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

An atom is the smallest unit of ordinary Template:Direct link that forms a Template:Direct link. Every Template:Direct link, Template:Direct link, Template:Direct link, and Template:Direct link is composed of neutral or Template:Direct link atoms. Atoms are extremely small, typically around 100 Template:Direct links across. They are so small that accurately predicting their behavior using Template:Direct link—as if they were tennis balls, for example—is not possible due to Template:Direct link.

Every atom is composed of a Template:Direct link and one or more Template:Direct links bound to the nucleus. The nucleus is made of one or more Template:Direct links and a number of Template:Direct links. Only the most common variety of Template:Direct link has no neutrons. More than 99.94% of an atom's Template:Direct link is in the nucleus. The protons have a positive Template:Direct link, the electrons have a negative electric charge, and the neutrons have no electric charge. If the number of protons and electrons are equal, then the atom is electrically neutral. If an atom has more or fewer electrons than protons, then it has an overall negative or positive charge, respectively – such atoms are called Template:Direct links.

The electrons of an atom are attracted to the protons in an atomic nucleus by the Template:Direct link. The protons and neutrons in the nucleus are attracted to each other by the Template:Direct link. This force is usually stronger than the electromagnetic force that repels the positively charged protons from one another. Under certain circumstances, the repelling electromagnetic force becomes stronger than the nuclear force. In this case, the nucleus Template:Direct link and Template:Direct link. This is a form of Template:Direct link.

The number of protons in the nucleus is the Template:Direct link and it defines to which chemical element the atom belongs. For example, any atom that contains 29 protons is Template:Direct link. The number of neutrons defines the Template:Direct link of the element. Atoms can attach to one or more other atoms by Template:Direct links to form Template:Direct links such as Template:Direct links or Template:Direct links. The ability of atoms to associate and dissociate is responsible for most of the physical changes observed in nature. Template:Direct link is the discipline that studies these changes.

History of atomic theory

Main article: Atomic theory

In philosophy

Main article: Atomism

The basic idea that matter is made up of tiny indivisible particles is very old, appearing in many ancient cultures such as Greece and India. This ancient idea was based in philosophical reasoning rather than scientific reasoning, and modern atomic theory is not based on these old concepts. The word atom is derived from the Greek word atomos, which means "uncuttable".[1][2]

Dalton's law of multiple proportions

Atoms and molecules as depicted in Template:Direct link's A New System of Chemical Philosophy vol. 1 (1808)

In the early 1800s, Template:Direct link compiled experimental data gathered by himself and other scientists and discovered a pattern now known as the "Template:Direct link". He noticed that in chemical compounds which contain a particular chemical element, the content of that element in these compounds will differ by ratios of small whole numbers. This pattern suggested to Dalton that each chemical element combines with others by some basic and consistent unit of mass.

For example, there are two types of Template:Direct link: one is a black powder that is 88.1% tin and 11.9% oxygen, and the other is a white powder that is 78.7% tin and 21.3% oxygen. Adjusting these figures, in the black oxide there is about 13.5 g of oxygen for every 100 g of tin, and in the white oxide there is about 27 g of oxygen for every 100 g of tin. 13.5 and 27 form a ratio of 1:2. In these oxides, for every tin atom there are one or two oxygen atoms respectively (Template:Direct link and Template:Direct link).[3][4]

As a second example, Dalton considered two Template:Direct links: a black powder which is 78.1% iron and 21.9% oxygen, and a red powder which is 70.4% iron and 29.6% oxygen. Adjusting these figures, in the black oxide there is about 28 g of oxygen for every 100 g of iron, and in the red oxide there is about 42 g of oxygen for every 100 g of iron. 28 and 42 form a ratio of 2:3. In these respective oxides, for every two atoms of iron, there are two or three atoms of oxygen (Template:Direct link and Template:Direct link).[a][5][6]

As a final example: Template:Direct link is 63.3% nitrogen and 36.7% oxygen, Template:Direct link is 44.05% nitrogen and 55.95% oxygen, and Template:Direct link is 29.5% nitrogen and 70.5% oxygen – adjusting the figures, for every 140 g of nitrogen, there is about 80 g, 160 g, and 320 g of oxygen in these oxides respectively, which gives a ratio of 1:2:4. The respective formulas for these oxides are Template:Direct link, Template:Direct link, and Template:Direct link.[7][8]

Kinetic theory of gases

Main article: Kinetic theory of gases

In the late 18th century, a number of scientists found that they could better explain the behavior of gases by describing them as collections of sub-microscopic particles and modelling their behavior using Template:Direct link and Template:Direct link. Unlike Dalton's atomic theory, the kinetic theory of gases describes not how gases react chemically with each other to form compounds, but how they behave physically: diffusion, viscosity, conductivity, pressure, etc.

Brownian motion

In 1827, Template:Direct link Template:Direct link used a microscope to look at dust grains floating in water and discovered that they moved about erratically, a phenomenon that became known as "Template:Direct link". This was thought to be caused by water molecules knocking the grains about. In 1905, Template:Direct link proved the reality of these molecules and their motions by producing the first Template:Direct link analysis of Template:Direct link.[9][10][11] French physicist Template:Direct link used Einstein's work to experimentally determine the mass and dimensions of molecules, thereby providing physical evidence for the particle nature of matter.[12]

Discovery of the electron

[[File:Geiger-Marsden experiment expectation and result.svg|thumb|right|The Template:Direct link In 1897, Template:Direct link discovered that Template:Direct links are not electromagnetic waves but made of particles that are 1,800 times lighter than Template:Direct link (the lightest atom). Thomson concluded that these particles came from the atoms within the cathode — they were subatomic particles. He called these new particles corpuscles but they were later renamed Template:Direct links. Thomson also showed that electrons were identical to particles given off by Template:Direct link and radioactive materials.[13] It was quickly recognized that electrons are the particles that carry Template:Direct links in metal wires. Thomson concluded that these electrons emerged from the very atoms of the cathode in his instruments, which meant that atoms are not indivisible as the name atomos suggests.

