A drawing of thehelium atom, showing thenucleus (pink) and theelectron cloud (black). The nucleus is made of two protons (red) and two neutrons (violet). The black bar is oneangstrom (10−10 m or0.1 nm).
Anatom is an extremely small piece ofmatter. All normal matter – everything that hasmass – is made of atoms. This includessolids,liquids, andgases. The atom cannot be broken to parts bychemistry, so people once thought it was the smallest piece of matter that could exist.[1] 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 0.5nanometers across.[2] One nanometer is about 100,000 times smaller than the width of a humanhair.[3] This makes one atom impossible to see without special tools.Scientists learn how they work by doingexperiments.
Atoms are made of three kinds ofsubatomic particles. These areprotons,neutrons, andelectrons. Protons and neutrons have much moremass. These are in the middle of the atom, called thenucleus. Lightweight electrons move quickly around them. Theelectromagnetic force holds the nucleus and electrons together.
Atoms with the same number of protons belong to the samechemical element. Examples of elements arecarbon andgold. Atoms with the same number of protons, but different numbers of neutrons, are calledisotopes. Usually an atom has the same number of electrons as protons. If an atom has more or less electrons than protons, it is called anion, and has an electric charge.
Atoms split if the forces inside are too weak to hold them together. This is what causesradioactivity. Atoms can also join to make larger atoms at very high temperatures, such as inside astar. These changes are studied innuclear physics. Most atoms on Earth are notradioactive. They are rarely made, destroyed, or changed into another kind of atom.
The word "atom" comes from theGreek (ἀτόμος) "atomos", which meansindivisible oruncuttable.[4] One of the first people to use the word "atom" is the GreekphilosopherDemocritus, around 400 BC. He thought that everything was made ofparticles called atoms, which could not be divided into smaller pieces. SomeHindu,Jain, andBuddhist philosophers also had ideas like this.[5] Atomic theory was a mostlyphilosophical subject, with not muchscientific investigation or study, until the early 1800s.[6]
In 1777French chemistAntoine Lavoisier defined the termelement as we now use it. He said that anelement was any substance that could not be broken down into other substances by the methods ofchemistry. Any substance which could be broken down was acompound.[7]
Dalton's drawings of atoms (1808)
In 1803,English philosopherJohn Dalton suggested that elements were made of tiny, solid balls called atoms. Dalton believed that all atoms of the same element have the samemass. 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.[6][8]
In 1827, British scientistRobert Brown looked atpollen grains in water under his microscope. The pollen grains appeared to be shaking.[9] Brown used Dalton's atomic theory to describe patterns in how they moved. This was calledBrownian 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.[10][11]
In 1869, Russian scientistDmitri Mendeleev published the firstperiodic table. The periodic table groups elements by theiratomic number (how manyprotons they have; this is usually the same as the number ofelectrons). Elements in the same column, or group, usually have similar qualities.[12] For example,helium,neon,argon,krypton, andxenon are all in the same column and are very similar. All these elements aregases that have no color or smell. Also, they cannot combine with other atoms to form compounds. Together they are known asnoble gases.
The physicistJ.J. Thomson was the first person to discover electrons. This happened while he was working withcathode rays in 1897. He learned they had a negativecharge, and the rest of the atom had a positive charge. Thomson made theplum 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 theGeiger–Marsden experiment to prove that most of an atom is in a very small space, theatomic nucleus. Rutherford took a photo plate and covered it with gold foil. He then shotalpha 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 discoveredprotons in the nucleus.[13]
TheBohr model is not completely true, but it is useful for the idea ofelectron shells. This atom has 28 electrons in three shells.
