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Encyclopedia > Atom
Helium atom
An illustration of the helium atom, depicting the nucleus (pink) and the electron cloud distribution (black). The nucleus (upper right) is in reality spherically symmetric, although for more complicated nuclei this is not always the case. The black bar is one ångström, equal to 10−10 m or 100,000 fm.
Classification
 Smallest recognized division of a chemical element
Properties
 Mass range: 1.67×10-24 to 4.52×10-22 g Electric charge: zero (neutral), or ion charge Diameter range: 62 pm (He) to 520 pm (Cs) (data page) Components: Electrons and a compact nucleus of protons and neutrons

Relative to everyday experience, atoms are minuscule objects with proportionately tiny masses that can only be observed individually using special instruments such as the scanning tunneling microscope. More than 99.9% of an atom's mass is concentrated in the nucleus,[3] with protons and neutrons having about equal mass. In atoms with too many or too few neutrons relative to the number of protons, the nucleus is unstable and subject to radioactive decay.[4] The electrons surrounding the nucleus occupy a set of stable energy levels, or orbitals, and they can transition between these states by the absorption or emission of photons that match the energy differences between the levels. The electrons determine the chemical properties of an element, and strongly influence an atom's magnetic properties. Image of reconstruction on a clean Au(100) surface. ... Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting radiation in the form of particles or electromagnetic waves. ... A quantum mechanical system can only be in certain states, so that only certain energy levels are possible. ... In chemistry, an atomic orbital is the region in which an electron may be found around a single atom. ... In modern physics the photon is the elementary particle responsible for electromagnetic phenomena. ... For other senses of this word, see magnetism (disambiguation). ...

Main articles: Atomic theory and Atomism

The concept that matter is composed of discrete units and cannot be divided into arbitrarily tiny quantities has been around for millennia, but these ideas were founded in abstract, philosophical reasoning rather than experimentation and empirical observation. The nature of atoms in philosophy varied considerably over time and between cultures and schools, and often had spiritual elements. Nevertheless, the basic idea of the atom was adopted by scientists thousands of years later because it elegantly explained new discoveries in the field of chemistry.[5] This article focuses on the historical models of the atom. ... Concern has been expressed that this article or section is missing information about: discussions of existence of atoms among prominent physicists up to the end of 19th century. ...

The earliest references to the concept of atoms date back to ancient India in the 6th century BCE.[6] The Nyaya and Vaisheshika schools developed elaborate theories of how atoms combined into more complex objects (first in pairs, then trios of pairs).[7] The references to atoms in the West emerged a century later from Leucippus whose student, Democritus, systemized his views. In approximately 450 BCE, Democritus coined the term átomos (Greek ἄτομος), which means "uncuttable" or "the smallest indivisible particle of matter", i.e., something that cannot be divided. Although the Indian and Greek concepts of the atom were based purely on philosophy, modern science has retained the name coined by Democritus.[5] The History of India begins with the Indus Valley Civilization, which flourished in the north-western part of the Indian subcontinent from 3300 to 1700 BCE. This Bronze Age civilization was followed by the Iron Age Vedic period, which witnessed the rise of major kingdoms known as the Mahajanapadas. ... BCE is a TLA that may stand for: Before the Common Era, date notation equivalent to BC (e. ... (Sanskrit ni-ÄyÃ¡, literally recursion, used in the sense of syllogism, inference)) is the name given to one of the six orthodox or astika schools of Hindu philosophyâ€”specifically the school of logic. ... Vaisheshika, also Vaisesika, (Sanskrit: à¤µà¥ˆà¤¶à¥†à¤·à¤¿à¤•)is one of the six Hindu schools of philosophy (orthodox Vedic systems) of India. ... Vaisheshika, also Vaisesika, (Sanskrit: à¤µà¥ˆà¤¶à¥†à¤·à¤¿à¤•)is one of the six Hindu schools of philosophy (orthodox Vedic systems) of India. ... This article is about the philosopher. ... â€Ž Democritus (Greek: ) was a pre-Socratic Greek materialist philosopher (born at Abdera in Thrace ca. ...

Further progress in the understanding of atoms did not occur until the science of chemistry began to develop. In 1661, the natural philosopher Robert Boyle published The Sceptical Chymist in which he argued that matter was composed of various combinations of different "corpuscules" or atoms, rather than the classical elements of air, earth, fire and water.[8] In 1789 the term element was defined by the French nobleman and scientific researcher Antoine Lavoisier to mean basic substances that could not be further broken down by the methods of chemistry.[9] For other uses, see Chemistry (disambiguation). ... Natural philosophy or the philosophy of nature, known in Latin as philosophia naturalis, is a term applied to the objective study of nature and the physical universe that was regnant before the development of modern science. ... For the American art director and production designer, see Robert F. Boyle Robert Boyle (25 January 1627 â€“ 30 December 1691) was a natural philosopher, chemist, physicist, inventor, and early gentleman scientist, noted for his work in physics and chemistry. ... This article or section does not adequately cite its references or sources. ... Many ancient philosophies used a set of archetypal classical elements to explain patterns in nature. ... Lavoisier redirects here. ...

Various atoms and molecules as depicted in John Dalton's A New System of Chemical Philosophy (1808)

In 1803, the Englishman John Dalton, an instructor and natural philosopher, used the concept of atoms to explain why elements always reacted in a ratio of small whole numbers—the law of multiple proportions—and why certain gases dissolved better in water than others. He proposed that each element consists of atoms of a single, unique type, and that these atoms could join to each other, to form chemical compounds.[10][11] Image File history File links A_New_System_of_Chemical_Philosophy_fp. ... Image File history File links A_New_System_of_Chemical_Philosophy_fp. ... John Dalton John Dalton (September 6, 1766 â€“ July 27, 1844) was an English chemist and physicist, born at Eaglesfield, near Cockermouth in Cumberland. ... John Dalton John Dalton (September 6, 1766 â€“ July 27, 1844) was an English chemist and physicist, born at Eaglesfield, near Cockermouth in Cumberland. ... In mathematics, a natural number can mean either an element of the set {1, 2, 3, ...} (i. ... In chemistry, the law of multiple proportions is one of the most basic laws of stoichiometry. ...

Additional validation of particle theory (and by extension atomic theory) occurred in 1827 when botanist Robert Brown used a microscope to look at dust grains floating in water and discovered that they moved about erratically—a phenomenon that became known as "Brownian motion". J. Desaulx suggested in 1877 that the phenomenon was caused by the thermal motion of water molecules, and in 1905 Albert Einstein produced the first mathematical analysis of the motion, thus confirming the hypothesis.[12][13] This article focuses on the historical models of the atom. ... Pinguicula grandiflora commonly known as a Butterwort Example of a cross section of a stem [1] Botany is the scientific study of plant life. ... Robert Brown (1773â€“1858) Robert Brown (December 21, 1773â€“June 10, 1858) is acknowledged as the leading British botanist to collect in Australia during the first half of the 19th century. ... A microscope (Greek: (micron) = small + (skopein) = to look at) is an instrument for viewing objects that are too small to be seen by the naked or unaided eye. ... Three different views of Brownian motion, with 32 steps, 256 steps, and 2048 steps denoted by progressively lighter colors. ... â€œEinsteinâ€ redirects here. ...

The physicist J. J. Thomson, through his work on cathode rays in 1897, discovered the electron and its subatomic nature, which destroyed the concept of atoms as being indivisible units.[14] Thomson believed that the electrons were distributed throughout the atom, with their charge balanced by the presence of a uniform sea of positive charge (the plum pudding model). Sir Joseph John â€œJ.J.â€ Thomson, OM, FRS (18 December 1856 â€“ 30 August 1940) was a British physicist and Nobel laureate, credited for the discovery of the electron and of isotopes, and the invention of the mass spectrometer. ... A schematic diagram of a Crookes tube apparatus. ... A schematic representation of the plum pudding model of the atom. ...