Discovery of the nucleus

Main article: Geiger–Marsden experiment

Template:Direct link thought that the negatively-charged electrons were distributed throughout the atom in a sea of positive charge that was distributed across the whole volume of the atom.[14] This model is sometimes known as the Template:Direct link.

Template:Direct link and his colleagues Template:Direct link and Template:Direct link came to have doubts about the Thomson model after they encountered difficulties when they tried to build an instrument to measure the charge-to-mass ratio of Template:Direct link (these are positively-charged particles emitted by certain radioactive substances such as Template:Direct link). The alpha particles were being scattered by the air in the detection chamber, which made the measurements unreliable. Thomson had encountered a similar problem in his work on cathode rays, which he solved by creating a near-perfect vacuum in his instruments. Rutherford didn't think he'd run into this same problem because alpha particles are much heavier than electrons. According to Thomson's model of the atom, the positive charge in the atom is not concentrated enough to produce an electric field strong enough to deflect an alpha particle, and the electrons are so lightweight they should be pushed aside effortlessly by the much heavier alpha particles. Yet there was scattering, so Rutherford and his colleagues decided to investigate this scattering carefully.[15]

Between 1908 and 1913, Rutheford and his colleagues performed a series of experiments in which they bombarded thin foils of metal with alpha particles. They spotted alpha particles being deflected by angles greater than 90°. To explain this, Rutherford proposed that the positive charge of the atom is not distributed throughout the atom's volume as Thomson believed, but is concentrated in a tiny nucleus at the center. Only such an intense concentration of charge could produce an electric field strong enough to deflect the alpha particles as observed.[15]

Discovery of isotopes

While experimenting with the products of Template:Direct link, in 1913 Template:Direct link Template:Direct link discovered that there appeared to be more than one type of atom at each position on the Template:Direct link.[16] The term Template:Direct link was coined by Template:Direct link as a suitable name for different atoms that belong to the same element. J. J. Thomson created a technique for Template:Direct link through his work on Template:Direct linkes, which subsequently led to the discovery of Template:Direct links.[17]

Bohr model

The Bohr model of the atom, with an electron making instantaneous "quantum leaps" from one orbit to another with gain or loss of energy. This model of electrons in orbits is obsolete.

Main article: Bohr model

In 1913 the physicist Template:Direct link proposed a model in which the electrons of an atom were assumed to orbit the nucleus but could only do so in a finite set of orbits, and could jump between these orbits only in discrete changes of energy corresponding to absorption or radiation of a photon.[18] This quantization was used to explain why the electrons' orbits are stable (given that normally, charges in acceleration, including circular motion, lose kinetic energy which is emitted as electromagnetic radiation, see Template:Direct link) and why elements absorb and emit electromagnetic radiation in discrete spectra.[19]

Later in the same year Template:Direct link provided additional experimental evidence in favor of Template:Direct link. These results refined Template:Direct link's and Template:Direct link's model, which proposed that the atom contains in its Template:Direct link a number of positive Template:Direct links that is equal to its (atomic) number in the periodic table. Until these experiments, Template:Direct link was not known to be a physical and experimental quantity. That it is equal to the atomic nuclear charge remains the accepted atomic model today.[20]

Template:Direct links between atoms were explained by Template:Direct link in 1916, as the interactions between their constituent electrons.[21] As the Template:Direct link of the elements were known to largely repeat themselves according to the Template:Direct link,[22] in 1919 the American chemist Template:Direct link suggested that this could be explained if the electrons in an atom were connected or clustered in some manner. Groups of electrons were thought to occupy a set of Template:Direct links about the nucleus.[23]

The Bohr model of the atom was the first complete physical model of the atom. It described the overall structure of the atom, how atoms bond to each other, and predicted the spectral lines of hydrogen. Bohr's model was not perfect and was soon superseded by the more accurate Schrödinger model, but it was sufficient to evaporate any remaining doubts that matter is composed of atoms. For chemists, the idea of the atom had been a useful heuristic tool, but physicists had doubts as to whether matter really is made up of atoms as nobody had yet developed a complete physical model of the atom.

The Schrödinger model

The Template:Direct link of 1922 provided further evidence of the quantum nature of atomic properties. When a beam of silver atoms was passed through a specially shaped magnetic field, the beam was split in a way correlated with the direction of an atom's angular momentum, or Template:Direct link. As this spin direction is initially random, the beam would be expected to deflect in a random direction. Instead, the beam was split into two directional components, corresponding to the atomic Template:Direct link being oriented up or down with respect to the magnetic field.[24]

In 1925 Template:Direct link published the first consistent mathematical formulation of quantum mechanics (Template:Direct link).[20] One year earlier, Template:Direct link had proposed the Template:Direct link: that all particles behave like waves to some extent,[25] and in 1926 Template:Direct link used this idea to develop the Template:Direct link, a mathematical model of the atom (wave mechanics) that described the electrons as three-dimensional Template:Direct links rather than point particles.[26]

A consequence of using waveforms to describe particles is that it is mathematically impossible to obtain precise values for both the Template:Direct link and Template:Direct link of a particle at a given point in time; this became known as the Template:Direct link, formulated by Template:Direct link in 1927.[20] In this concept, for a given accuracy in measuring a position one could only obtain a range of probable values for momentum, and vice versa.[27] This model was able to explain observations of atomic behavior that previous models could not, such as certain structural and Template:Direct link patterns of atoms larger than hydrogen. Thus, the planetary model of the atom was discarded in favor of one that described Template:Direct link zones around the nucleus where a given electron is most likely to be observed.[28][29]

