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| {{Infobox atom}}
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| The '''atom''' is a basic unit of [[matter]] that consists of a dense central [[atomic nucleus|nucleus]] surrounded by a [[electron cloud|cloud]] of [[electric charge|negatively charged]] [[electrons]]. The [[atomic nucleus]] contains a mix of positively charged [[proton]]s and electrically neutral [[neutron]]s (except in the case of [[hydrogen-1]], which is the only stable [[nuclide]] with no neutrons). The electrons of an atom are bound to the nucleus by the [[electromagnetic force]]. Likewise, a group of atoms can remain bound to each other by [[chemical bond]]s based on the same force, forming a [[molecule]]. An atom containing an equal number of protons and electrons is electrically neutral, otherwise it is positively or negatively charged and is known as an [[ion]]. An atom is [[Periodic table|classified]] according to the number of protons and neutrons in its nucleus: the [[atomic number|number of protons]] determines the [[chemical element]], and the [[neutron number|number of neutrons]] determines the [[isotope]] of the element.<ref name=leigh1990/>
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| Chemical atoms, which in science now carry the simple name of "atom," are minuscule objects with diameters of a few tenths of a [[nanometer]] and tiny masses proportional to the volume implied by these dimensions. Atoms can only be observed individually using special instruments such as the [[scanning tunneling microscope]]. Over 99.94% of an atom's mass is concentrated in the nucleus,<ref group=note>In the case of hydrogen-1, with a single electron and nucleon, the proton is <math>\begin{smallmatrix}\frac{1836}{1837} \approx 0.99946\end{smallmatrix}</math>, or 99.946% of the total atomic mass. All other nuclides (isotopes of hydrogen and all other elements) have more nucleons than electrons, so the fraction of mass taken by the nucleus is significantly closer to 100% for all of these types of atoms, than for hydrogen-1.</ref> with protons and neutrons having roughly equal mass. Each element has at least one isotope with an unstable nucleus that can undergo [[radioactive decay]]. This can result in a [[Nuclear transmutation|transmutation]] that changes the number of protons or neutrons in a nucleus.<ref name=slac_20090615/> Electrons that are bound to atoms possess a set of stable [[energy level]]s, or [[Atomic orbital|orbitals]], and can undergo transitions between them by absorbing or emitting [[photon]]s that match the energy differences between the levels. The electrons determine the chemical properties of an element, and strongly influence an atom's [[Magnetism|magnetic]] properties. The principles of [[quantum mechanics]] have been successfully used to [[Scientific modelling|model]] the observed properties of the atom.
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| ==Etymology==
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| The name atom comes from the [[Greek language|Greek]] [[wikt:ἄτομος#Ancient Greek|ἄτομος]] (''atomos'', "indivisible") from ''[[wikt:ἀ-#Ancient Greek|ἀ-]]'' (''a-'', "not") and ''[[wikt:τέμνω#Ancient Greek|τέμνω]]'' (''temnō'', "I cut"),<ref name=liddell_scott_to_cut/> which means uncuttable, or indivisible, something that cannot be divided further.<ref name=liddell_scott_uncuttable/> The concept of an atom as an indivisible component of matter was first proposed by early [[Indian philosophy|Indian]] and [[Greek philosophy|Greek]] philosophers. In the 18th and 19th centuries, [[chemist]]s provided a physical basis for this idea by showing that certain substances could not be further broken down by chemical methods, and they applied the ancient philosophical name of ''atom'' to the chemical entity. During the late 19th and early 20th centuries, [[physicist]]s discovered subatomic components and structure inside the atom, thereby demonstrating that the chemical "atom" was divisible and that the name might not be appropriate.<ref name=haubold_mathai1998/>{{sfn|Harrison|2003|pp=123–139}} However, it was retained. This has led to some debate about whether the ancient philosophers, who intended to refer to fundamental individual objects with their concept of "atoms," were referring to modern chemical atoms, or something more like indivisible subatomic particles such as [[lepton]]s or [[quark]]s, or even some more fundamental particle that has yet to be discovered.<ref>{{cite book| title = The God Particle: If the Universe is the Answer, What is the Question? | year = 1993, reprint in 2006| publisher = [[Houghton Mifflin Company]]| location=Boston| author = Leon M. Lederman and Dick Teresi|isbn=0-618-71168-6|url=http://books.google.com/books?id=-v84Bp-LNNIC&printsec=frontcover}} Lederman provides an excellent discussion of this point, and this debate.</ref>
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| ==History of atomic theory==
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| {{Main|Atomic theory}}
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| ===Atomism===
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| {{Main|Atomism}}
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| The concept that matter is composed of [[wikt:discrete|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 [[experiment]]ation and [[Empirical|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.{{sfn|Ponomarev|1993|pp=14–15}} The ancient name of "atom" from atomism had already been nearly universally used to describe chemical atoms by that time, and it was therefore retained as a term, long after chemical atoms were found to be divisible, and even after smaller, [[elementary particle|truly indivisible particles]] were identified.
