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A simplified picture of a helium atom, with two protons (red), two neutrons (white), and two electrons (blue)

An atom is the most basic unit of matter. All normal matter on Earth and everywhere in the Universe is made of atoms. This includes solids, liquids, gases, and plasma. There are a fixed number of different types of atoms, called chemical elements. An atom is the smallest unit of matter that can combine with other atoms to make molecules and more complex matter that have specific chemical qualities.

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 discover how they work by doing experiments.

Atoms are made of three types of subatomic particles. These are protons, neutrons, and electrons. Protons and neutrons are heavier and are in the middle of the atom, which is called the nucleus. The nucleus is very small and dense. It is surrounded by light-weight electrons. Electrons are attracted to the nucleus by the electromagnetic force because they have opposite electric charges.

Atoms with the same number of protons are the same chemical element. They have very similar properties. Examples of elements are hydrogen and gold. About 92 elements occur in the natural world. (More have been made artificially in a laboratory). 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.

Many things are made of more than one type of atom. These are chemical compounds. Atoms can join by making chemical bonds. 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.

Atoms are only rarely made, destroyed, or changed into another type of atom. This happens if the forces inside are too weak to hold them together. Atoms can also join to make larger atoms at very high temperatures, such as inside a star. These changes are studied in nuclear physics.

History

The periodic table organizes all known chemical elements.

The word "atom" comes from the Greek (ἀτόμος) "atomos", indivisible, from (ἀ)-, not, and τόμος, a cut. The first person we know used the word "atom" is the Greek philosopher Democritus, around 400 BC. He believed that everything was made of atoms, and atoms could not be broken into smaller pieces. Atomic theory initially was a philosophical subject, with not much actual scientific investigation or study, until the development of chemistry in the 1650s.

In 1777 French chemist Antoine Lavoisier defined the term element for the first time. He said that an element was any basic substance that could not be broken down into other substances by the methods of chemistry. Any substance that 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. According to Dalton, in a certain compound, the atoms of the compound's elements always combine in the same way.

In 1827, British scientist Robert Brown looked at pollen grains in water under his microscope. The pollen grains appeared to be jiggling. 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 conclusively proved the existence of the atom. In 1869, Russian scientist Dmitri Mendeleev published the first version of the 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 period, usually have similar properties. For example, helium, neon, argon, krypton and xenon are all in the same column and have very similar properties. All these elements are gases that have no color or smell. Also, they are unable to combine with other atoms to form compounds. Together they are known as the noble gases.

Ernest Rutherford

The physicist J.J. Thomson was the first person to discover electrons. This happened while he was working with cathode rays in 1897. He realized they had a negative charge, and the atomic nucleus 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, a scientist named 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 introduced the Bohr model. This model showed that electrons travel around the nucleus in fixed circular orbits. This was more accurate than the Rutherford model. However, it was still not completely right. Improvements to the Bohr model have been made after it was first introduced.

The Bohr model is not accurate, but it is useful for the idea of electron shells. This atom has 28 electrons in three shells.

In 1925, chemist Frederick Soddy discovered that some elements in the periodic table had more than one kind of atom, called isotopes. Soddy believed that isotopes of an element have different mass. To prove this, chemist Francis W. Aston built the mass spectrometer, which measures the mass of individual 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 particle in the nucleus besides 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 most accurate 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 is used to determine 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, he noticed that 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 eventually led to the creation of the atomic bomb and nuclear power, where fission occurs repeatedly, creating a chain reaction.

Further, into 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.

Classification

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.

Because protons and neutrons have very similar 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. The average mass of all atoms of a particular element is called its atomic mass or atomic weight.

If the protons, neutrons, or electrons of an atom are switched with other particles, exotic atoms can be made. Experiments have showed that every particle has an opposite called an antiparticle. Together, these particles make up antimatter. An antimatter atom would be made from antiprotons, antineutrons, and antielectrons (positrons). When a particle meets its antiparticle, they are both destroyed. Aside from that, antimatter atoms could be very much like normal atoms.