A Bohr model of the hydrogen atom, showing an electron jumping between fixed orbits and emitting a photon of energy with a specific frequency

However, in 1909, researchers under the direction of physicist Ernest Rutherford bombarded a sheet of gold foil with helium ions and discovered that a small percentage were deflected through much larger angles than was predicted using Thomson's proposal. Rutherford interpreted the gold foil experiment as suggesting that the positive charge of an atom and most of its mass was concentrated in a nucleus at the center of the atom (the Rutherford model), with the electrons orbiting it like planets around a sun. Positively charged helium ions passing close to this dense nucleus would then be deflected away at much sharper angles.[15] Image File history File links Bohr_model. ... Image File history File links Bohr_model. ... In modern physics the photon is the elementary particle responsible for electromagnetic phenomena. ... Ernest Rutherford, 1st Baron Rutherford of Nelson OM PC FRS (30 August 1871 â€“ 19 October 1937), widely referred to as Lord Rutherford, was a chemist (B.Sc. ... Top: Expected results: alpha particles passing through the plum pudding model of the atom undisturbed. ... A stylised representation of the Rutherford model of a lithium atom (nuclear structure anachronistic) The Rutherford model or planetary model was a model of the atom devised by Ernest Rutherford. ...

While experimenting with the products of radioactive decay, in 1913 radiochemist Frederick Soddy discovered that there appeared to be more than one type of atom at each position on the periodic table.[16] The term isotope was coined by Margaret Todd as a suitable name for different atoms that belong to the same element. J.J. Thomson created a technique for separating atom types through his work on ionized gases, which subsequently led to the discovery of stable isotopes.[17] Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting radiation in the form of particles or electromagnetic waves. ... Radiochemistry deals with the use of radioactivity to study ordinary chemical reactions. ... Frederick Soddy in 1922. ... For other uses, see Isotope (disambiguation). ... It has been suggested that this article or section be merged into Isotope. ...

Meanwhile, in 1913, physicist Niels Bohr revised Rutherford's model by suggesting that the electrons were confined into clearly defined orbits, and could jump between these, but could not freely spiral inward or outward in intermediate states.[18] An electron must absorb or emit specific amounts of energy to transition between these fixed orbits. When the light from a heated material is passed through a prism, it produced a multi-colored spectrum. The appearance of fixed lines in this spectrum was successfully explained by the orbital transitions.[19] Niels Henrik David Bohr (October 7, 1885 â€“ November 18, 1962) was a Danish physicist who made fundamental contributions to understanding atomic structure and quantum mechanics, for which he received the Nobel Prize in Physics in 1922. ... For other uses, see Light (disambiguation). ... If a shaft of light entering a prism is sufficiently narrow, a spectrum results. ... This article deals with the general meaning of spectrum and the history of its use. ... A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range, compared with the nearby frequencies. ...

In 1926, Erwin Schrödinger, using Louis de Broglie's 1924 proposal that particles behave to an extent like waves, developed a mathematical model of the atom that described the electrons as three-dimensional waveforms, rather than point particles. A consequence of using waveforms to describe electrons is that it is mathematically impossible to obtain precise values for both the position and momentum of a particle at the same time; this became known as the uncertainty principle. In this concept, for each measurement of a position one could only obtain a range of probable values for momentum, and vice versa. Although this model was difficult to visually conceptualize, it was able to explain observations of atomic behavior that previous models could not, such as certain structural and spectral patterns of atoms larger than hydrogen. Thus, the planetary model of the atom was discarded in favor of one that described orbital zones around the nucleus where a given electron is most likely to exist.[20][21] SchrÃ¶dinger in 1933, when he was awarded the Nobel Prize in Physics Bust of SchrÃ¶dinger, in the courtyard arcade of the main building, University of Vienna, Austria. ... Louis-Victor-Pierre-Raymond, 7th duc de Broglie, generally known as Louis de Broglie (August 15, 1892â€“March 19, 1987), was a French physicist and Nobel Prize laureate. ... Waveform quite literally means the shape and form of a signal, such as a wave moving across the surface of water, or the vibration of a plucked string. ... A spatial point is an entity with a location in space but no extent (volume, area or length). ... This article is about momentum in physics. ... In quantum physics, the outcome of even an ideal measurement of a system is not deterministic, but instead is characterized by a probability distribution, and the larger the associated standard deviation is, the more uncertain we might say that that characteristic is for the system. ... A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range, compared with the nearby frequencies. ...

Schematic diagram of a simple mass spectrometer

The development of the mass spectrometer allowed the exact mass of atoms to be measured. 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 Francis William Aston used this instrument to demonstrate that isotopes had different masses. The mass of these isotopes varied by integer amounts, called the whole number rule.[22] The explanation for these different atomic isotopes awaited the discovery of the neutron, a neutral-charged particle with a mass similar to the proton, by the physicist James Chadwick in 1932. Isotopes were then explained as elements with the same number of protons, but different numbers of neutrons within the nucleus.[23] Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... Mass spectrometry (previously called mass spectroscopy (deprecated) or informally, mass-spec and MS) is an analytical technique that measures the mass-to-charge ratio of ions. ... Francis William Aston (born Harborne, Birmingham, September 1, 1877; died Cambridge, November 20, 1945) was a British chemist and physicist who won the 1922 Nobel Prize in Chemistry for his discovery, by means of his mass spectrograph, of isotopes, in a large number of non-radioactive elements, and for his... The Whole Number Rule states that the masses of the elements are whole number multiples of the mass of the hydrogen atom. ... This article or section does not adequately cite its references or sources. ... For other uses, see Proton (disambiguation). ... Sir James Chadwick, CH (20 October 1891 â€“ 24 July 1974) was an English physicist and Nobel laureate who is best known for discovering the neutron. ...

In the 1950s, the development of improved particle accelerator and particle detectors allowed scientists to study the impacts of atoms moving at high energies.[24] Neutrons and protons were found to be hadrons, or composites of smaller particles called quarks. Standard models of nuclear physics were developed that successfully explained the properties of the nucleus in terms of these sub-atomic particles and the forces that govern their interactions.[25] Atom Smasher redirects here. ... The Compact Muon Solenoid (CMS) is an example of a large particle detector. ... A hadron, in particle physics, is a subatomic particle which experiences the nuclear force. ... For other uses, see Quark (disambiguation). ...

Around 1985, Steven Chu and co-workers at Bell Labs developed a technique for lowering the temperatures of atoms using lasers. In the same year, a team led by William D. Phillips managed to contain atoms of sodium in a magnetic trap. The combination of these two techniques and a method based on the Doppler effect, developed by Claude Cohen-Tannoudji and his group, allows small numbers of atoms to be cooled to several microkelvin. This allows the atoms to be studied with great precision, and later led to the discovery of Bose-Einstein condensation.[26] Steven Chu (Chinese: ; pinyin: ), born 1948 in St. ... Bell Laboratories (also known as Bell Labs and formerly known as AT&T Bell Laboratories and Bell Telephone Laboratories) was the main research and development arm of the United States Bell System. ... For other uses, see Laser (disambiguation). ... Photograph of William Daniel Phillips William Daniel Phillips (born November 5, 1948 in Wilkes-Barre, Pennsylvania) is an American physicist. ... A magnetic trap uses a magnetic gradient in order to trap neutral particles with a magnetic moment. ... A source of waves moving to the left. ... Claude Cohen-Tannoudji (born April 1, 1933) is a French physicist working at the Ã‰cole Normale SupÃ©rieure in Paris, France, where he has also studied physics. ... For other uses, see Kelvin (disambiguation). ... A Boseâ€“Einstein condensate is a phase of matter formed by bosons cooled to temperatures very near to absolute zero. ...

Historically, single atoms have been prohibitively small for scientific applications. Recently, devices have been constructed that use a single metal atom connected through organic ligands to construct a single electron transistor.[27] Experiments have been carried out by trapping and slowing single atoms using laser cooling in a cavity to gain a better physical understanding of matter.[28] In chemistry, a ligand is an atom, ion, or molecule (see also: functional group) that generally donates one or more of its electrons through a coordinate covalent bond to, or shares its electrons through a covalent bond with, one or more central atoms or ions (these ligands act as a... In physics, a Coulomb blockade, named after Charles-Augustin de Coulomb, is the increased resistance at small bias voltages of an electronic device comprising at least one low-capacitance tunnel junction. ... Laser cooling is a technique that uses light to cool atoms to a very low temperature. ...