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| References to the concept of atoms date back to ancient [[History of Greece|Greece]] and [[History of India|India]]. In India, the [[Ājīvika]], [[Jain]], and [[Cārvāka]] schools of atomism may date back to the 6th century BCE.{{sfn|McEvilley|2002|p=317}} The [[Nyaya]] and [[Vaisheshika]] schools later developed theories on how atoms combined into more complex objects.{{sfn|King|1999|pp=105–107}} In the West, the references to atoms emerged in the 5th century BCE with [[Leucippus]], whose student, [[Democritus]], systematized his views. In approximately 450 BCE, Democritus coined the term ''átomos'' ({{lang-el|ἄτομος}}), which means "uncuttable" or "the smallest indivisible particle of matter". Although the [[Atomism#Indian atomism|Indian]] and [[Atomism#Greek atomism|Greek concepts]] of the atom were based purely on philosophy, modern science has retained the name coined by [[Democritus]].{{sfn|Ponomarev|1993|pp=14–15}}
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| [[Corpuscularianism]] is the postulate, expounded
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| in the 13th-century by the [[alchemy|alchemist]] [[Pseudo-Geber]] (Geber),{{sfn|Moran|2005|p=146}} sometimes identified with [[Paul of Taranto]], that all physical bodies possess an inner and outer layer of minute [[particle]]s or corpuscles.{{sfn|Levere|2001|p=7}} Corpuscularianism is similar to the theory of atomism, except that where atoms were supposed to be indivisible, corpuscles could in principle be divided. In this manner, for example, it was theorized that [[Mercury (element)|mercury]] could penetrate into metals and modify their inner structure.<ref name=pratt20070928/> Corpuscularianism stayed a dominant theory over the next several hundred years.
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| In 1661, [[Natural philosophy|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 element]]s of air, earth, fire and water.{{sfn|Siegfried|2002|pp=42–55}} During the 1670s corpuscularianism was used by [[Isaac Newton]] in his development of the [[corpuscular theory of light]].{{sfn|Levere|2001|p=7}}<ref name=kemerling20020808/>
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| ===Origin of scientific theory===
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| [[File:Daltons symbols.gif|right|thumb|Various atoms and molecules as depicted in [[John Dalton]]'s ''A New System of Chemical Philosophy'' (1808), one of the earliest scientific works on atomic theory]]
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| Further progress in the understanding of atoms did not occur until the science of [[chemistry]] began to develop. In 1789, French nobleman and scientific researcher [[Antoine Lavoisier]] discovered the [[law of conservation of mass]] and defined an [[Chemical element|element]] as a basic substance that could not be further broken down by the methods of chemistry.<ref name=lavoisier_eoc/>
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| In 1805, English instructor and natural philosopher [[John Dalton]] used the concept of atoms to explain why elements always react in ratios of small [[Natural number|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 can join together to form chemical compounds.{{sfn|Wurtz|1881|pp=1–2}}{{sfn|Dalton|1808}} Dalton is considered the originator of modern [[atomic theory]].{{sfn|Roscoe|1895|pp=129}}
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| Dalton's atomic hypothesis did not specify the size of atoms. Common sense indicated they must be very small, but nobody knew how small. Therefore it was a major landmark when in 1865 [[Johann Josef Loschmidt]] measured the size of the molecules that make up air.