Discovery of the neutron

The development of the Template:Direct link allowed the mass of atoms to be measured with increased accuracy. The device uses a magnet to bend the trajectory of a beam of ions, and the amount of deflection is determined by the ratio of an atom's mass to its charge. The chemist Template:Direct link used this instrument to show that isotopes had different masses. The Template:Direct link of these isotopes varied by integer amounts, called the Template:Direct link.[30] The explanation for these different isotopes awaited the discovery of the Template:Direct link, an uncharged particle with a mass similar to the Template:Direct link, by the physicist Template:Direct link in 1932. Isotopes were then explained as elements with the same number of protons, but different numbers of neutrons within the nucleus.[31]

Fission, high-energy physics and condensed matter

In 1938, the German chemist Template:Direct link, a student of Rutherford, directed neutrons onto uranium atoms expecting to get Template:Direct links. Instead, his chemical experiments showed Template:Direct link as a product.[32][33] A year later, Template:Direct link and her nephew Template:Direct link verified that Hahn's result were the first experimental nuclear fission.[34][35] In 1944, Hahn received the Template:Direct link. Despite Hahn's efforts, the contributions of Meitner and Frisch were not recognized.[36]

In the 1950s, the development of improved Template:Direct links and Template:Direct links allowed scientists to study the impacts of atoms moving at high energies.[37] Neutrons and protons were found to be Template:Direct links, or composites of smaller particles called Template:Direct links. The Template:Direct link was developed that so far has successfully explained the properties of the nucleus in terms of these sub-atomic particles and the forces that govern their interactions.[38]


Subatomic particles

Main article: Subatomic particle

Though the word atom originally denoted a particle that cannot be cut into smaller particles, in modern scientific usage the atom is composed of various Template:Direct links. The constituent particles of an atom are the Template:Direct link, the Template:Direct link and the Template:Direct link.

The electron is by far the least massive of these particles at 9.11×10−31 kg, with a negative Template:Direct link and a size that is too small to be measured using available techniques.[39] It was the lightest particle with a positive rest mass measured, until the discovery of Template:Direct link mass. Under ordinary conditions, electrons are bound to the positively charged nucleus by the attraction created from opposite electric charges. If an atom has more or fewer electrons than its atomic number, then it becomes respectively negatively or positively charged as a whole; a charged atom is called an Template:Direct link. Electrons have been known since the late 19th century, mostly thanks to Template:Direct link; see Template:Direct link for details.

Protons have a positive charge and a mass 1,836 times that of the electron, at 1.6726×10−27 kg. The number of protons in an atom is called its Template:Direct link. Template:Direct link (1919) observed that nitrogen under alpha-particle bombardment ejects what appeared to be hydrogen nuclei. By 1920 he had accepted that the hydrogen nucleus is a distinct particle within the atom and named it Template:Direct link.

Neutrons have no electrical charge and have a free mass of 1,839 times the mass of the electron, or 1.6749×10−27 kg.[40][41] Neutrons are the heaviest of the three constituent particles, but their mass can be reduced by the Template:Direct link. Neutrons and protons (collectively known as Template:Direct links) have comparable dimensions—on the order of 2.5×10−15 m—although the 'surface' of these particles is not sharply defined.[42] The neutron was discovered in 1932 by the English physicist Template:Direct link.

In the Template:Direct link of physics, electrons are truly elementary particles with no internal structure, whereas protons and neutrons are composite particles composed of Template:Direct links called Template:Direct links. There are two types of quarks in atoms, each having a fractional electric charge. Protons are composed of two Template:Direct links (each with charge +2/3) and one Template:Direct link (with a charge of −1/3). Neutrons consist of one up quark and two down quarks. This distinction accounts for the difference in mass and charge between the two particles.[43][44]

The quarks are held together by the Template:Direct link (or strong force), which is mediated by Template:Direct links. The protons and neutrons, in turn, are held to each other in the nucleus by the Template:Direct link, which is a residuum of the strong force that has somewhat different range-properties (see the article on the nuclear force for more). The gluon is a member of the family of Template:Direct links, which are elementary particles that mediate physical forces.[43][44]


Main article: Atomic nucleus

[[File:Binding energy curve - common isotopes.svg|thumb|The Template:Direct link needed for a nucleon to escape the nucleus, for various isotopes]]

All the bound protons and neutrons in an atom make up a tiny Template:Direct link, and are collectively called Template:Direct links. The radius of a nucleus is approximately equal to  Template:Direct links, where is the total number of nucleons.[45] This is much smaller than the radius of the atom, which is on the order of 105 fm. The nucleons are bound together by a short-ranged attractive potential called the Template:Direct link. At distances smaller than 2.5 fm this force is much more powerful than the Template:Direct link that causes positively charged protons to repel each other.[46]

Atoms of the same Template:Direct link have the same number of protons, called the Template:Direct link. Within a single element, the number of neutrons may vary, determining the Template:Direct link of that element. The total number of protons and neutrons determine the Template:Direct link. The number of neutrons relative to the protons determines the stability of the nucleus, with certain isotopes undergoing Template:Direct link.[47]

The proton, the electron, and the neutron are classified as Template:Direct links. Fermions obey the Template:Direct link which prohibits Template:Direct link fermions, such as multiple protons, from occupying the same quantum state at the same time. Thus, every proton in the nucleus must occupy a quantum state different from all other protons, and the same applies to all neutrons of the nucleus and to all electrons of the electron cloud.[48]

A nucleus that has a different number of protons than neutrons can potentially drop to a lower energy state through a radioactive decay that causes the number of protons and neutrons to more closely match. As a result, atoms with matching numbers of protons and neutrons are more stable against decay, but with increasing atomic number, the mutual repulsion of the protons requires an increasing proportion of neutrons to maintain the stability of the nucleus.[48]

Illustration of a nuclear fusion process that forms a deuterium nucleus, consisting of a proton and a neutron, from two protons. A Template:Direct link (e+)—an Template:Direct link electron—is emitted along with an electron Template:Direct link.