Structure and parts

Parts

An atom is made of three main particles: the proton, the neutron and the electron. Hydrogen-1, an isotope of hydrogen, has no neutrons, just one proton and one electron. A positive hydrogen ion has no electrons, just one proton. All other atoms have at least one proton, one neutron, and one electron each.

Electrons are by far the smallest of the three atomic particles. Their size is too small to be measured using current technology, and their mass is about 9.1×10⁻²⁸ grams (0.00055 atomic mass units). They have a negative charge. Protons and neutrons are of similar size and weight to each other, with a mass of about 1.7×10⁻²⁴ grams (1 atomic mass unit). Protons have a positive charge, and neutrons have no charge. Most atoms have a neutral charge. The number of protons (positive) and electrons (negative) are the same, so the charges balance out to zero. However, in ions (different number of electrons), this is not the case, and they can have a positive or a negative charge. Protons and neutrons are made out of quarks of two types; 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

See the main article: Atomic nucleus

The nucleus is in the middle of an atom. It 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. It is made of protons and neutrons. 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 interaction.

The strong interaction is strongest over a very short distance. At this distance, it is carried by a particle called a gluon. Gluons act like glue to stick quarks together within a proton or neutron. Over a longer distance, the strong interaction is weaker. It is carried by a particle called a meson. Mesons attract the protons and neutrons using the strong interaction. They hold the nucleus together. This special form of the strong interaction is called the nuclear force.

A picture showing the main difficulty in nuclear fusion: Protons, which have positive charges, repel each other when forced together.

The number of neutrons in relation to protons defines whether the nucleus is stable 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 does this by emitting radiation in the form of alpha, beta or gamma decay. Nuclei can change through other means too. Nuclear fission is when the nucleus breaks into two smaller nuclei, releasing a lot of energy. This release of energy is what 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 extreme amounts of energy to overcome the electrostatic 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 of the conversion of some of the mass into energy.

Electrons

See the main article: Electron

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.

According to the Bohr model, some electrons are farther from the nucleus than others in different layers. These are called electron shells. We have learned that 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 specific 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 type 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 similar reasons, 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.

Shapes of different orbitals around an atom

For each shell, numbered 1, 2, 3, and so on, there can be several orbitals with different shapes. The types of orbitals are given letters: s, p, d, and f. There can also be several orbitals for one number and letter combination. 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, and 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

Size and mass

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. However, no electron shells are added. 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.

The mass of atoms is from 1.7×10⁻²⁴ to 4.9×10⁻²² grams. Usually, the mass is measured using 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 1.7×10⁻²⁴ grams. Hydrogen-1 has a mass of about 1 amu, and the heaviest atom known, oganesson, has a mass of about 294 amu.

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 constantly 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 weaker still, they will form a gas.

Graphite is made of carbon atoms in layers. Each layer is held together by covalent bonds. The attraction between different layers is a Van der Waals force.

Chemical bonds are the strongest kinds of attraction between atoms. All chemical bonds involve the movement of electrons. Atoms usually bond with each other in a way that fills or empties their outer electron shell. The most reactive elements need to lose or gain a small number of electrons to have a full 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 lattice, 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 reshape metals.

All atoms attract each other by Van der Waals forces, which are weaker than chemical bonds. These forces 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 charge, they will attract.

Despite being mostly empty space, atoms cannot pass through each other. When two atoms are very close, their electron clouds will repel each other by the electromagnetic force.

Magnetism

An electron has a property called a magnetic moment, which has a direction and a magnitude (or size). The magnetic moment comes from the electron's individual spin and its orbit around the nucleus. Together, the magnetic moments for the electrons add up to a magnetic moment for the whole atom. This explains the behavior of atoms in a magnetic field.

Each electron in an atom has one of two kinds of spin. If every electron is paired with an electron with the opposite spin, the spins will cancel out, so the atom will have no lasting magnetic moment. 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. In a magnetic field, most atoms will line up in the direction of the field. Ferromagnetic materials, such as iron, cobalt, and nickel, are strongly attracted to a magnetic field.