## Components

### 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 subatomic particles. The constituent particles of an atom consist of the electron, the proton and, for atoms other than hydrogen-1, the neutron. Helium atom (schematic) Showing two protons (red), two neutrons (green) and two electrons (yellow). ... Helium atom (schematic) Showing two protons (red), two neutrons (green) and two electrons (yellow). ... For other uses, see Electron (disambiguation). ... For other uses, see Proton (disambiguation). ... This article is about the chemistry of hydrogen. ... This article or section does not adequately cite its references or sources. ...

The electron is by far the least massive of these particles at 9.11×10−28 g, with a negative electrical charge and a size that is too small to be measured using available techniques.[29] Protons have a positive charge and a mass 1,836 times that of the electron, at 1.6726×10−24 g, although this can be reduced by changes to the atomic binding energy. Neutrons have no electrical charge and have a free mass of 1,839 times the mass of electrons,[30] or 1.6929×10−24 g. Neutrons and protons have comparable dimensions—on the order of 2.5×10−15 m—although the 'surface' of these particles is not sharply defined.[31] This box:      Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. ... Binding energy is the energy required to disassemble a whole into separate parts. ... This article is about the unit of length. ...

In the Standard Model of physics, both protons and neutrons are composed of elementary particles called quarks. The quark is a type of fermion, one of the two basic constituents of matter—the other being the lepton, of which the electron is an example. There are six types of quarks, and each has a fractional electric charge of either +2/3 or −1/3. Protons are composed of two up quarks and one down quark, while a neutron consists of one up quark and two down quarks. This distinction accounts for the difference in mass and charge between the two particles. The quarks are held together by the strong nuclear force, which is mediated by gluons. The gluon is a member of the family of bosons, which are elementary particles that mediate physical forces.[32][33] The Standard Model of Fundamental Particles and Interactions For the Standard Model in Cryptography, see Standard Model (cryptography). ... For the novel, see The Elementary Particles. ... For other uses, see Quark (disambiguation). ... In particle physics, fermions are particles with half-integer spin, such as protons and electrons. ... For the former Greek currency unit, see Greek drachma. ... The up quark is a first-generation quark with a charge of +(2/3)e. ... The down quark is a first-generation quark with a charge of -(1/3)e. ... The strong nuclear force or strong interaction (also called color force or colour force) is a fundamental force of nature which affects only quarks and antiquarks, and is mediated by gluons in a similar fashion to how the electromagnetic force is mediated by photons. ... In particle physics, gluons are subatomic particles that cause quarks to interact, and are indirectly responsible for the binding of protons and neutrons together in atomic nuclei. ... In particle physics, bosons are particles with an integer spin, as opposed to fermions which have half-integer spin. ... For other uses, see Force (disambiguation). ...

### Nucleus

Main article: Atomic nucleus

All of the bound protons and neutrons in an atom make up a tiny atomic nucleus, and are collectively called nucleons. The radius of a nucleus is approximately equal to $begin{smallmatrix}1.07 cdot sqrt[3]{A}end{smallmatrix}$ fm, where A is the total number of nucleons.[34] 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 residual strong force. At distances smaller than 2.5 fm, this force is much more powerful than the electrostatic force that causes positively charged protons to repel each other.[35] The nucleus of an atom is the very small dense region, of positive charge, in its centre consisting of nucleons (protons and neutrons). ... The nucleus of an atom is the very small dense region, of positive charge, in its centre consisting of nucleons (protons and neutrons). ... In physics a nucleon is a collective name for two baryons: the neutron and the proton. ... This article is about the force sometimes called the strong nuclear force. For the weak nuclear force or weak interaction, see that article. ... Femtometre (American spelling: femtometer) is an SI measure of length that is equal to 10−15 (femto) of a metre. ... In physics, the electrostatic force is the force arising between static (that is, non-moving) electric charges. ...

Atoms of the same element have the same number of protons, called the atomic number. Within a single element, the number of neutrons may vary, determining the isotope of that element. The number of neutrons relative to the protons determines the stability of the nucleus, with certain isotopes undergoing radioactive decay.[36] The periodic table of the chemical elements A chemical element, or element, is a type of atom that is distinguished by its atomic number; that is, by the number of protons in its nucleus. ... See also: List of elements by atomic number In chemistry and physics, the atomic number (also known as the proton number) is the number of protons found in the nucleus of an atom. ... For other uses, see Isotope (disambiguation). ... Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting radiation in the form of particles or electromagnetic waves. ...

The neutron and the proton are different types of fermions. The Pauli exclusion principle is a quantum mechanical effect that prohibits identical fermions (such as multiple protons) from occupying the same quantum physical state at the same time. Thus every proton in the nucleus must occupy a different state, with its own energy level, and the same rule applies to all of the neutrons. (This prohibition does not apply to a proton and neutron occupying the same quantum state.)[37] In particle physics, fermions are particles with half-integer spin, such as protons and electrons. ... The Pauli exclusion principle is a quantum mechanical principle formulated by Wolfgang Pauli in 1925. ... For a generally accessible and less technical introduction to the topic, see Introduction to quantum mechanics. ...

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. However, with increasing atomic number, the mutual repulsion of the protons requires an increasing proportion of neutrons to maintain the stability of the nucleus, which slightly modifies this trend of equal numbers of protons to neutrons.[37]

This diagram illustrates a nuclear fusion process that forms a deuterium nucleus, consisting of a proton and a neutron, from two protons. A positron (e+)—an antimatter electron—is emitted along with an electron neutrino.

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. Nuclear fusion occurs when multiple atomic particles join to form a heavier nucleus, such as through the energetic collision of two nuclei. At the core of the Sun, protons require energies of 3–10 KeV to overcome their mutual repulsion—the coulomb barrier—and fuse together into a single nucleus.[38] Nuclear fission 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. In such processes that change the number of protons in a nucleus, the atom becomes an atom of a different chemical element.[39][40] Image File history File links Wpdms_physics_proton_proton_chain_1. ... Image File history File links Wpdms_physics_proton_proton_chain_1. ... The first detection of the positron in 1932 by Carl D. Anderson The positron is the antiparticle or the antimatter counterpart of the electron. ... For other senses of this term, see antimatter (disambiguation). ... For other uses, see Neutrino (disambiguation). ... The deuterium-tritium (D-T) fusion reaction is considered the most promising for producing sustainable fusion power. ... The Coulomb barrier, named after physicist Charles-Augustin de Coulomb (1736â€”1806), is the energy barrier due to electrostatic interaction that two nuclei need to overcome so they can get close enough to undergo nuclear fusion. ... For the generation of electrical power by fission, see Nuclear power plant. ...

The mass of the nucleus following a fusion reaction is less than the sum of the masses of the separate particles. The difference between these two values is emitted as energy, as described by Albert Einstein's mass–energy equivalence formula, E = mc², where m is the mass loss and c is the speed of light. This deficit is the binding energy of the nucleus.[41] â€œEinsteinâ€ redirects here. ... 5-meter-tall sculpture of Einsteins 1905 E = mc2 formula at the 2006 Walk of Ideas, Germany In physics, massâ€“energy equivalence is the concept that any mass has an associated energy and vice versa. ... A line showing the speed of light on a scale model of Earth and the Moon, taking about 1â…“ seconds to traverse that distance. ... Binding energy is the energy required to disassemble a whole into separate parts. ...

The fusion of two nuclei that have lower atomic numbers than iron and nickel is an exothermic process that releases more energy than is required to bring them together.[42] It is this energy-releasing process that makes nuclear fusion in stars a self-sustaining reaction. For heavier nuclei, the total binding energy begins to decrease. That means fusion processes with nuclei that have higher atomic numbers is an endothermic process. These more massive nuclei can not undergo an energy-producing fusion reaction that can sustain the hydrostatic equilibrium of a star.[37] General Name, symbol, number iron, Fe, 26 Chemical series transition metals Group, period, block 8, 4, d Appearance lustrous metallic with a grayish tinge Standard atomic weight 55. ... For other uses, see Nickel (disambiguation). ... In chemistry, an exothermic reaction is one that releases heat. ... This article is about the astronomical object. ... In Chemistry an endothermic reaction is one in which the reactants have less energy than the products, and thus a net input of energy, usually in the form of heat, is required. ... Hydrostatic equilibrium occurs when compression due to gravity is balanced by a pressure gradient which creates a pressure gradient force in the opposite direction. ...