The number of protons and neutrons in the atomic nucleus can be modified, although this can require very high energies because of the strong force. Template:Direct link occurs when multiple atomic particles join to form a heavier nucleus, such as through the energetic collision of two nuclei. For example, at the core of the Sun protons require energies of 3 to 10 keV to overcome their mutual repulsion—the Template:Direct link—and fuse together into a single nucleus.[49] Template:Direct link is the opposite process, causing a nucleus to split into two smaller nuclei—usually through radioactive decay. The nucleus can also be modified through bombardment by high energy subatomic particles or photons. If this modifies the number of protons in a nucleus, the atom changes to a different chemical element.[50][51]

If the mass of the nucleus following a fusion reaction is less than the sum of the masses of the separate particles, then the difference between these two values can be emitted as a type of usable energy (such as a Template:Direct link, or the kinetic energy of a Template:Direct link), as described by Template:Direct link's Template:Direct link formula, , where is the mass loss and is the Template:Direct link. This deficit is part of the Template:Direct link of the new nucleus, and it is the non-recoverable loss of the energy that causes the fused particles to remain together in a state that requires this energy to separate.[52]

The fusion of two nuclei that create larger nuclei with lower atomic numbers than Template:Direct link and Template:Direct link—a total nucleon number of about 60—is usually an Template:Direct link that releases more energy than is required to bring them together.[53] It is this energy-releasing process that makes nuclear fusion in Template:Direct links a self-sustaining reaction. For heavier nuclei, the binding energy per Template:Direct link in the nucleus begins to decrease. That means fusion processes producing nuclei that have atomic numbers higher than about 26, and Template:Direct linkes higher than about 60, is an Template:Direct link. These more massive nuclei can not undergo an energy-producing fusion reaction that can sustain the Template:Direct link of a star.[48]

Electron cloud

Main articles: Atomic orbital and Electron configuration

A potential well, showing, according to Template:Direct link, the minimum energy V(x) needed to reach each position x. Classically, a particle with energy E is constrained to a range of positions between x1 and x2.

The electrons in an atom are attracted to the protons in the nucleus by the Template:Direct link. This force binds the electrons inside an Template:Direct link Template:Direct link surrounding the smaller nucleus, which means that an external source of energy is needed for the electron to escape. The closer an electron is to the nucleus, the greater the attractive force. Hence electrons bound near the center of the potential well require more energy to escape than those at greater separations.

Electrons, like other particles, have properties of both a Template:Direct link. The electron cloud is a region inside the potential well where each electron forms a type of three-dimensional Template:Direct link—a wave form that does not move relative to the nucleus. This behavior is defined by an Template:Direct link, a mathematical function that characterises the probability that an electron appears to be at a particular location when its position is measured.[54] Only a discrete (or Template:Direct link) set of these orbitals exist around the nucleus, as other possible wave patterns rapidly decay into a more stable form.[55] Orbitals can have one or more ring or node structures, and differ from each other in size, shape and orientation.[56]

3D views of some Template:Direct link atomic orbitals showing probability density and phase (g orbitals and higher are not shown)

Each atomic orbital corresponds to a particular Template:Direct link of the electron. The electron can change its state to a higher energy level by absorbing a Template:Direct link with sufficient energy to boost it into the new quantum state. Likewise, through Template:Direct link, an electron in a higher energy state can drop to a lower energy state while radiating the excess energy as a photon. These characteristic energy values, defined by the differences in the energies of the quantum states, are responsible for Template:Direct links.[55]

The amount of energy needed to remove or add an electron—the Template:Direct link—is far less than the Template:Direct link. For example, it requires only 13.6 eV to strip a Template:Direct link electron from a hydrogen atom,[57] compared to 2.23 million eV for splitting a Template:Direct link nucleus.[58] Atoms are Template:Direct link neutral if they have an equal number of protons and electrons. Atoms that have either a deficit or a surplus of electrons are called Template:Direct links. Electrons that are farthest from the nucleus may be transferred to other nearby atoms or shared between atoms. By this mechanism, atoms are able to Template:Direct link into Template:Direct links and other types of Template:Direct links like Template:Direct link and Template:Direct link network Template:Direct link.[59]


Nuclear properties

Main articles: Isotope, Stable isotope, List of nuclides, and List of elements by stability of isotopes

By definition, any two atoms with an identical number of protons in their nuclei belong to the same Template:Direct link. Atoms with equal numbers of protons but a different number of neutrons are different isotopes of the same element. For example, all hydrogen atoms admit exactly one proton, but isotopes exist with no neutrons (Template:Direct link, by far the most common form,[60] also called protium), one neutron (Template:Direct link), two neutrons (Template:Direct link) and Template:Direct link. The known elements form a set of atomic numbers, from the single-proton element Template:Direct link up to the 118-proton element Template:Direct link.[61] All known isotopes of elements with atomic numbers greater than 82 are radioactive, although the radioactivity of element 83 (Template:Direct link) is so slight as to be practically negligible.[62][63]

About 339 nuclides occur naturally on Template:Direct link,[64] of which 252 (about 74%) have not been observed to decay, and are referred to as "Template:Direct links". Only 90 nuclides are stable Template:Direct link, while another 162 (bringing the total to 252) have not been observed to decay, even though in theory it is energetically possible. These are also formally classified as "stable". An additional 34 radioactive nuclides have half-lives longer than 100 million years, and are long-lived enough to have been present since the birth of the Template:Direct link. This collection of 286 nuclides are known as Template:Direct links. Finally, an additional 53 short-lived nuclides are known to occur naturally, as daughter products of primordial nuclide decay (such as Template:Direct link from Template:Direct link), or as products of natural energetic processes on Earth, such as cosmic ray bombardment (for example, carbon-14).[65][note 1]

For 80 of the chemical elements, at least one Template:Direct link exists. As a rule, there is only a handful of stable isotopes for each of these elements, the average being 3.2 stable isotopes per element. Twenty-six elements have only a single stable isotope, while the largest number of stable isotopes observed for any element is ten, for the element Template:Direct link. Elements Template:Direct link, Template:Direct link, and all elements numbered Template:Direct link or higher have no stable isotopes.[66]: 1–12 