In 1913,Niels Bohr created theBohr 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.[14]
In 1925, chemistFrederick Soddy discovered that some elements had more than one kind of atom, calledisotopes. Soddy believed that each different isotope of an element has a different mass.[15] To prove this, chemistFrancis William Aston built themass 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.[16] This meant that there must be some particles in the nucleus other than protons. In 1932, physicistJames 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 theneutron.[17]
The best model so far comes from theSchrödinger equation. Schrödinger learned that the electrons exist in a cloud around the nucleus, called theelectron 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'sorbital.[18]
In 1937,German chemistOtto Hahn became the first person to makenuclear fission in a laboratory. He discovered this by chance when shooting neutrons at auranium atom, hoping to make a new isotope. However, instead of a new isotope, the uranium changed into abarium atom, a smaller atom than uranium. Hahn had "broken" the uranium atom. This was the world's first recorded nuclear fission reaction.[19] This discovery led to the creation of theatomic bomb andnuclear 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. Usingparticle accelerators, they discovered that protons and neutrons were made of other particles, calledquarks.[20]
This picture shows how small the nucleus is. The electrons are somewhere in the black cloud.
An atom is made of three mainparticles: theproton, theneutron, and theelectron. Protons and neutrons have nearly the same size and mass (about1.7×10−24grams). The mass of an electron is about 1800 times smaller (about9.1×10−28 grams). Protons have a positivecharge, 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.[21][1]
Scientists believe that electrons areelementary particles: they are not made of any smaller pieces. Protons and neutrons are made ofquarks 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.[20]
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 1femtometer (10−15 m) across, which is around 100,000 times smaller than the width of an atom, so it has a very highdensity.[22]
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 thestrong nuclear force. This force also holds together the quarks that make up the protons and neutrons. Particles calledmesons travel back and forth between protons and neutrons, and carry the force.[23][24]
A picture showing the main difficulty innuclear fusion: Protons, which have positive charges, repel each other when forced together.
The number of neutrons in relation to protons defines whether the nucleus stays together or goes throughradioactive 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 ofalpha,beta, orgamma decay.[25] Nuclei can also change in other ways.Nuclear fission is when the nucleus breaks into two smaller nuclei, releasing a lot ofenergy. This release of energy makes nuclear fission useful for makingbombs, andelectricity in the form ofnuclear power.[26]The other way nuclei can change is throughnuclear 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 instars like ourSun, which fuses hydrogen for fuel. However, once fusion happens, far more energy is released, because some of the mass becomes energy.[27]
The energy needed to break a nucleus into protons and neutrons is called itsnuclear binding energy. This energy can be converted to mass, as stated by Einstein's famous formulaE = mc2. Medium-sized nuclei, such asiron-56 andnickel-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.[28]
Electrons orbit, or travel around, the nucleus. They are called the atom'selectron cloud. They are attracted to the nucleus because of theelectromagnetic force. Electrons have a negative charge, and the nucleus always has a positive charge, so they attract each other.[29]
TheBohr model shows that some electrons are farther from the nucleus than others in different levels. These are calledelectron shells.[29] Only the electrons in the outer shell can makechemical bonds. The number of electrons in the outer shell determines whether the atom is stable or which atoms it will bond with in achemical 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.[30]
The Bohr model is important because it has the idea ofenergy 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 aphoton hits an electron, the electron can jump into ahigher-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 willabsorb certain colors of light, called theabsorption spectrum. An electron can also send out, or emit, a photon, and fall into alower energy shell. For the same reason, the atom will only send out certain colors of light, called theemission spectrum.[29]
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 theprobability, or chance, that it will be in any place. Each electron is part of anorbital, which describes where it is likely to be. No more than two electrons can be in one orbital; these two electrons have differentspin.
Shapes of different orbitals around an atom
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 threequantum numbers. Theprincipal quantum number is the electron shell number. Theazimuthal 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 amagnetic 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.[31]
Theperiodic table organizes all known chemical elements.
The number of protons in an atom is called itsatomic 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.[32] 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.[33] These elements are organized on theperiodic table.