### Electron cloud

Main article: Electron cloud
This is an example of a potential well, showing the minimum energy V(x) needed to reach each position x. 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 electromagnetic force. This force binds the electrons inside an electrostatic potential well surrounding the smaller nucleus, which means that an external source of energy is needed in order 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 the exterior. Electron cloud is a term used- if not originally coined- by the nobelaurate and acclaimed educator Richard Feynman in The Feynman Lectures on Physics, for discussing exactly what is an electron?. This intuitive model provides a simplified way of visualizing an electron as a solution of the SchrÃ¶dinger equation. ... In physics, the electromagnetic force is the force that the electromagnetic field exerts on electrically charged particles. ... Electrostatics is the branch of physics that deals with the force exerted by a static (i. ... A potential well is the region surrounding a local minimum of potential energy. ...

Electrons, like other particles, have properties of both a particle and a wave. The electron cloud is a region inside the potential well where each electron forms a type of three-dimensional standing wave—a wave form that does not move relative to the nucleus. This behavior is defined by an atomic orbital, a mathematical function that characterises the probability that an electron will appear to be at a particular location when its position is measured. Only a discrete (or quantized) set of these orbitals exist around the nucleus, as other possible wave patterns will rapidly decay into a more stable form.[43] Orbitals can have one or more ring or node structures, and they differ from each other in size, shape and orientation.[44] This box:      In physics and chemistry, waveâ€“particle duality is the concept that all matter exhibits both wave-like and particle-like properties. ... Vibration and standing waves in a string, The fundamental and the first 6 overtones A standing wave, also known as a stationary wave, is a wave that remains in a constant position. ... In chemistry, an atomic orbital is the region in which an electron may be found around a single atom. ...

This illustration shows the wave functions of the first five atomic orbitals. Note how each of the three 2p orbitals display a single angular node that has an orientation and a minimum at the center.

Each atomic orbital corresponds to a particular energy level of the electron. The electron can change its state to a higher energy level by absorbing a photon with sufficient energy to boost it into the new quantum state. Likewise, through spontaneous emission, 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 atomic spectral lines.[43] A standing wave. ... A quantum mechanical system can only be in certain states, so that only certain energy levels are possible. ... In modern physics the photon is the elementary particle responsible for electromagnetic phenomena. ... Spontaneous emission is the process by which a molecule in an excited state drops to the ground state, resulting in the creation of a photon. ... In physics, atomic spectral lines are of two types: An emission line is formed when an electron makes a transition from a particular discrete energy level of an atom, to a lower energy state, emitting a photon of a particular energy and wavelength. ...

The amount of energy needed to remove or add an electron (the electron binding energy) is far less than the binding energy of nucleons. For example, it requires only 13.6 eV to strip a ground-state electron from a Hydrogen atom.[45] Atoms are electrically neutral if they have an equal number of protons and electrons. Atoms that have either a deficit or a surplus of electrons are called ions. 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 bond into molecules and other types of chemical compounds like ionic and covalent network crystals.[46] Electron binding energy (BE) is the energy required to release an electron from its atomic or molecular orbital. ... Binding energy is the energy required to disassemble a whole into separate parts. ... In quantum mechanics, a stationary state is an eigenstate of a Hamiltonian, or in other words, a state of definite energy. ... This box:      Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. ... This article is about the electrically charged particle. ... A chemical bond is the physical process responsible for the attractive interactions between atoms and molecules, and that which confers stability to diatomic and polyatomic chemical compounds. ... 3D (left and center) and 2D (right) representations of the terpenoid molecule atisane. ... Look up chemical compound in Wiktionary, the free dictionary. ... An ionic crystal is a crystal consisting of ions bound together by their electrostatic attraction. ... Covalent redirects here. ... Frost crystallization on a shrub. ...

## Properties

By definition, any two atoms with an identical number of protons in their nuclei belong to the same chemical element. Atoms with the same number of protons but a different number of neutrons are different isotopes of the same element. Hydrogen atoms, for example, always have only a single proton, but isotopes exist with no neutrons (hydrogen-1, sometimes called protium, by far the most common form), one neutron (deuterium) and two neutrons (tritium).[47] The known elements form a continuous range of atomic numbers from hydrogen with a single proton up to the 118-proton element ununoctium.[48] All known isotopes of elements with atomic numbers greater than 82 are radioactive.[49][50] The periodic table of the chemical elements A chemical element, or element, is a type of atom that is distinguished by its atomic number; that is, by the number of protons in its nucleus. ... For other uses, see Isotope (disambiguation). ... Depiction of a hydrogen atom showing the diameter as about twice the Bohr model radius. ... Deuterium, also called heavy hydrogen, is a stable isotope of hydrogen with a natural abundance in the oceans of Earth of approximately one atom in 6500 of hydrogen (~154 PPM). ... Tritium (symbol T or Â³H) is a radioactive isotope of hydrogen. ... General Name, Symbol, Number ununoctium, Uuo, 118 Chemical series noble gases Group, Period, Block 18, 7, p Appearance unknown, probably colorless Atomic mass predicted, (314) g/mol Electron configuration perhaps [Rn] 5f14 6d10 7s2 7p6 (guess based on radon) Electrons per shell 2, 8, 18, 32, 32, 18, 8 Phase...

### Mass

Main article: Atomic mass

Because the large majority of an atom's mass comes from the protons and neutrons, the total number of these particles in an atom is called the mass number. The mass of an atom at rest is often expressed using the unified atomic mass unit (u), which is also called a Dalton (Da). This unit is defined as a twelfth of the mass of a free neutral atom of carbon-12, which is approximately 1.66×10−24 g.[51] hydrogen-1, the lightest isotope of hydrogen and the atom with the lowest mass, has an atomic weight of 1.007825 u.[52] An atom has a mass approximately equal to the mass number times the atomic mass unit.[53] The heaviest stable atom is lead-208,[49] with a mass of 207.9766521 u.[54] Stylized lithium-7 atom: 3 protons, 4 neutrons & 3 electrons (~1800 times smaller than protons/neutrons). ... The mass number (A), also called atomic mass number (not to be confused with atomic number (Z) which denotes the number of protons in a nucleus) or nucleon number, is the number of nucleons (protons and neutrons) in an atomic nucleus. ... The invariant mass or intrinsic mass or proper mass or just mass is a measurement or calculation of the mass of an object that is the same for all frames of reference. ... The unified atomic mass unit (u), or dalton (Da), is a small unit of mass used to express atomic and molecular masses. ... Carbon 12 is a stable isotope of the element carbon. ... Depiction of a hydrogen atom showing the diameter as about twice the Bohr model radius. ...

As even the most massive atoms are far too light to work with directly, chemists instead use the unit of moles. The mole is defined such that one mole of any element will always have the same number of atoms (about 6.022×1023). This number was chosen so that if an element has an atomic mass of 1 u, a mole of atoms of that element will have a mass of 1 g. Carbon, for example, has an atomic mass of 12 u, so a mole of carbon atoms weighs 12 g.[51] The mole (symbol: mol) is the SI base unit that measures an amount of substance. ... The Avogadro constant (symbols: L, NA), also called the Avogadro number and, in German scientific literature, sometimes also known as the Loschmidt constant/number, is formally defined to be the number of entities in one mole,[1][2] that is the number of carbon-12 atoms in 12 grams (0. ... For other uses, see Carbon (disambiguation). ...