Stability of isotopes is affected by the ratio of protons to neutrons, and also by the presence of certain "magic numbers" of neutrons or protons that represent closed and filled quantum shells. These quantum shells correspond to a set of energy levels within the Template:Direct link of the nucleus; filled shells, such as the filled shell of 50 protons for tin, confers unusual stability on the nuclide. Of the 252 known stable nuclides, only four have both an odd number of protons and odd number of neutrons: Template:Direct link (Template:Direct link), Template:Direct link, Template:Direct link and Template:Direct link. Also, only four naturally occurring, radioactive odd-odd nuclides have a half-life over a billion years: Template:Direct link, Template:Direct link, Template:Direct link and Template:Direct link. Most odd-odd nuclei are highly unstable with respect to Template:Direct link, because the decay products are even-even, and are therefore more strongly bound, due to Template:Direct link.[67]


Main articles: Atomic mass and mass number

The large majority of an atom's mass comes from the protons and neutrons that make it up. The total number of these particles (called "nucleons") in a given atom is called the Template:Direct link. It is a positive integer and dimensionless (instead of having dimension of mass), because it expresses a count. An example of use of a mass number is "carbon-12," which has 12 nucleons (six protons and six neutrons).

The actual Template:Direct link is often expressed in Template:Direct links (Da), also called the unified atomic mass unit (u). This unit is defined as a twelfth of the mass of a free neutral atom of Template:Direct link, which is approximately 1.66×10−27 kg.[68] Template:Direct link (the lightest isotope of hydrogen which is also the nuclide with the lowest mass) has an atomic weight of 1.007825 Da.[69] The value of this number is called the Template:Direct link. A given atom has an atomic mass approximately equal (within 1%) to its mass number times the atomic mass unit (for example the mass of a nitrogen-14 is roughly 14 Da), but this number will not be exactly an integer except (by definition) in the case of carbon-12.[70] The heaviest Template:Direct link is lead-208,[62] with a mass of 207.9766521 Da.[71]

As even the most massive atoms are far too light to work with directly, chemists instead use the unit of Template:Direct link. One mole of atoms of any element always has the same number of atoms (about Template:Direct link). This number was chosen so that if an element has an atomic mass of 1 u, a mole of atoms of that element has a mass close to one gram. Because of the definition of the Template:Direct link, each carbon-12 atom has an atomic mass of exactly 12 Da, and so a mole of carbon-12 atoms weighs exactly 0.012 kg.[68]

Shape and size

Main article: Atomic radius

Atoms lack a well-defined outer boundary, so their dimensions are usually described in terms of an Template:Direct link. This is a measure of the distance out to which the electron cloud extends from the nucleus.[72] This assumes the atom to exhibit a spherical shape, which is only obeyed for atoms in vacuum or free space. Atomic radii may be derived from the distances between two nuclei when the two atoms are joined in a Template:Direct link. The radius varies with the location of an atom on the atomic chart, the type of chemical bond, the number of neighboring atoms (Template:Direct link) and a Template:Direct link property known as Template:Direct link.[73] On the Template:Direct link of the elements, atom size tends to increase when moving down columns, but decrease when moving across rows (left to right).[74] Consequently, the smallest atom is helium with a radius of 32 Template:Direct link, while one of the largest is Template:Direct link at 225 pm.[75]

When subjected to external forces, like Template:Direct links, the shape of an atom may deviate from Template:Direct link. The deformation depends on the field magnitude and the orbital type of outer shell electrons, as shown by Template:Direct link considerations. Aspherical deviations might be elicited for instance in Template:Direct links, where large crystal-electrical fields may occur at Template:Direct link lattice sites.[76][77] Significant Template:Direct linkal deformations have been shown to occur for sulfur ions[78] and Template:Direct link ions[79] in Template:Direct link-type compounds.

Atomic dimensions are thousands of times smaller than the wavelengths of Template:Direct link (400–700 Template:Direct link) so they cannot be viewed using an Template:Direct link, although individual atoms can be observed using a Template:Direct link. To visualize the minuteness of the atom, consider that a typical human hair is about 1 million carbon atoms in width.[80] A single drop of water contains about 2 Template:Direct link (2×1021) atoms of oxygen, and twice the number of hydrogen atoms.[81] A single Template:Direct link Template:Direct link with a mass of 2×10−4 kg contains about 10 sextillion (1022) atoms of Template:Direct link.[note 2] If an apple were magnified to the size of the Earth, then the atoms in the apple would be approximately the size of the original apple.[82]

Radioactive decay

Main article: Radioactive decay

This diagram shows the Template:Direct link (T½) of various isotopes with Z protons and N neutrons.

Every element has one or more isotopes that have unstable nuclei that are subject to radioactive decay, causing the nucleus to emit particles or electromagnetic radiation. Radioactivity can occur when the radius of a nucleus is large compared with the radius of the strong force, which only acts over distances on the order of 1 fm.[83]

The most common forms of radioactive decay are:[84][85]

Other more rare types of Template:Direct link include ejection of neutrons or protons or clusters of Template:Direct links from a nucleus, or more than one Template:Direct link. An analog of gamma emission which allows excited nuclei to lose energy in a different way, is Template:Direct link—a process that produces high-speed electrons that are not beta rays, followed by production of high-energy photons that are not gamma rays. A few large nuclei explode into two or more charged fragments of varying masses plus several neutrons, in a decay called spontaneous Template:Direct link.