Radioactive decay

See the main article: Radioactive decay

An alpha particle shoots out of a nucleus.

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 has too many protons or neutrons. When this happens, the nucleus has to eliminate the excess mass of particles. It does this through radiation. An atom that does this can be 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 types 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 essentially a helium nucleus. The result is an element with an atomic number two less than before. So, for example, if a beryllium atom (atomic number 4) went through alpha decay, it would become helium (atomic number 2). Alpha decay happens when an atom is too big and needs to get rid of 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 is 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.

Every radioactive element or isotope has a half-life. This is how long it takes half of any sample of atoms of that type to decay until they become a different isotope or element.

Fission and fusion

Devices that use nuclear fission start by shooting neutrons at atoms. This causes the atom to break apart quickly. The fission of one atom shoots off more neutrons, which then break other atoms, creating chain reactions. This process makes huge amounts of heat energy. The chain reaction of fission powered the first nuclear weapons (fission bombs). Nuclear power stations are a bit different: things called control rods are used to slow down the fission. Control rods collect some of the neutrons, which stops a chain reaction from happening.

Nuclear fusion mostly occurs in the Sun and other stars. It requires a hot place but makes even more energy than fission. This explains the heat and light of the Sun. The Sun now fuses hydrogen into helium, while bigger and hotter stars make heavier atoms. Fusion bombs, or thermonuclear weapons, are the most powerful nuclear weapons. Scientists are trying to make fusion reactors for nuclear power stations, but none exists yet.

Nuclear fusion and nuclear fission make energy for similar reasons. According to Einstein's famous formula E = mc², a small amount of mass can transform into a large amount of energy. When protons and neutrons come together in nuclear fusion, they lose some mass, which they send out as energy. The nucleus can split into its protons and neutrons only if the same amount of energy is added. This property of each type of nucleus is called its nuclear binding energy. Fusion or fission send out energy if the total binding energy increases. Nuclei in the range of iron-56 and nickel-62 have the highest binding energy divided by their number of protons and neutrons, so they generally do not go through fission or fusion. Bigger atoms, such as uranium, go through nuclear fission, because it increases their binding energy.

Formation and occurrence

Nearly all the hydrogen atoms in the Universe, most of the helium atoms, and some of the lithium atoms were made soon after the Big Bang. Even today, about 90% of all atoms in the Universe are hydrogen. Larger atoms are made in stars by nuclear fusion, while the largest atoms are made in very massive stars or supernovae. Most atoms on Earth were made by a star that existed before the Sun.

People make very large atoms by smashing together smaller atoms in particle accelerators. However, these atoms often decay very quickly. Oganesson (element 118) has a half-life of 0.89 milliseconds. It is possible that even larger atoms will be created in the future.

Related pages

• Atomic physics, for more detail about the physics of atoms
• Chemistry, the field which studies all things that are made from atoms
• Elementary particle, the particles that make up atoms and everything else
• Quantum mechanics, the study of small particles and how they interact with energy
• States of matter, the different forms in which atoms or molecules can be found

Sources

Bibliography

• Flowers, Paul; Theopold, Klaus; Langley, Richard; Neth, Edward J.; Robinson, William R. (2019). Chemistry: Atoms First (2nd ed.). Houston, Texas: OpenStax, Rice University. ISBN 978-1-947172-63-0. OCLC 1089692119.
• Iliadis, Christian (2007). Nuclear Physics of Stars. Weinheim: Wiley-VCH. doi:10.1002/9783527618750. ISBN 978-3-527-40602-9.
• Serway, Raymond A.; Moses, Clement; Moyer, Curt A. (1997). Modern Physics (2nd ed.). Saunders College Publishing. ISBN 0-03-001547-2.

Other websites

• General information on atomic structure Archived December 23, 2012, at the Wayback Machine
• Atomic structure timeline Archived September 26, 2007, at the Wayback Machine
• Atom (science) -Citizendium