### Size

Atoms lack a well-defined outer boundary, so the dimensions are usually described in terms of the distances between two nuclei when the two atoms are joined in a chemical bond. The radius varies with the location of an atom on the atomic chart, the type of chemical bond, the number of neighboring atoms (coordination number) and a quantum mechanical property known as spin.[55] On the periodic table of the elements, atom size tends to increase when moving down columns, but decrease when moving across rows (left to right).[56] Consequently, the smallest atom is helium with a radius of 32 pm, while one of the largest is caesium at 225 pm.[57] These dimensions are thousands of times smaller than the wavelengths of light (400–700 nm) so they can not be viewed using an optical microscope. However, individual atoms can be observed using a scanning tunneling microscope. Atomic radius: Ionic radius Covalent radius Metallic radius van der Waals radius edit Atomic radius, and more generally the size of an atom, is not a precisely defined physical quantity, nor is it constant in all circumstances. ... A chemical bond is the physical process responsible for the attractive interactions between atoms and molecules, and that which confers stability to diatomic and polyatomic chemical compounds. ... In chemistry coordination number (c. ... For a generally accessible and less technical introduction to the topic, see Introduction to quantum mechanics. ... In physics, spin refers to the angular momentum intrinsic to a body, as opposed to orbital angular momentum, which is the motion of its center of mass about an external point. ... The Periodic Table redirects here. ... One picometre is defined as 1x10-12 metres, in standard units. ... General Name, Symbol, Number caesium, Cs, 55 Chemical series alkali metals Group, Period, Block 1, 6, s Appearance silvery gold Standard atomic weight 132. ... For other uses, see Light (disambiguation). ... A nanometre (American spelling: nanometer, symbol nm) (Greek: Î½Î¬Î½Î¿Ï‚, nanos, dwarf; Î¼ÎµÏ„ÏÏŽ, metrÏŒ, count) is a unit of length in the metric system, equal to one billionth of a metre (or one millionth of a millimetre), which is the current SI base unit of length. ... An 1879 Carl Zeiss Jena Optical microscope. ... Image of reconstruction on a clean Au(100) surface. ...

Some examples will demonstrate the minuteness of the atom. A typical human hair is about 1 million carbon atoms in width.[58] A single drop of water contains about 2 sextillion (2×1021) atoms of oxygen, and twice the number of hydrogen atoms.[59] A single carat diamond with a mass of 0.2 g contains about 10 sextillion atoms of carbon.[60] If an apple was magnified to the size of the Earth, then the atoms in the apple would be approximately the size of the original apple.[61] Main article: Names of large numbers A sextillion is a number written as either: a 1 followed by 21 zeros (10 to the 21st power, as used in the short scale system of numeration. ... The carat is a unit of mass used for measuring gems and pearls, and is exactly 200 milligrams. ... This article is about the mineral. ... Main article: Names of large numbers A sextillion is a number written as either: a 1 followed by 21 zeros (10 to the 21st power, as used in the short scale system of numeration. ... For other uses, see Carbon (disambiguation). ...

This diagram shows the half-life (T½) in seconds 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.[62] Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting radiation in the form of particles or electromagnetic waves. ... Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting radiation in the form of particles or electromagnetic waves. ...

There are three primary forms of radioactive decay:[63][64]

• Alpha decay is caused when the nucleus emits an alpha particle, which is a helium nucleus consisting of two protons and two neutrons. The result of the emission is a new element with a lower atomic number.
• Beta decay is regulated by the weak force, and results from a transformation of a neutron into a proton, or a proton into a neutron. The first is accompanied by the emission of an electron and an antineutrino, while the second causes the emission of a positron and a neutrino. The electron or positron emissions are called beta particles. Beta decay either increases or decreases the atomic number of the nucleus by one.
• Gamma decay results from a change in the energy level of the nucleus to a lower state, resulting in the emission of electromagnetic radiation. This can occur following the emission of an alpha or a beta particle from radioactive decay.

Each radioactive isotope has a characteristic decay time period—the half-life—that is determined by the amount of time needed for half of a sample to decay. This is an exponential decay 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 will be present, and so forth.[62] Alpha decay Alpha decay is a type of radioactive decay in which an atom emits an alpha particle (two protons and two neutrons bound together into a particle identical to a helium nucleus) and transforms (or decays) into an atom with a mass number 4 less and atomic number 2... See also: List of elements by atomic number In chemistry and physics, the atomic number (also known as the proton number) is the number of protons found in the nucleus of an atom. ... In nuclear physics, beta decay (sometimes called neutron decay) is a type of radioactive decay in which a beta particle (an electron or a positron) is emitted. ... The weak nuclear force or weak interaction is one of the four fundamental forces of nature. ... Antineutrinos, the antiparticles of neutrinos, are neutral particles produced in nuclear beta decay. ... The first detection of the positron in 1932 by Carl D. Anderson The positron is the antiparticle or the antimatter counterpart of the electron. ... For other uses, see Neutrino (disambiguation). ... This article is about electromagnetic radiation. ... Half-Life For a quantity subject to exponential decay, the half-life is the time required for the quantity to fall to half of its initial value. ... A quantity is said to be subject to exponential decay if it decreases at a rate proportional to its value. ...

### Magnetic moment

Elementary particles possess an intrinsic quantum mechanical property known as spin. This is analogous to the angular momentum of an object that is spinning around its center of mass, 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 Planck constant ($hbar$), with electrons, protons and neutrons all having spin ½ $hbar$, or "spin-½". In an atom, electrons in motion around the nucleus possess orbital angular momentum in addition to their spin, while the nucleus itself possesses angular momentum due to its nuclear spin.[65] In atomic physics, the magnetic dipole moment of an electron is involved in a variety of important atomic processes and effects. ... The nuclear magnetic moment is the magnetic moment of an atomic nucleus and arises from the spin of the protons and neutrons. ... In physics, spin refers to the angular momentum intrinsic to a body, as opposed to orbital angular momentum, which is the motion of its center of mass about an external point. ... This gyroscope remains upright while spinning due to its angular momentum. ... In physics, the center of mass of a system of particles is a specific point at which, for many purposes, the systems mass behaves as if it were concentrated. ... A commemoration plaque for Max Planck on his discovery of Plancks constant, in front of Humboldt University, Berlin. ... The nucleus of an atom is the very small dense region, of positive charge, in its centre consisting of nucleons (protons and neutrons). ... The Azimuthal quantum number (or orbital angular momentum quantum number) l is a quantum number for an atomic orbital which determines its orbital angular momentum. ...

The magnetic field produced by an atom—its magnetic moment—is determined by these various forms of angular momentum, just as a rotating charged object classically produces a magnetic field. However, the most dominant contribution comes from spin. Due to the nature of electrons to obey the Pauli exclusion principle, in which no two electrons may be found in the same quantum state, 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.[66] For the indie-pop band, see The Magnetic Fields. ... A bar magnet. ... The Pauli exclusion principle is a quantum mechanical principle formulated by Wolfgang Pauli in 1925. ... Probability densities for the electron at different quantum numbers (l) In quantum mechanics, the quantum state of a system is a set of numbers that fully describe a quantum system. ...

In ferromagnetic elements such as iron, 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 process is known as an exchange interaction. When the magnetic moments of ferromagnetic atoms are lined up, the material can produce a measurable macroscopic field. Paramagnetic materials 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.[67][66] Ferromagnetism is the phenomenon by which materials, such as iron, in an external magnetic field become magnetized and remain magnetized for a period after the material is no longer in the field. ... In physics, the exchange interaction is a quantum mechanical effect which increases or decreases the energy of two or more electrons when their wave functions overlap. ... Simple Illustration of a paramagnetic probe made up from miniature magnets. ...

The nucleus of an atom can also have a net spin. Normally these nuclei are aligned in random directions because of thermal equilibrium. However, for certain elements (such as xenon-129) it is possible to polarize a significant proportion of the nuclear spin states so that they are aligned in the same direction—a condition called hyperpolarization. This has important applications in magnetic resonance imaging.[68][69] In thermodynamics, a thermodynamic system is in thermodynamic equilibrium if its energy distribution equals a Maxwell-Boltzmann-distribution. ... General Name, Symbol, Number xenon, Xe, 54 Chemical series noble gases Group, Period, Block 18, 5, p Appearance colorless Standard atomic weight 131. ... In electrodynamics, polarization (also spelled polarisation) is the property of electromagnetic waves, such as light, that describes the direction of their transverse electric field. ... Hyperpolarization is the nuclear spin polarization of a material far beyond thermal equilibrium conditions. ... MRI redirects here. ...