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| An additional line of reasoning in support of particle theory (and by extension [[atomic theory]]) began in 1827 when [[Botany|botanist]] [[Robert Brown (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.<ref name=adp322_8_549/>{{sfn|Mazo|2002|pp=1–7}}<ref name=lee_hoon1995/> French physicist [[Jean Perrin]] used Einstein's work to experimentally determine the mass and dimensions of atoms, thereby conclusively verifying Dalton's atomic theory.<ref name=e31_2_50/>
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| [[File:Mendeleev's 1869 periodic table.png|thumb|left|[[Dmitri Mendeleev|Mendeleev's]] first periodic table (1869)]]
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| In 1869, building upon earlier discoveries by such scientists as Lavoisier, [[Dmitri Mendeleev]] published the first functional [[periodic table]].<ref name=pte20071101/> The table itself is a visual representation of the periodic law, which states that certain chemical properties of [[chemical element|elements]] repeat ''periodically'' when arranged by [[atomic number]].{{sfn|Scerri|2007|pp=10–17}}
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| ===Subcomponents and quantum theory===
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| [[File:Atom diagram.png|thumb|right|A generic atomic planetary model, or the [[Rutherford model]]]]
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| The physicist [[J. J. Thomson]], through his work on [[cathode ray]]s in 1897, discovered the electron, and concluded that they were a component of every atom. Thus he overturned the belief that atoms are the indivisible, ultimate particles of matter.<ref name=nobel1096/> Thomson postulated that the low mass, negatively charged electrons were distributed throughout the atom, possibly rotating in rings, with their charge balanced by the presence of a uniform sea of positive charge. This later became known as the [[plum pudding model]].
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| In 1909, [[Hans Geiger]] and [[Ernest Marsden]], under the direction of physicist [[Ernest Rutherford]], bombarded a sheet of gold foil with [[alpha particle|alpha rays]]—by then known to be positively charged helium atoms—and discovered that a small percentage of these particles 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 a heavy gold atom and most of its mass was concentrated in a nucleus at the center of the atom—the [[Rutherford model]].<ref name=pm21_669/>
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| While experimenting with the products of [[radioactive decay]], in 1913 [[radiochemistry|radiochemist]] [[Frederick Soddy]] discovered that there appeared to be more than one type of atom at each position on the periodic table.<ref name=npc1921/> The term [[isotope]] was coined by [[Margaret Todd (doctor)|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 isotope]]s.<ref name=prsA_89_1_1913/>
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| [[File:Bohr Model.svg|left|thumb|A [[Bohr model]] of the hydrogen atom, showing an electron jumping between fixed orbits and emitting a [[photon]] of energy with a specific frequency]]
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| Meanwhile, in 1913, physicist [[Niels Bohr]] suggested that the electrons were confined into clearly defined, quantized orbits, and could jump between these, but could not freely spiral inward or outward in intermediate states.<ref name=stern20050516/> An electron must absorb or emit specific amounts of energy to transition between these fixed orbits. When the [[light]] from a heated material was passed through a [[Prism (optics)|prism]], it produced a multi-colored [[spectrum]]. The appearance of fixed [[Spectral line|lines in this spectrum]] was successfully explained by these orbital transitions.<ref name=bohr19221211/>
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| Later in the same year [[Henry Moseley]] provided additional experimental evidence in favor of [[Bohr model|Niels Bohr's theory]]. These results refined [[Ernest Rutherford]]'s and [[Antonius Van den Broek]]'s model, which proposed that the atom contains in its [[atomic nucleus|nucleus]] a number of positive [[nuclear charge]]s that is equal to its (atomic) number in the periodic table. Until these experiments, [[atomic number]] 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.{{sfn|Pais|1986|pp=228–230}}
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| [[Chemical bond]]s between atoms were now explained, by [[Gilbert Newton Lewis]] in 1916, as the interactions between their constituent electrons.<ref name=jacs38_4_762/> As the [[Chemical property|chemical properties]] of the elements were known to largely repeat themselves according to the [[periodic law]],{{sfn|Scerri|2007|pp=205–226}} in 1919 the American chemist [[Irving Langmuir]] 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 [[electron shell]]s about the nucleus.<ref name=jacs41_6_868/>
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| The [[Stern–Gerlach experiment]] of 1922 provided further evidence of the quantum nature of the atom. When a beam of silver atoms was passed through a specially shaped magnetic field, the beam was split based on the direction of an atom's angular momentum, or spin. As this direction is random, the beam could be expected to spread into a line. Instead, the beam was split into two parts, depending on whether the atomic spin was oriented up or down.<ref name=fop17_6_575/>