Each Template:Direct link has a characteristic decay time period—the Template:Direct link—that is determined by the amount of time needed for half of a sample to decay. This is an Template:Direct link process that steadily decreases the proportion of the remaining isotope by 50% every half-life. Hence after two half-lives have passed only 25% of the isotope is present, and so forth.[83]

Magnetic moment

Main articles: Electron magnetic moment and Nuclear magnetic moment

Elementary particles possess an intrinsic quantum mechanical property known as Template:Direct link. This is analogous to the Template:Direct link of an object that is spinning around its Template:Direct link, although strictly speaking these particles are believed to be point-like and cannot be said to be rotating. Spin is measured in units of the reduced Template:Direct link (ħ), with electrons, protons and neutrons all having spin ½ ħ, or "spin-½". In an atom, electrons in motion around the Template:Direct link possess orbital Template:Direct link in addition to their spin, while the nucleus itself possesses angular momentum due to its nuclear spin.[86]

The Template:Direct link produced by an atom—its Template:Direct link—is determined by these various forms of angular momentum, just as a rotating charged object classically produces a magnetic field, but the most dominant contribution comes from electron spin. Due to the nature of electrons to obey the Template:Direct link, in which no two electrons may be found in the same Template:Direct link, bound electrons pair up with each other, with one member of each pair in a spin up state and the other in the opposite, spin down state. Thus these spins cancel each other out, reducing the total magnetic dipole moment to zero in some atoms with even number of electrons.[87]

In Template:Direct link elements such as iron, cobalt and nickel, an odd number of electrons leads to an unpaired electron and a net overall magnetic moment. The orbitals of neighboring atoms overlap and a lower energy state is achieved when the spins of unpaired electrons are aligned with each other, a spontaneous process known as an Template:Direct link. When the magnetic moments of ferromagnetic atoms are lined up, the material can produce a measurable macroscopic field. Template:Direct link have atoms with magnetic moments that line up in random directions when no magnetic field is present, but the magnetic moments of the individual atoms line up in the presence of a field.[87][88]

The nucleus of an atom will have no spin when it has even numbers of both neutrons and protons, but for other cases of odd numbers, the nucleus may have a spin. Normally nuclei with spin are aligned in random directions because of Template:Direct link, but for certain elements (such as Template:Direct link) it is possible to Template:Direct link a significant proportion of the nuclear spin states so that they are aligned in the same direction—a condition called Template:Direct link. This has important applications in Template:Direct link.[89][90]

Energy levels

Template:Direct link The Template:Direct link of an electron in an atom is Template:Direct link relative to when the Template:Direct link from the nucleus Template:Direct link; its dependence on the electron's Template:Direct link reaches the Template:Direct link inside the nucleus, roughly in Template:Direct link to the distance. In the quantum-mechanical model, a bound electron can occupy only a set of Template:Direct link centered on the nucleus, and each state corresponds to a specific Template:Direct link; see Template:Direct link for a theoretical explanation. An energy level can be measured by the Template:Direct link the electron from the atom, and is usually given in units of Template:Direct links (eV). The lowest energy state of a bound electron is called the ground state, i.e. Template:Direct link, while an electron transition to a higher level results in an excited state.[91] The electron's energy increases along with Template:Direct link because the (average) distance to the nucleus increases. Dependence of the energy on Template:Direct link is caused not by the Template:Direct link of the nucleus, but by interaction between electrons.

For an electron to Template:Direct link, e.g. Template:Direct link to first Template:Direct link, it must absorb or emit a Template:Direct link at an energy matching the difference in the potential energy of those levels, according to the Template:Direct link model, what can be precisely calculated by the Template:Direct link. Electrons jump between orbitals in a particle-like fashion. For example, if a single photon strikes the electrons, only a single electron changes states in response to the photon; see Template:Direct link.

The energy of an emitted photon is proportional to its Template:Direct link, so these specific energy levels appear as distinct bands in the Template:Direct link.[92] Each element has a characteristic spectrum that can depend on the nuclear charge, subshells filled by electrons, the electromagnetic interactions between the electrons and other factors.[93]

An example of absorption lines in a spectrum

When a continuous Template:Direct link is passed through a gas or plasma, some of the photons are absorbed by atoms, causing electrons to change their energy level. Those excited electrons that remain bound to their atom spontaneously emit this energy as a photon, traveling in a random direction, and so drop back to lower energy levels. Thus the atoms behave like a filter that forms a series of dark Template:Direct links in the energy output. (An observer viewing the atoms from a view that does not include the continuous spectrum in the background, instead sees a series of Template:Direct links from the photons emitted by the atoms.) Template:Direct link measurements of the strength and width of Template:Direct links allow the composition and physical properties of a substance to be determined.[94]

Close examination of the spectral lines reveals that some display a Template:Direct link splitting. This occurs because of Template:Direct link, which is an interaction between the spin and motion of the outermost electron.[95] When an atom is in an external magnetic field, spectral lines become split into three or more components; a phenomenon called the Template:Direct link. This is caused by the interaction of the magnetic field with the magnetic moment of the atom and its electrons. Some atoms can have multiple Template:Direct links with the same energy level, which thus appear as a single spectral line. The interaction of the magnetic field with the atom shifts these electron configurations to slightly different energy levels, resulting in multiple spectral lines.[96] The presence of an external Template:Direct link can cause a comparable splitting and shifting of spectral lines by modifying the electron energy levels, a phenomenon called the Template:Direct link.[97]

If a bound electron is in an excited state, an interacting photon with the proper energy can cause Template:Direct link of a photon with a matching energy level. For this to occur, the electron must drop to a lower energy state that has an energy difference matching the energy of the interacting photon. The emitted photon and the interacting photon then move off in parallel and with matching phases. That is, the wave patterns of the two photons are synchronized. This physical property is used to make Template:Direct links, which can emit a coherent beam of light energy in a narrow frequency band.[98]

Valence and bonding behavior

Main articles: Valence (chemistry) and Chemical bond

Valency is the combining power of an element. It is determined by the number of bonds it can form to other atoms or groups.[99] The outermost electron shell of an atom in its uncombined state is known as the Template:Direct link, and the electrons in that shell are called Template:Direct links. The number of valence electrons determines the Template:Direct link behavior with other atoms. Atoms tend to Template:Direct link with each other in a manner that fills (or empties) their outer valence shells.[100] For example, a transfer of a single electron between atoms is a useful approximation for bonds that form between atoms with one-electron more than a filled shell, and others that are one-electron short of a full shell, such as occurs in the compound Template:Direct link and other chemical ionic salts. Many elements display multiple valences, or tendencies to share differing numbers of electrons in different compounds. Thus, Template:Direct linking between these elements takes many forms of electron-sharing that are more than simple electron transfers. Examples include the element carbon and the Template:Direct link.[101]