### Energy levels

When an electron is bound to an atom, it has a potential energy that is inversely proportional to its distance from the nucleus. This is measured by the amount of energy needed to unbind the electron from the atom, and is usually given in units of electronvolts (eV). In the quantum mechanical model, a bound electron can only occupy a set of states centered on the nucleus, and each state corresponds to a specific energy level. The lowest energy state of a bound electron is called the ground state, while an electron at a higher energy level is in an excited state.[70] A quantum mechanical system can only be in certain states, so that only certain energy levels are possible. ... In physics, atomic spectral lines are of two types: An emission line is formed when an electron makes a transition from a particular discrete energy level of an atom, to a lower energy state, emitting a photon of a particular energy and wavelength. ... Potential energy can be thought of as energy stored within a physical system. ... The electronvolt (symbol eV) is a unit of energy. ...

In order for an electron to transition between two different states, it must absorb or emit a photon at an energy matching the difference in the potential energy of those levels. The energy of an emitted photon is proportional to its frequency, so these specific energy levels appear as distinct bands in the electromagnetic spectrum.[71] 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.[72] In modern physics the photon is the elementary particle responsible for electromagnetic phenomena. ... For other uses, see Frequency (disambiguation). ... Although some radiations are marked as N for no in the diagram, some waves do in fact penetrate the atmosphere, although extremely minimally compared to the other radiations The electromagnetic (EM) spectrum is the range of all possible electromagnetic radiation. ...

An example of absorption lines in a spectrum

When a continuous spectrum of energy 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 will 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 absorption bands in the energy output. (An observer viewing the atoms from a different direction, which does not include the continuous spectrum in the background, will instead see a series of emission lines from the photons emitted by the atoms.) Spectroscopic measurements of the strength and width of spectral lines allow the composition and physical properties of a substance to be determined.[73] An absorption band is a range of wavelengths (or, equivalently, frequencies) in the electromagnetic spectrum within which electromagnetic energy is absorbed by a substance. ... A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range, compared with the nearby frequencies. ... Animation of the dispersion of light as it travels through a triangular prism. ... A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range, compared with the nearby frequencies. ...

Close examination of the spectral lines reveals that some display a fine structure splitting. This occurs because of spin-orbit coupling, which is an interaction between the spin and motion of the outermost electron.[74] When an atom is in an external magnetic field, spectral lines become split into three or more components; a phenomenon called the Zeeman effect. 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 electron configurations 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.[75] The presence of an external electric field can cause a comparable splitting and shifting of spectral lines by modifying the electron energy levels, a phenomenon called the Stark effect.[76] In atomic physics, the fine structure describes the splitting of the spectral lines of atoms. ... ... The Zeeman effect (IPA ) is the splitting of a spectral line into several components in the presence of a magnetic field. ... Electron atomic and molecular orbitals In atomic physics and quantum chemistry, the electron configuration is the arrangement of electrons in an atom, molecule, or other physical structure (, a crystal). ... In physics, the space surrounding an electric charge or in the presence of a time-varying magnetic field has a property called an electric field. ... The Stark effect is the splitting of a spectral line into several components in the presence of an electric field. ...

If a bound electron is in an excited state, an interacting photon with the proper energy can cause stimulated emission 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 will then move off in parallel and with matching phases. That is, the wave patterns of the two photons will be synchronized. This physical property is used to make lasers, which can emit a coherent beam of light energy in a narrow frequency band.[77] In optics, stimulated emission is the process by which, when perturbed by a photon, matter may lose energy resulting in the creation of another photon. ... For other uses, see Laser (disambiguation). ...

### Valence

Main article: Valence (chemistry)

The outermost electron shell of an atom in its uncombined state is known as the valence shell, and the electrons in that shell are called valence electrons. The number of valence electrons determines the bonding behavior with other atoms. Atoms tend to chemically react with each other in a manner that will fill (or empty) their outer valence shells.[78] For other uses, see Valence. ... In chemistry, valence electrons are the electrons contained in the outermost, or valence, electron shell of an atom. ... A chemical bond is the physical process responsible for the attractive interactions between atoms and molecules, and that which confers stability to diatomic and polyatomic chemical compounds. ... For other uses, see Chemical reaction (disambiguation). ...

The chemical elements are often displayed in a periodic table 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 noble gases.[79][80] The periodic table of the chemical elements A chemical element, or element, is a type of atom that is distinguished by its atomic number; that is, by the number of protons in its nucleus. ... The Periodic Table redirects here. ... This article is about the chemical series. ...

### States

Main articles: State of matter and Phase (matter)
These snapshots illustrate the formation of a bose-einstein condensate.

Quantities of atoms are found in different states of matter that depend on the physical conditions, such as temperature and pressure. By varying the conditions, materials can transition between solids, liquids, gases and plasmas.[81] Within a state, a material can also exist in different phases. An example of this is solid carbon, which can exist as graphite or diamond.[82] A State of matter is a class of materials, for example solid, liquid, or gas, also called a physical state or phase. ... In the physical sciences, a phase is a set of states of a macroscopic physical system that have relatively uniform chemical composition and physical properties (i. ... Bose-Einstein condensate from http://www. ... Bose-Einstein condensate from http://www. ... For other uses, see Temperature (disambiguation). ... This article is about pressure in the physical sciences. ... This box:      For other uses, see Solid (disambiguation). ... For other uses, see Liquid (disambiguation). ... For other uses, see Gas (disambiguation). ... For other uses, see Plasma. ... For other uses, see Graphite (disambiguation). ... This article is about the mineral. ...

At temperatures close to absolute zero, atoms can form a Bose–Einstein condensate, at which point quantum mechanical effects, which are normally only observed at the atomic scale, become apparent on a macroscopic scale.[83][84] This super-cooled collection of atoms then behaves as a single Super Atom, which may allow fundamental checks of quantum mechanical behavior.[85] For other uses, see Absolute Zero (disambiguation). ... A Boseâ€“Einstein condensate (BEC) is a state of matter formed by a system of bosons confined in an external potential and cooled to temperatures very near to absolute zero (0 kelvin or âˆ’273. ...

## Identification

This scanning tunneling microscope image clearly shows the individual atoms that make up this gold(100) surface. Reconstruction causes the surface atoms to deviate from the bulk crystal structure and arrange in columns several atoms wide with pits between them.

The scanning tunneling microscope is a device for viewing surfaces at the atomic level. It uses the quantum tunneling phenomenon, which allows particles to pass through a barrier that would normally be insurmountable. Electrons tunnel through the vacuum between two planar metal electrodes, on each of which is an adsorbed atom, providing a tunneling-current density that can be measured. Scanning one atom (taken as the tip) as it moves past the other (the sample) permits plotting of tip displacement versus lateral separation for a constant current. The calculation shows the extent to which scanning-tunneling-microscope images of an individual atom are visible. It confirms that for low bias, the microscope images the space-averaged dimensions of the electron orbitals across closely packed energy levels—the Fermi level local density of states.[86][87] Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... Image of reconstruction on a clean Au(100) surface. ... GOLD refers to one of the following: GOLD (IEEE) is an IEEE program designed to garner more student members at the university level (Graduates of the Last Decade). ... Examples of directions Miller indices are a notation used to describe lattice planes and directions in a crystal. ... In surface physics, surface reconstruction is the name given to the process by which the atoms at the surface of a crystal rearrange themselves to form a structure with a different periodicity and/or symmetry than that of the bulk crystal. ... Enargite crystals In mineralogy and crystallography, a crystal structure is a unique arrangement of atoms in a crystal. ... Image of reconstruction on a clean Au(100) surface. ... Quantum tunneling is the quantum-mechanical effect of transitioning through a classically-forbidden energy state. ... In quantum mechanics, particles with a half-integer spin, usually spin 1/2 (for example electrons) follow the Pauli exclusion principle, which states that no two particles may occupy the same quantum state. ...