Because protons and neutrons have nearly the samemass, and the mass of electrons is very small, we can call the number of protons and neutrons in an atom itsmass 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 itsatomic mass. This is usually measured with theatomic mass unit (amu), also called the dalton. One amu is exactly 1/12 of the mass of a carbon-12 atom, which is1.7×10−24grams. Hydrogen-1 has a mass of about 1 amu. The heaviest atom known,oganesson, has a mass of about 294 amu, or4.9×10−22 grams.[34] The average mass of all atoms of a particular element is called itsatomic weight.[32]
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.[35] The biggest atom iscaesium, which is about 0.596 nanometers wide according to one model. The smallest atom ishelium, which is about 0.062 nanometers wide.[36]
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, orkinetic energy, of atoms makes them always move. If the attraction is strong enough, relative to the amount of heat, atoms will form asolid. If the attraction is weaker, they will form aliquid, and if it is even weaker, they will form agas.
Graphite is made of carbon atoms in layers. Covalent bonds hold each layer together. The attraction between different layers is a Van der Waals force.[37]
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, callednoble gases, do not usually form bonds.[38]
In an ionic bond, one atom gives electrons to another atom. Each atom becomes anion: an atom or group of atoms with a positive or negative charge. The positive ion (which has lost electrons) is called acation; it is usually ametal. The negative ion (which has gained electrons) is called ananion; it is usually anonmetal. Ionic bonding usually results in a regular network, orcrystal, of ions held together.
In a covalent bond, two atoms share electrons. This usually happens when both atoms are nonmetals. Covalent bonds often formmolecules, ranging in size from two atoms to many more. They can also form large networks, such asglass orgraphite. The number of bonds that an atom makes (itsvalency) 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 conductelectric 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.[39]
All atoms attract each other byVan 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.[40]
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.[41]
To understand howmagnets 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 itsmagnetic moment. Every electron also has a magnetic moment, like a tiny magnet. This comes from the electron'sspin 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 amagnetic 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 sameorbital, the magnetic moments will cancel out to zero. Atoms like this are calleddiamagnetic. 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 beparamagnetic orferromagnetic. 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.[42]
Some elements, and many isotopes, have what is called anunstable nucleus. This means the nucleus is either too big to hold itself together, or it has too many protons or neutrons.[43] When a nucleus is unstable, it has to eliminate the excess mass of particles. It does this throughradiation. An atom that does this is calledradioactive. 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.[44]
The nuclei in black are stable. All others areradioactive. The left axis is the number of protons, and the bottom axis is the number of neutrons.
There are three main kinds of radioactive decay:alpha,beta, andgamma.[25]
Alpha decay is when the atom shoots out a particle having two protons and two neutrons. This is ahelium-4 nucleus. The result is an element with an atomic number of two less than before. So, for example, if auranium atom (atomic number 92) went through alpha decay, it would becomethorium (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 apositron (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 agamma ray, or wave. It happens when there is a change in theenergy 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 ahalf-life. This is how long it takes half of any sample of atoms of that type to decay into a different isotope or element.[45]
Nearly all the hydrogen atoms in theUniverse, most of the helium atoms, and some of the lithium atoms were made soon after theBig Bang. Even today, about 90% of all atoms in the Universe are hydrogen.[46]
All other atoms come fromnuclear fusion in stars, or sometimes fromcosmic rays that hit atoms. At the start of their life, all stars fuse hydrogen to make helium. The least massive stars,red dwarfs, are expected to stop there. All other stars will then fuse helium to make carbon and oxygen. In stars like the Sun, the temperature and pressure are too low to make larger atoms. But more massive stars continue fusion, until they create iron (atomic number 26) or nickel (atomic number 28).[47] Atoms can also grow larger when neutrons or protons hit them. This could happen inside stars or insupernovae. Most atoms on Earth were made by a star that existed before the Sun.[48]
People make very large atoms by smashing together smaller atoms inparticle accelerators. However, these atoms often decay very quickly.Oganesson (element 118) has a half-life of 0.00089 seconds. Even larger atoms may be created in the future.[34]
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