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| In 1924, [[Louis de Broglie]] proposed that all particles behave to an extent like waves. In 1926, [[Erwin Schrödinger]] used this idea to develop a mathematical model of the atom that described the electrons as three-dimensional [[waveform]]s rather than point particles. A consequence of using waveforms to describe particles is that it is mathematically impossible to obtain precise values for both the [[Point (geometry)|position]] and [[momentum]] of a particle at the same time; this became known as the [[uncertainty principle]], formulated by [[Werner Heisenberg]] in 1926. 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. This model was able to explain observations of atomic behavior that previous models could not, such as certain structural and [[Spectral line|spectral]] patterns of atoms larger than hydrogen. Thus, the planetary model of the atom was discarded in favor of one that described [[atomic orbital]] zones around the nucleus where a given electron is most likely to be observed.<ref name=brown2007/><ref name=harrison2000/>
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| [[File:Mass Spectrometer Schematic.svg|right|thumb|Schematic diagram of a simple mass spectrometer]]
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| The development of the [[mass spectrometry|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 show that isotopes had different masses. The [[atomic mass]] of these isotopes varied by integer amounts, called the [[whole number rule]].<ref name=pm39_6_449/> The explanation for these different 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.<ref name=chadwick1935/>
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| ===Fission, high-energy physics and condensed matter===
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| In 1938, the German chemist [[Otto Hahn]], a student of Rutherford, directed neutrons onto uranium atoms expecting to get [[transuranium element]]s. Instead, his chemical experiments showed [[barium]] as a product.<ref name=hahn_meitner_strassmann/> A year later, [[Lise Meitner]] and her nephew [[Otto Frisch]] verified that Hahn's result were the first experimental ''nuclear fission''.<ref name=nature143_3615_239/><ref name=schroeder/> In 1944, Hahn received the [[Nobel prize]] in chemistry. Despite Hahn's efforts, the contributions of Meitner and Frisch were not recognized.<ref name=pt50_9_26/>
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| In the 1950s, the development of improved [[particle accelerator]]s and [[particle detector]]s allowed scientists to study the impacts of atoms moving at high energies.<ref name=kullander2001/> Neutrons and protons were found to be [[hadron]]s, or composites of smaller particles called [[quark]]s. The [[standard model of particle physics]] 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.<ref name=npp1990/>
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| ===Subatomic particles===
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| {{Main|Subatomic particle}}
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| 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 particle]]s. The constituent particles of an atom are the [[electron]], the [[proton]] and the [[neutron]]; all three are [[fermion]]s. However, the [[hydrogen|hydrogen-1]] atom has no neutrons and the [[hydron (chemistry)|hydron ion]] has no electrons.
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| The electron is by far the least massive of these particles at {{val|9.11|e=-31|u=kg}}, with a negative [[Electric charge|electrical charge]] and a size that is too small to be measured using available techniques.{{sfn|Demtröder|2002|pp=39–42}} It is the lightest particle with a positive rest mass measured. Under ordinary conditions, electrons are bound to the positively charged nucleus by the attraction created from opposite electric charges. If an atom have more or fewer electrons than its atomic number, then is become respectively negatively or positively charged as a whole; a charged atom is called [[ion]]. Electrons are known since the late 19th century, mostly thanks to [[J.J. Thomson]]; see [[history of subatomic physics]] for details.
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| Protons have a positive charge and a mass 1,836 times that of the electron, at {{val|1.6726|e=-27|u=kg}}. The number of protons in an atom is called its [[atomic number]]. [[Ernest Rutherford]] (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 [[proton]].
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| Neutrons have no electrical charge and have a free mass of 1,839 times the mass of the electron,{{sfn|Woan|2000|p=8}} or {{val|1.6929|e=-27|u=kg}}, the heaviest of the three constituent particles, but it can be reduced by the [[nuclear binding energy]]. Neutrons and protons (collectively known as [[nucleon]]s) have comparable dimensions—on the order of {{val|2.5|e=-15|u=m}}—although the 'surface' of these particles is not sharply defined.{{sfn|MacGregor|1992|pp=33–37}} The neutron was discovered in 1932 by the English physicist [[James Chadwick]].