An atom can be ionized by removing one of its electrons. The electric charge causes the trajectory of an atom to bend when it passes through a magnetic field. 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 mass spectrometer uses this principle to measure the mass-to-charge ratio 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 inductively coupled plasma atomic emission spectroscopy and inductively coupled plasma mass spectrometry, both of which use a plasma to vaporize samples for analysis.[88] This article is about the electrically charged particle. ... This box:      Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. ... For the indie-pop band, see The Magnetic Fields. ... Mass spectrometry (previously called mass spectroscopy (deprecated) or informally, mass-spec and MS) is an analytical technique that measures the mass-to-charge ratio of ions. ... It has been suggested that Charge-to-mass ratio be merged into this article or section. ... Inductively coupled plasma atomic emission spectroscopy (ICP-AES), also referred to as Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES), is a type of emission spectroscopy that uses a plasma (e. ... ICP-MS (Inductively coupled plasma mass spectrometry) is a type of mass spectrometry that is highly sensitive and capable of the determination of a range of metals and several non-metals at concentrations below one part in 1012. ...

A more area-selective method is electron energy loss spectroscopy, which measures the energy loss of an electron beam within a transmission electron microscope when it interacts with a portion of a sample. The atom-probe tomograph has sub-nanometer resolution in 3-D and can chemically identify individual atoms using time-of-flight mass spectrometry.[89] See also HREELS. In electron energy loss spectroscopy (EELS) a material is exposed to a beam of electrons with a known, narrow range of kinetic energies. ... A charged particle beam is a group of electrically charged particles that have approximately the same kinetic energy and move in approximately the same direction. ... Transmission electron microscopy (TEM) is an imaging technique whereby a beam of electrons is focused onto a specimen causing an enlarged version to appear on a fluorescent screen or layer of photographic film (see electron microscope), or can be detected by a CCD camera. ... The atom probe is an atomic-resolution microscope used in materials science that was invented in 1967 by Erwin MÃ¼ller. ...

Spectra of excited states can be used to analyze the atomic composition of distant stars. Specific light wavelengths 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 gas-discharge lamp containing the same element.[90] Helium was discovered in this way in the spectrum of the Sun 23 years before it was found on Earth.[91] After absorbing energy, an electron may jump from the ground state to a higher energy excited state. ... This article is about the astronomical object. ... For other uses, see Wavelength (disambiguation). ... Germicidal lamps are simple low pressure mercury vapor discharges in a fused quartz envelope. ... General Name, symbol, number helium, He, 2 Chemical series noble gases Group, period, block 18, 1, s Appearance colorless Standard atomic weight 4. ...

## Origin and current state

Atoms form about 4% of the total mass density of the observable universe, with an average density of about 0.25 atoms/m3.[92] Within a galaxy such as the Milky Way, atoms have a much higher concentration, with the density of matter in the interstellar medium (ISM) ranging from 105 to 109 atoms/m3.[93] The Sun is believed to be inside the Local Bubble, a region of highly ionized gas, so the density in the solar neighborhood is only about 103 atoms/m3.[94] 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 atoms are concentrated inside stars and the total mass of atoms forms about 10% of the mass of the galaxy.[95] (The remainder of the mass is an unknown dark matter.[96]) For other uses, see Universe (disambiguation). ... For other uses, see Milky Way (disambiguation). ... The interstellar medium (or ISM) is the name astronomers give to the tenuous gas and dust that pervade interstellar space. ... The Local Bubble is a cavity in the local interstellar medium (ISM) at least 300 light years across containing a neutral hydrogen density that is approximately one tenth of that of the average ISM in the Milky Way (approximately 0. ... For other uses, see Dark matter (disambiguation). ...

### Nucleosynthesis

Main article: Nucleosynthesis

Stable protons and electrons appeared one second after the Big Bang. During the following three minutes, Big Bang nucleosynthesis produced most of the helium, lithium, and deuterium atoms in the universe, and perhaps some of the beryllium and boron.[97][98][99] The first atoms (complete with bound electrons) were theoretically created 380,000 years after the Big Bang—an epoch called recombination, when the expanding universe cooled enough to allow electrons to become attached to nuclei.[100] Since then, atomic nuclei have been combined in stars through the process of nuclear fusion to produce elements up to iron.[101] Nucleosynthesis is the process of creating new atomic nuclei from preexisting nucleons (protons and neutrons). ... For other uses, see Big Bang (disambiguation). ... In cosmology, Big Bang nucleosynthesis (or primordial nucleosynthesis) refers to the production of nuclei other than H-1, the normal, light hydrogen, during the early phases of the universe, shortly after the Big Bang. ... General Name, symbol, number helium, He, 2 Chemical series noble gases Group, period, block 18, 1, s Appearance colorless Standard atomic weight 4. ... This article is about the chemical element. ... Deuterium, also called heavy hydrogen, is a stable isotope of hydrogen with a natural abundance in the oceans of Earth of approximately one atom in 6500 of hydrogen (~154 PPM). ... General Name, symbol, number beryllium, Be, 4 Chemical series alkaline earth metals Group, period, block 2, 2, s Appearance white-gray metallic Standard atomic weight 9. ... For other uses, see Boron (disambiguation). ... This box:      A graphical timeline is available here: Graphical timeline of the Big Bang This timeline of the Big Bang describes the events according to the scientific theory of the Big Bang, using the cosmological time parameter of comoving coordinates. ... This article is about the astronomical object. ... The deuterium-tritium (D-T) fusion reaction is considered the most promising for producing sustainable fusion power. ...

Isotopes such as lithium-6 are generated in space through cosmic ray spallation.[102] 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 supernovae through the r-process and in AGB stars through the s-process, both of which involve the capture of neutrons by atomic nuclei.[103] Elements such as lead formed largely through the radioactive decay of heavier elements.[104] Cosmic ray spallation is a form of naturally occuring nuclear fission and nucleosynthesis. ... For other uses, see Supernova (disambiguation). ... The R process (R for rapid) is a neutron capture process for radioactive elements which occurs in high neutron density, high temperature conditions. ... A period of Stellar evolution undertaken by all low to intermediate mass stars (0. ... This article or section does not cite its references or sources. ... General Name, Symbol, Number lead, Pb, 82 Chemical series Post-transition metals or poor metals Group, Period, Block 14, 6, p Appearance bluish gray Standard atomic weight 207. ...

### Earth

Most of the atoms that make up the Earth and its inhabitants were present in their current form in the nebula that collapsed out of a molecular cloud to form the solar system. The rest are the result of radioactive decay, and their relative proportion can be used to determine the age of the Earth through radiometric dating.[105][106] Most of the helium in the crust of the Earth (about 99% of the helium from gas wells, as shown by its lower abundance of helium-3) is a product of alpha decay.[107] The Triangulum Emission Nebula NGC 604 The Pillars of Creation from the Eagle Nebula For other uses, see Nebula (disambiguation). ... A molecular cloud is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen (H2). ... Earth as seen from Apollo 17 Modern geologists consider the age of the Earth to be around 4. ... Radiometric dating (often called radioactive dating) is a technique used to date materials, based on a comparison between the observed abundance of particular naturally occurring radioactive isotopes and their known decay rates. ... General Name, symbol, number helium, He, 2 Chemical series noble gases Group, period, block 18, 1, s Appearance colorless Standard atomic weight 4. ... Helium-3 is a non-radioactive and light isotope of helium. ... Alpha decay Alpha decay is a type of radioactive decay in which an atom emits an alpha particle (two protons and two neutrons bound together into a particle identical to a helium nucleus) and transforms (or decays) into an atom with a mass number 4 less and atomic number 2...

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. Carbon-14 is continuously generated by cosmic rays in the atmosphere.[108] Some atoms on Earth have been artificially generated either deliberately or as by-products of nuclear reactors or explosions.[109][110] Of the transuranic elements—those with atomic numbers greater than 92—only plutonium and neptunium occur naturally on Earth.[111][112] Transuranic elements have radioactive lifetimes shorter than the current age of the Earth[113] and thus identifiable quantities of these elements have long since decayed, with the exception of traces of plutonium-244 possibly deposited by cosmic dust.[105] Natural deposits of plutonium and neptunium are produced by neutron capture in uranium ore.[114] Carbon-14 is the radioactive isotope of carbon discovered February 27, 1940, by Martin Kamen and Sam Ruben. ... This article needs additional references or sources to facilitate its verification. ... General Name, Symbol, Number neptunium, Np, 93 Chemical series actinides Group, Period, Block n/a, 7, f Appearance silvery metallic Standard atomic weight (237) gÂ·molâˆ’1 Electron configuration [Rn] 5f4 6d1 7s2 Electrons per shell 2, 8, 18, 32, 22, 9, 2 Physical properties Phase solid Density (near r. ... The process of neutron capture can proceed in two ways - as a rapid process (an r-process) or a slow process (an s-process). ...

