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Helium atom
Helium atom ground state.
An illustration of the helium atom, depicting the nucleus (pink) and the electron cloud distribution (black). The nucleus (upper right) in helium-4 is in reality spherically symmetric and closely resembles the electron cloud, although for more complicated nuclei this is not always the case. The black bar is one angstrom (10−10 m or 100 pm).
Smallest recognized division of a chemical element
Mass range1.67×10−27 to 4.52×10−25 kg
Electric chargezero (neutral), or ion charge
Diameter range62 pm (He) to 520 pm (Cs) (data page)
ComponentsElectrons and a compact nucleus of protons and neutrons

Atoms are very small pieces of matter. There are many different types of atoms. Each type has its own mass, size and number of parts (protons, neutrons, electrons).

Atoms are very small. Their exact size depends on the element. Atoms range from 0.1 to 0.5 nanometers in width.[1] One nanometer is about 100,000 times smaller than the width of a human hair.[2] This makes one atom impossible to see without special tools. Scientists discover how they work and interact with other atoms through experiments.

Atoms can join to make molecules. For example, two hydrogen atoms and one oxygen atom combine to make a water molecule. When atoms join, it is called a chemical reaction. Atoms can join without forming separate molecules. In this case, every atom is connected to a giant web of atoms. Other chemical reactions are breaking a molecule into atoms or exchanging atoms between different molecules.

Atoms are made up of three kinds of smaller particles, called protons, neutrons, and electrons. The protons and neutrons are heavier and stay in the middle of the atom, which is called the nucleus. The nucleus is surrounded by light-weight electrons. The electrons are attracted to the protons in the nucleus by the electromagnetic force because they have opposite electric charges.

Atoms with the same number of protons are said to belong to the same chemical element. They have very similar properties. Chemical elements are types of atoms. Examples of elements are hydrogen, carbon, chlorine, and gold. Chemical elements with the same number of protons but different types of neutrons are called isotopes.

The number of protons an atom has is called its atomic number. For example, a hydrogen atom has one proton and a sulfur atom has 16 protons. Because the mass of neutrons and protons is very similar, and the mass of electrons is very small, we can call the number of protons and neutrons in an atom its atomic mass.[3]

Atoms are only rarely made, destroyed, or changed into another kind of atom. Most kinds of atoms were made by stars. A few kinds are as old as the universe.[4] Almost all atoms on Earth have been on Earth for billions of years. A person can have more than 1027 atoms in their body. Some of these were once a part of every person who has ever lived.[5]

History[change | change source]

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. 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.[6]

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.[7]

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.[8] 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.[6]

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 stated 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.[9] Many of the particles went through the gold foil, which proved that atoms are mostly empty space. Electrons are so small they make up only 1% of an atom's mass.[10]

Ernest Rutherford

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 since it was first introduced.[6]

In 1925, chemist Frederick Soddy found that some elements in the periodic table had more than one kind of atom.[11] For example, an atom with 2 protons should be a helium atom. Usually, a helium nucleus also contains two neutrons. However, some helium atoms have only one neutron. This means they are helium because the number of protons defines an element, but they are not normal helium. Soddy called an atom like this, with a different number of neutrons, an isotope. To get the name of the isotope, we look at how many protons, and neutrons it has in its nucleus and add this to the name of the element. So a helium atom with two protons and one neutron is called helium-3, and a carbon atom with six protons and six neutrons is called carbon-12. However, when he developed his theory, Soddy could not be certain neutrons existed. To prove they were real, physicist James Chadwick and a team of others built the mass spectrometer.[12] The mass spectrometer measures the mass and weight of individual atoms. By doing this, Chadwick proved that neutrons must exist to account for all the atom's weight.

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.[13] 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.

The most accurate model so far comes from the Schrödinger equation. Schrödinger realized 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.

Structure and parts[change | change source]

Parts[change | change source]

An atom is made up of three main particles; the proton, the neutron and the electron. Hydrogen-1, an isotope of hydrogen, has no neutrons, just the one proton and one electron. A positive hydrogen ion has no electrons, just one proton. These two examples are the only exceptions to the rule that 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,[14] and their mass is about 9.1 x 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 x 10−24 grams (1 atomic mass unit). Protons are positively charged, and neutrons have no charge.[15] Most atoms have a neutral charge; because the number of protons (positive) and electrons (negative) are the same, the charges balance out to zero. However, in ions (different number of electrons), this is not always 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[change | change source]

The nucleus is in the middle of an atom. While it makes up almost all of the atom's mass, it is very small: about 1 femtometre (10−15 m) across, which is around 100,000 times smaller than the width of an atom, so it has a very high density.[15] It is made up 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 it was a mystery to scientists how the positively charged protons in the nucleus stayed together. They solved this by finding particles called mesons that hold together these protons and neutrons.[16][17] Later, scientists found that the quarks in a proton or neutron are held together by a particle called a gluon. Its name comes from the word glue as gluons act like atomic glue, sticking the quarks together using the strong interaction.[18] Mesons are also made of quarks, so the strong interaction explains how mesons hold the nucleus together.[17]

A graph showing the main difficulty in nuclear fusion, the fact that 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 the same by removing the extra particles. It does this by emitting radiation in the form of alpha, beta or gamma decay.[19] 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 heavier 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[change | change source]

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. Each electron is found in an area of space called an orbital. An atom can have many orbitals with different shapes and sizes. No more than two electrons can be in one orbital; these two electrons have different spin.