The Template:Direct links are often displayed in a Template:Direct link that is laid out to display recurring chemical properties, and elements with the same number of valence electrons form a group that is aligned in the same column of the table. (The horizontal rows correspond to the filling of a quantum shell of electrons.) The elements at the far right of the table have their outer shell completely filled with electrons, which results in chemically inert elements known as the Template:Direct linkes.[102][103]


Main articles: State of matter and Phase (matter)

Graphic illustrating the formation of a Template:Direct link

Quantities of atoms are found in different states of matter that depend on the physical conditions, such as Template:Direct link and Template:Direct link. By varying the conditions, materials can transition between Template:Direct links, Template:Direct links, Template:Direct linkes and Template:Direct link.[104] Within a state, a material can also exist in different Template:Direct link. An example of this is solid carbon, which can exist as Template:Direct link or Template:Direct link.[105] Gaseous allotropes exist as well, such as Template:Direct link and Template:Direct link.

At temperatures close to Template:Direct link, atoms can form a Template:Direct link, at which point quantum mechanical effects, which are normally only observed at the atomic scale, become apparent on a macroscopic scale.[106][107] This super-cooled collection of atoms then behaves as a single Template:Direct link, which may allow fundamental checks of quantum mechanical behavior.[108]


Template:Direct link image showing the individual atoms making up this Template:Direct link (Template:Direct link) surface. The surface atoms deviate from the bulk Template:Direct link and arrange in columns several atoms wide with pits between them (See Template:Direct link).

While atoms are too small to be seen, devices such as the Template:Direct link (STM) enable their visualization at the surfaces of solids. The microscope uses the Template:Direct link phenomenon, which allows particles to pass through a barrier that would be insurmountable in the classical perspective. Electrons tunnel through the vacuum between two Template:Direct link electrodes, providing a tunneling current that is exponentially dependent on their separation. One electrode is a sharp tip ideally ending with a single atom. At each point of the scan of the surface the tip's height is adjusted so as to keep the tunneling current at a set value. How much the tip moves to and away from the surface is interpreted as the height profile. For low bias, the microscope images the averaged electron orbitals across closely packed energy levels—the local Template:Direct link near the Template:Direct link.[109][110] Because of the distances involved, both electrodes need to be extremely stable; only then periodicities can be observed that correspond to individual atoms. The method alone is not chemically specific, and cannot identify the atomic species present at the surface.

Atoms can be easily identified by their mass. If an atom is Template:Direct linkized by removing one of its electrons, its trajectory when it passes through a Template:Direct link will bend. The radius by which the trajectory of a moving ion is turned by the magnetic field is determined by the mass of the atom. The Template:Direct link uses this principle to measure the Template:Direct link of ions. If a sample contains multiple isotopes, the mass spectrometer can determine the proportion of each isotope in the sample by measuring the intensity of the different beams of ions. Techniques to vaporize atoms include Template:Direct link and Template:Direct link, both of which use a plasma to vaporize samples for analysis.[111]

The Template:Direct link has sub-nanometer resolution in 3-D and can chemically identify individual atoms using time-of-flight mass spectrometry.[112]

Electron emission techniques such as Template:Direct link (XPS) and Template:Direct link (AES), which measure the binding energies of the Template:Direct links, are used to identify the atomic species present in a sample in a non-destructive way. With proper focusing both can be made area-specific. Another such method is Template:Direct link (EELS), which measures the energy loss of an Template:Direct link within a Template:Direct link when it interacts with a portion of a sample.

Spectra of Template:Direct links can be used to analyze the atomic composition of distant Template:Direct links. Specific light Template:Direct links contained in the observed light from stars can be separated out and related to the quantized transitions in free gas atoms. These colors can be replicated using a Template:Direct link containing the same element.[113] Template:Direct link was discovered in this way in the spectrum of the Sun 23 years before it was found on Earth.[114]

Origin and current state

Template:Direct link forms about 4% of the total energy density of the Template:Direct link, with an average density of about 0.25 particles/m3 (mostly Template:Direct links and electrons).[115] Within a galaxy such as the Template:Direct link, particles have a much higher concentration, with the density of matter in the Template:Direct link (ISM) ranging from 105 to 109 atoms/m3.[116] The Sun is believed to be inside the Template:Direct link, so the density in the solar neighborhood is only about 103 atoms/m3.[117] Stars form from dense clouds in the ISM, and the evolutionary processes of stars result in the steady enrichment of the ISM with elements more massive than hydrogen and helium.

Up to 95% of the Milky Way's baryonic matter are concentrated inside stars, where conditions are unfavorable for atomic matter. The total baryonic mass is about 10% of the mass of the galaxy;[118] the remainder of the mass is an unknown Template:Direct link.[119] High Template:Direct link inside stars makes most "atoms" fully ionized, that is, separates all electrons from the nuclei. In Template:Direct links—with exception of their surface layers—an immense Template:Direct link make electron shells impossible.


Main article: Nucleosynthesis

Periodic table showing the origin of each element. Elements from carbon up to sulfur may be made in small stars by the Template:Direct link. Elements beyond iron are made in large stars with slow neutron capture (Template:Direct link). Elements heavier than iron may be made in neutron star mergers or supernovae after the Template:Direct link.

