment. Every electron also has a magnetic moment, like a tiny magnet. This comes from the electron's spin and its orbit around the nucleus. The magnetic moments for the electrons add up to a magnetic moment for the whole atom. This tells us how atoms act in a magnetic field. 

Every electron has one of two opposite spins. We can think of one as turning to the right, and the other as turning to the left. If every electron is paired with an electron with the opposite spin in the same orbital, the magnetic moments will cancel out to zero. Atoms like this are called diamagnetic. They are only weakly repelled by a magnetic field.
However, if some electrons are not paired, the atom will have a lasting magnetic moment: it will be paramagnetic or ferromagnetic. When atoms are paramagnetic, the magnetic moment of each atom points in a random direction. They are weakly attracted to a magnetic field. When atoms are ferromagnetic, the magnetic moments of nearby atoms act on each other. They point in the same direction. This means that the whole object is a magnet, and it can point in the direction of a magnetic field. Ferromagnetic materials, such as iron, cobalt, and nickel, are strongly attracted to a magnetic field.

Radioactive decay
Radioactive decay 

Some elements, and many isotopes, have what is called an unstable nucleus. This means the nucleus is either too big to hold itself together, or it has too many protons or neutrons. When a nucleus is unstable, it has to eliminate the excess mass of particles. It does this through radiation. An atom that does this is called radioactive. Unstable atoms emit radiation until they lose enough particles in the nucleus to become stable. All atoms above atomic number 82 (82 protons, lead) are radioactive.
There are three main kinds of radioactive decay: alpha, beta, and gamma.
 Alpha decay is when the atom shoots out a particle having two protons and two neutrons. This is a helium-4 nucleus. The result is an element with an atomic number of two less than before. So, for example, if a uranium atom (atomic number 92) went through alpha decay, it would become thorium (atomic number 90). Alpha decay happens when an atom is too big and needs to lose some mass.
 Beta decay is when a neutron turns into a proton, or a proton turns into a neutron. In the first case, the atom shoots out an electron. In the second case, it shoots out a positron (like an electron but with a positive charge). The result is an element with one higher or one lower atomic number than before. Beta decay happens when an atom has either too many protons or too many neutrons.
Gamma decay is when an atom shoots out a gamma ray, or wave. It happens when there is a change in the energy of the nucleus. This is usually after a nucleus has gone through alpha or beta decay. There is no change in the atom's mass, or atomic number, only in the stored energy inside the nucleus, in the form of particle spin.