Shapes of different orbitals around an atom

Around the nucleus, some electron orbitals are further out than others in different layers. These are called electron shells. In most atoms, the first shell has two electrons, and all after that have eight. Exceptions are rare, but they do happen and are difficult to predict.[20] The electron shell that is farthest away from the nucleus determines how atoms combine or bond together to form molecules. The number of electrons in the outermost shell determines whether the atom is stable or which atoms it will bond with in a chemical reaction.[21]

Electrons that are further from the nucleus generally have more energy. When a small burst of energy called a photon hits an electron, the electron can jump into a higher energy shell. An electron can also send out a photon and fall into a lower energy shell. For each kind of atom, the possible energy level is restricted to one of a fixed set of possible energies. This is due to quantum mechanics, the physics of very tiny particles. This means that only photons with very specific amounts of energy can do this: these appear as specific colors of light because the photon energy level causes the color. The absorption spectrum of each element shows the colors of light that cause its electrons to jump. The emission spectrum shows the colors of light that its electrons send out when they fall.[22]

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.[15] The biggest atom is caesium, which is about 0.596 nanometres wide according to one model. The smallest atom is helium, which is about 0.062 nanometres wide.[23]

How atoms interact[change | change source]

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.[24]

Chemical bonds are the strongest kinds of attraction between atoms. All chemical bonds involve the movement of electrons. Atoms tend to 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.[25]

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 with 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.[26]

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.[27]

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.[28]

Magnetism[change | change source]

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.

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.[29]

Radioactive decay[change | change source]

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.[30] 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.[31]

There are three main types of radioactive decay; alpha, beta and gamma.[32]

  • 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 what is named 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 stable isotope or element.[33]

Atoms in nature have up to 94 protons.[34] Scientists make atoms with more than 94 protons, by smashing together smaller atoms at very fast speeds. All chemical elements with up to 118 protons have been found in nature or made by scientists. However, elements with more protons tend to have shorter half-lives: the half-lives of elements 115-118 are all less than one second. Some of the biggest atoms break apart by nuclear fission.[35]

Fission and fusion[change | change source]

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).[36] 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.[15]

Nuclear fusion mostly occurs in the Sun and other stars. It requires a hot place but makes even more energy than fission. This accounts for the heat and light of the Sun. The Sun now fuses hydrogen into helium, while bigger and hotter stars make heavier atoms. Almost all atoms in the Universe, except for hydrogen and helium, were made by nuclear fusion in stars.[4] Fusion bombs, or thermonuclear weapons, are the most powerful nuclear weapons.[36] Scientists are trying to make fusion reactors for nuclear power stations, but none exists yet.[37]

Nuclear fusion and nuclear fission make energy for similar reasons. Strangely, the mass of a nucleus is less than the masses of its protons and neutrons added together. For example, a helium-4 nucleus is made of 2 protons and two neutrons; it weighs about 0.8% less than 2 protons and two neutrons alone. This is explained by Einstein's famous formula E = mc2. The formula tells us that mass can transform into energy, or energy can transform into mass. A proton or neutron loses some of its mass when it joins a nucleus. This mass becomes energy, which as the strong force holds the nucleus together.

The mass per nucleon of a nucleus is its mass, divided by its number of protons and neutrons (nucleons). For any atom bigger than hydrogen, the mass per nucleon is less than the mass of a proton or neutron. Yet, mass per nucleon is higher for some atoms than others. As the nucleus gets bigger, it first goes down, then goes up. The lowest mass per nucleon is Iron-56. Atoms that are used in nuclear fission, such as uranium, are heavier than iron atoms. So when a heavy atom breaks into two smaller nuclei, some of the mass is lost. This mass becomes energy.[4]

Unusual forms and kinds of atoms[change | change source]

On Earth, atoms are usually in molecules that are in solid, liquid, or gas form. But atoms can also be found in other forms. Plasma is a mixture of ions and electrons that is usually very hot — it is found in stars. In the strange Bose–Einstein condensate, a group of atoms moves together like one big super-atom. To make a Bose-Einstein condensate, atoms must be very cold. Their total number of protons, neutrons, and electrons must be even. If this number is odd, they can make a Fermionic condensate. Some very cold atoms can make superfluids, which flow without losing any energy or slowing down; or superconductors, which have no electrical resistance.[38]

If the protons, neutrons, or electrons of an atom are switched with other particles, exotic atoms can be made.[39] 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 — they could make antimatter molecules.[40]

Related pages[change | change source]

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