Electrons are thought to exist in the Universe since early stages of the Template:Direct link. Atomic nuclei forms in Template:Direct link reactions. In about three minutes Template:Direct link produced most of the Template:Direct link, Template:Direct link, and Template:Direct link in the Universe, and perhaps some of the Template:Direct link and Template:Direct link.[120][121][122]

Ubiquitousness and stability of atoms relies on their Template:Direct link, which means that an atom has a lower energy than an unbound system of the nucleus and electrons. Where the Template:Direct link is much higher than Template:Direct link, the matter exists in the form of Template:Direct link—a gas of positively charged ions (possibly, bare nuclei) and electrons. When the temperature drops below the ionization potential, atoms become Template:Direct link favorable. Atoms (complete with bound electrons) became to dominate over Template:Direct link Template:Direct links 380,000 years after the Big Bang—an epoch called Template:Direct link, when the expanding Universe cooled enough to allow electrons to become attached to nuclei.[123]

Since the Big Bang, which produced no Template:Direct link or Template:Direct link, atomic nuclei have been combined in Template:Direct links through the process of Template:Direct link to produce more of the element Template:Direct link, and (via the Template:Direct link) the sequence of elements from carbon up to Template:Direct link;[124] see Template:Direct link for details.

Isotopes such as lithium-6, as well as some beryllium and boron are generated in space through Template:Direct link.[125] This occurs when a high-energy proton strikes an atomic nucleus, causing large numbers of nucleons to be ejected.

Elements heavier than iron were produced in Template:Direct linke and colliding Template:Direct links through the Template:Direct link, and in Template:Direct link through the Template:Direct link, both of which involve the capture of neutrons by atomic nuclei.[126] Elements such as Template:Direct link formed largely through the radioactive decay of heavier elements.[127]


Most of the atoms that make up the Template:Direct link and its inhabitants were present in their current form in the Template:Direct link that collapsed out of a Template:Direct link to form the Template:Direct link. The rest are the result of radioactive decay, and their relative proportion can be used to determine the Template:Direct link through Template:Direct link.[128][129] Most of the Template:Direct link in the crust of the Earth (about 99% of the helium from gas wells, as shown by its lower abundance of Template:Direct link) is a product of Template:Direct link.[130]

There are a few trace atoms on Earth that were not present at the beginning (i.e., not "primordial"), nor are results of radioactive decay. Template:Direct link is continuously generated by cosmic rays in the atmosphere.[131] Some atoms on Earth have been artificially generated either deliberately or as by-products of nuclear reactors or explosions.[132][133] Of the Template:Direct link—those with atomic numbers greater than 92—only Template:Direct link and Template:Direct link occur naturally on Earth.[134][135] Transuranic elements have radioactive lifetimes shorter than the current age of the Earth[136] and thus identifiable quantities of these elements have long since decayed, with the exception of traces of Template:Direct link possibly deposited by cosmic dust.[128] Natural deposits of plutonium and neptunium are produced by Template:Direct link in uranium ore.[137]

The Earth contains approximately 1.33×1050 atoms.[138] Although small numbers of independent atoms of Template:Direct linkes exist, such as Template:Direct link, Template:Direct link, and Template:Direct link, 99% of Template:Direct link is bound in the form of molecules, including Template:Direct link and Template:Direct link Template:Direct link and Template:Direct link. At the surface of the Earth, an overwhelming majority of atoms combine to form various compounds, including Template:Direct link, Template:Direct link, Template:Direct links and Template:Direct links. Atoms can also combine to create materials that do not consist of discrete molecules, including Template:Direct links and liquid or solid Template:Direct links.[139][140] This atomic matter forms networked arrangements that lack the particular type of small-scale interrupted order associated with molecular matter.[141]

Rare and theoretical forms

Superheavy elements

Main article: Superheavy element

All nuclides with atomic numbers higher than 82 (Template:Direct link) are known to be radioactive. No nuclide with an atomic number exceeding 92 (Template:Direct link) exists on Earth as a Template:Direct link, and heavier elements generally have shorter half-lives. Nevertheless, an "Template:Direct link" encompassing relatively long-lived isotopes of superheavy elements[142] with atomic numbers Template:Direct link to Template:Direct link might exist.[143] Predictions for the half-life of the most stable nuclide on the island range from a few minutes to millions of years.[144] In any case, superheavy elements (with Z > 104) would not exist due to increasing Template:Direct link repulsion (which results in Template:Direct link with increasingly short half-lives) in the absence of any stabilizing effects.[145]

Exotic matter

Main article: Exotic matter

Each particle of matter has a corresponding Template:Direct link particle with the opposite electrical charge. Thus, the Template:Direct link is a positively charged Template:Direct link and the Template:Direct link is a negatively charged equivalent of a Template:Direct link. When a matter and corresponding antimatter particle meet, they annihilate each other. Because of this, along with an imbalance between the number of matter and antimatter particles, the latter are rare in the universe. The first causes of this imbalance are not yet fully understood, although theories of Template:Direct link may offer an explanation. As a result, no antimatter atoms have been discovered in nature.[146][147] In 1996 the antimatter counterpart of the hydrogen atom (Template:Direct link) was synthesized at the Template:Direct link laboratory in Template:Direct link.[148][149]

Other Template:Direct links have been created by replacing one of the protons, neutrons or electrons with other particles that have the same charge. For example, an electron can be replaced by a more massive Template:Direct link, forming a Template:Direct link. These types of atoms can be used to test fundamental predictions of physics.[150][151][152]

See also

  • Template:Direct link
  • Template:Direct link
  • Template:Direct link
  • Template:Direct link
  • Template:Direct link
  • Template:Direct link
  • Template:Direct link
  • Template:Direct link


  1. ^ For more recent updates see Template:Direct link's Interactive Chart of Nuclides ] Archived 25 July 2020 at the Wayback Machine.
  2. ^ A carat is 200 milligrams. Template:Direct link, carbon-12 has 0.012 kg per mole. The Template:Direct link defines 6×1023 atoms per mole.
  1. ^ Iron(II) oxide's formula is written here as Fe2O2 rather than the more conventional FeO because this better illustrates the explanation.


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Further reading