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| In the [[Standard Model]] of physics, electrons are truly elementary particles with no internal structure. However, both protons and neutrons are composite particles composed of [[elementary particle]]s called [[quark]]s. There are two types of quarks in atoms, each having a fractional electric charge. Protons are composed of two [[up quark]]s (each with charge +{{frac|2|3}}) and one [[down quark]] (with a charge of −{{frac|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.<ref name=pdg2002/><ref name=schombert2006/>
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| The quarks are held together by the [[strong interaction]] (or strong force), which is mediated by [[gluon]]s. The protons and neutrons, in turn, are held to each other in the nucleus by the [[nuclear force]], 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 [[gauge boson]]s, which are elementary particles that mediate physical forces.<ref name=pdg2002/><ref name=schombert2006/>
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| ===Nucleus===
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| {{Main|Atomic nucleus}}
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| [[File:Binding energy curve - common isotopes.svg|thumb|350px|The [[binding energy]] needed for a nucleon to escape the nucleus, for various isotopes]]<!-- A brief explanation is provided here because 'binding energy' is not explained until the end of the setion. -->
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| All the bound protons and neutrons in an atom make up a tiny [[atomic nucleus]], and are collectively called [[nucleon]]s. The radius of a nucleus is approximately equal to 1.07 {{radic|''A''|3}} [[femtometre|fm]], where ''A'' is the total number of nucleons.{{sfn|Jevremovic|2005|p=63}} This is much smaller than the radius of the atom, which is on the order of 10<sup>5</sup> 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.{{sfn|Pfeffer|2000|pp=330–336}}
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| Atoms of the same [[chemical element|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 total number of protons and neutrons determine the [[nuclide]]. The number of neutrons relative to the protons determines the stability of the nucleus, with certain isotopes undergoing [[radioactive decay]].<ref name=wenner2007/>
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| The proton, the electron, and the neutron are classified as [[fermion]]s. Fermions obey the [[Pauli exclusion principle]] which prohibits ''[[identical particles|identical]]'' 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. However, a proton and a neutron are allowed to occupy the same quantum state.<ref name="raymond"/>
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| For atoms with low atomic numbers, a nucleus that has more neutrons than protons tends to drop to a lower energy state through radioactive decay so that the [[neutron-proton ratio]] is closer to one. However, as the atomic number increases, a higher proportion of neutrons is required to offset the mutual repulsion of the protons. Thus, there are no stable nuclei with equal proton and neutron numbers above atomic number ''Z'' = 20 (calcium) and as ''Z'' increases, the neutron-proton ratio of stable isotopes increases.<ref name="raymond"/> The stable isotope with the highest proton-neutron ration is [[lead-208]] (about 1.5).
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| [[File:Wpdms physics proton proton chain 1.svg|right|thumb|Illustration of a nuclear fusion process that forms a deuterium nucleus, consisting of a proton and a neutron, from two protons. A [[positron]] (e<sup>+</sup>)—an [[antimatter]] electron—is emitted along with an electron [[neutrino]].]]
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| 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. For example, 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.<ref name=mihos2002/> [[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. If this modifies the number of protons in a nucleus, the atom changes to a different chemical element.<ref name=lbnl20070330/><ref name=makhijani_saleska2001/>
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| 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 [[gamma ray]], or the kinetic energy of a [[beta particle]]), as described by [[Albert Einstein]]'s [[mass–energy equivalence]] formula, ''E'' = ''mc''<sup>2</sup>, where ''m'' is the mass loss and ''c'' is the [[speed of light]]. This deficit is part of the [[binding energy]] 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.{{sfn|Shultis|Faw|2002|pp=10–17}}
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| The fusion of two nuclei that create larger nuclei with lower atomic numbers than [[iron]] and [[nickel]]—a total nucleon number of about 60—is usually an [[exothermic reaction|exothermic process]] that releases more energy than is required to bring them together.<ref name=ajp63_7_653/> It is this energy-releasing process that makes nuclear fusion in [[star]]s a self-sustaining reaction. For heavier nuclei, the binding energy per [[nucleon]] in the nucleus begins to decrease. That means fusion processes producing nuclei that have atomic numbers higher than about 26, and [[atomic mass]]es higher than about 60, is an [[endothermic reaction|endothermic process]]. These more massive nuclei can not undergo an energy-producing fusion reaction that can sustain the [[hydrostatic equilibrium]] of a star.<ref name="raymond"/>
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| ===Electron cloud===
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| {{Main|Atomic orbital|Electron configuration}}
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| [[File:Potential energy well.svg|right|thumb|A potential well, showing, according to [[classical mechanics]], 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 ''x''<sub>1</sub> and ''x''<sub>2</sub>.]]
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| 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 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.