### Rare and theoretical forms

While isotopes with atomic numbers higher than lead (82) are known to be radioactive, an "island of stability" has been proposed for some elements with atomic numbers above 103. These superheavy elements may have a nucleus that is relatively stable against radioactive decay.[119] The most likely candidate for a stable superheavy atom, unbihexium, has 126 protons and 184 neutrons.[120] General Name, Symbol, Number lead, Pb, 82 Chemical series Post-transition metals or poor metals Group, Period, Block 14, 6, p Appearance bluish gray Standard atomic weight 207. ... 3-dimensional rendering of the theoretical Island of Stability. ... In chemistry, transuranium elements (also known as transuranic elements) are the chemical elements with atomic numbers greater than 92, the atomic number of Uranium. ... General Name, Symbol, Number unbihexium, Ubh, 126 Chemical series Superactinides Group, Period, Block g6, 8, g Appearance unknown - silvery or grey in color Image:.jpg Atomic mass [334] gÂ·molâˆ’1 Electron configuration [Uuo] 5g6 8s2 Electrons per shell 2, 8, 18, 32, 38, 18, 8, 2 Physical properties Phase...

Each particle of matter has a corresponding antimatter particle with the opposite electrical charge. Thus, the positron is a positively charged antielectron and the antiproton is a negatively charged equivalent of a proton. For unknown reasons, antimatter particles are rare in the universe, hence, no antimatter atoms have been discovered.[121][122] Antihydrogen, the antimatter counterpart of hydrogen, was first produced at the CERN laboratory in Geneva in 1996.[123][124] For other senses of this term, see antimatter (disambiguation). ... The first detection of the positron in 1932 by Carl D. Anderson The positron is the antiparticle or the antimatter counterpart of the electron. ... Antihydrogen is the antimatter counterpart of hydrogen. ... CERN logo The European Organization for Nuclear Research (French: ), commonly known as CERN (see Naming), pronounced (or in French), is the worlds largest particle physics laboratory, situated just northwest of Geneva on the border between France and Switzerland. ... For other uses, see Geneva (disambiguation). ...

Other exotic atoms 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 muon, forming a muonic atom. These types of atoms can be used to test the fundamental predictions of physics.[125][126][127] An exotic atom is the anologue of a normal atom in which one or more of the electrons are replaced by other negative particles, such as a muon or a pion, or the positively charged nucleus is replaced by other positively charged elementary particles, or both. ... The muon (from the letter mu (Î¼)--used to represent it) is an elementary particle with negative electric charge and a spin of 1/2. ...

 Introduction to quantum mechanics History of quantum mechanics Infinite divisibility List of basic chemistry topics

This box:      Werner Heisenberg and Erwin SchrÃ¶dinger, founders of Quantum Mechanics. ... Niels Bohrâ€™s 1913 quantum model of the atom, which incorporated an explanation of Johannes Rydbergs 1888 formula, Max Planckâ€™s 1900 quantum hypothesis, i. ... The concept of infinite divisibility arises in different ways in philosophy, physics, economics, order theory (a branch of mathematics), and probability theory (also a branch of mathematics). ... Below is a list of basic topics in chemistry -- topics which will help the beginner become familiar with the field of chemistry. ... This is a list of particles in particle physics, including currently known and hypothetical elementary particles, as well as the composite particles that can be built up from them. ... A nuclear model is any model that attempts to describe the atomic nucleus. ... A radionuclide is an atom with an unstable nucleus. ... This article needs additional references or sources to facilitate its verification. ...

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### Book references

• L'Annunziata, Michael F. (2003). Handbook of Radioactivity Analysis. Academic Press. ISBN 0124366031.
• Beyer, H. F.; Shevelko, V. P. (2003). Introduction to the Physics of Highly Charged Ions. CRC Press. ISBN 0750304812.
• Choppin, Gregory R.; Liljenzin, Jan-Olov; Rydberg, Jan (2001). Radiochemistry and Nuclear Chemistry. Elsevier. ISBN 0750674636.
• Dalton, J. (1808). A New System of Chemical Philosophy, Part 1. London and Manchester: S. Russell.
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• Feynman, Richard (1995). Six Easy Pieces. The Penguin Group. ISBN 978-0-140-27666-4.
• Fowles, Grant R. (1989). Introduction to Modern Optics. Courier Dover Publications. ISBN 0486659577.
• Gangopadhyaya, Mrinalkanti (1981). Indian Atomism: History and Sources. Atlantic Highlands, New Jersey: Humanities Press. ISBN 0-391-02177-X.
• Goodstein, David L. (2002). States of Matter. Courier Dover Publications. ISBN 048649506X.
• Harrison, Edward Robert (2003). Masks of the Universe: Changing Ideas on the Nature of the Cosmos. Cambridge University Press. ISBN 0521773512.
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• Shultis, J. Kenneth; Faw, Richard E. (2002). Fundamentals of Nuclear Science and Engineering. CRC Press. ISBN 0824708342.
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• Sills, Alan D. (2003). Earth Science the Easy Way. Barron's Educational Series. ISBN 0764121464.
• Smirnov, Boris M. (2003). Physics of Atoms and Ions. Springer. ISBN 038795550X.
• Teresi, Dick (2003). Lost Discoveries: The Ancient Roots of Modern Science. Simon & Schuster, 213–214. ISBN 074324379X.
• Woan, Graham (2000). The Cambridge Handbook of Physics. Cambridge University Press. ISBN 0521575079.
• Wurtz, Charles Adolphe (1881). The Atomic Theory. New York: D. Appleton and company.
• Zaider, Marco; Rossi, Harald H. (2001). Radiation Science for Physicians and Public Health Workers. Springer. ISBN 0306464039.
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John Dalton John Dalton (September 6, 1766 â€“ July 27, 1844) was an English chemist and physicist, born at Eaglesfield, near Cockermouth in Cumberland. ... 2008 (MMVIII) is the current year, a leap year that started on Tuesday of the Anno Domini (or common era), in accordance to the Gregorian calendar. ... is the 9th day of the year in the Gregorian calendar. ... IUPAC logo The International Union of Pure and Applied Chemistry (IUPAC) (Pronounced as eye-you-pack) is an international non-governmental organization established in 1919 devoted to the advancement of chemistry. ... Year 2007 (MMVII) was a common year starting on Monday of the Gregorian calendar in the 21st century. ... December 17 is the 351st day of the year (352nd in leap years) in the Gregorian calendar. ... 2008 (MMVIII) is the current year, a leap year that started on Tuesday of the Anno Domini (or common era), in accordance to the Gregorian calendar. ... is the 36th day of the year in the Gregorian calendar. ...

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 AtomEnabled / Developers / Syndication / Atom Syndication Format Spec (6990 words) Atom Processors MUST NOT reject an Atom Document containing such a signature because they are not capable of verifying it; they MUST continue processing and MAY inform the user of their failure to validate the signature. The root of an Atom Document (i.e., atom:feed in an Atom Feed Document, atom:entry in an Atom Entry Document) MAY be encrypted, using the mechanisms described by XML Encryption Syntax and Processing [W3C.REC-xmlenc-core-20021210]. Atom Processors should be aware of the potential for spoofing attacks where the attacker publishes an atom:entry with the atom:id value of an entry from another feed, perhaps with a falsified atom:source element duplicating the atom:id of the other feed.
 ietf-atompub-protocol-15.txt (10597 words) The Atom Protocol does not specify a means to create multiple representations of the same Resource (for example a PNG and a JPG of the same image) either on creation or editing. Atom processors that do not recognize the "type" parameter MUST ignore its value and examine the root element to determine the document type. Security Considerations The Atom Publishing Protocol is based on HTTP and thus subject to the security considerations found in Section 15 of [RFC2616].
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