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| Electrons, like other particles, have properties of both a [[Wave–particle duality|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 appears to be at a particular location when its position is measured.<ref name=science157_3784_13/> Only a discrete (or [[wikt:quantize|quantized]]) set of these orbitals exist around the nucleus, as other possible wave patterns rapidly decay into a more stable form.<ref name=Brucat2008/> Orbitals can have one or more ring or node structures, and they differ from each other in size, shape and orientation.<ref name=manthey2001/>
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| [[File:S-p-Orbitals.svg|left|thumb|Wave functions of the first five atomic orbitals. The three 2p orbitals each display a single angular [[Node (physics)|node]] that has an orientation and a minimum at the center.]] [[File:Atomic orbitals and periodic table construction.ogv|left|thumb|How atoms are constructed from electron orbitals and link to the periodic table]]
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| 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 line]]s.<ref name=Brucat2008/>
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| The amount of energy needed to remove or add an electron—the [[electron binding energy]]—is far less than the [[binding energy|binding energy of nucleons]]. For example, it requires only 13.6 eV to strip a [[Stationary state|ground-state]] electron from a hydrogen atom,<ref name=herter_8/> compared to 2.23 ''million'' eV for splitting a [[deuterium]] nucleus.<ref name=pr79_2_282/> Atoms are [[electric charge|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 [[ion]]s. 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 [[chemical bond|bond]] into [[molecule]]s and other types of [[chemical compound]]s like [[Ionic crystal|ionic]] and [[Covalent bond|covalent]] network [[Crystallization|crystals]].{{sfn|Smirnov|2003|pp=249–272}}
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| ==Properties==
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| ===Nuclear properties===
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| {{Main|Isotope|Stable isotope|List of nuclides|List of elements by stability of isotopes}}
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| By definition, any two atoms with an identical number of ''protons'' in their nuclei belong to the same [[chemical element]]. 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 ([[hydrogen-1]], by far the most common form,<ref name=matis2000/> also called protium), one neutron ([[deuterium]]), two neutrons ([[tritium]]) and [[isotopes of hydrogen|more than two neutrons]]. The known elements form a set of atomic numbers, from the single proton element [[hydrogen]] up to the 118-proton element [[ununoctium]].<ref name=weiss20061017/> All known isotopes of elements with atomic numbers greater than 82 are radioactive.{{sfn|Sills|2003|pp=131–134}}<ref name=dume20030423/>
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| About 339 nuclides occur naturally on [[Earth]],<ref name=lidsay20000730/> of which 254 (about 75%) have not been observed to decay, and are referred to as "[[stable isotope]]s". However, only 90 of these nuclides are stable to all decay, even [[list of nuclides|in theory]]. Another 164 (bringing the total to 254) 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 80 million years, and are long-lived enough to be present from the birth of the [[solar system]]. This collection of 288 nuclides are known as [[primordial nuclide]]s. Finally, an additional 51 short-lived nuclides are known to occur naturally, as daughter products of primordial nuclide decay (such as [[radium]] from [[uranium]]), or else as products of natural energetic processes on Earth, such as cosmic ray bombardment (for example, carbon-14).<ref name=tuli2005/><ref group=note>For more recent updates see [http://www.nndc.bnl.gov/chart Interactive Chart of Nuclides (Brookhaven National Laboratory)].</ref><!-- See article [[list of nuclides]]. The numbers are derived by [[WP:CALC]] (counting the table), which is not [[WP:OR]]-->
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| For 80 of the chemical elements, at least one [[stable isotope]] 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 [[tin]]. Elements [[technetium|43]], [[promethium|61]], and all elements numbered [[bismuth|83]] or higher have no stable isotopes.<ref name=CRC>CRC Handbook (2002).</ref>{{Page needed|date=April 2011}}
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| 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 [[Nuclear shell model|shell model]] of the nucleus; filled shells, such as the filled shell of 50 protons for tin, confers unusual stability on the nuclide. Of the 254 known stable nuclides, only four have both an odd number of protons ''and'' odd number of neutrons: [[hydrogen-2]] ([[deuterium]]), [[lithium-6]], [[boron-10]] and [[nitrogen-14]]. Also, only four naturally occurring, radioactive odd-odd nuclides have a half-life over a billion years: [[potassium-40]], [[vanadium-50]], [[lanthanum-138]] and [[tantalum-180m]]. Most odd-odd nuclei are highly unstable with respect to [[beta decay]], because the decay products are even-even, and are therefore more strongly bound, due to [[Semi-empirical mass formula#Pairing term|nuclear pairing effects]].<ref name=CRC/>{{Page needed|date=April 2011}}
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| ===Mass===
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| {{Main|Atomic mass|mass number}}
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| 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 [[mass number]]. The mass number is a simple whole number, and has units of "nucleons." An example of use of a mass number is "carbon-12," which has 12 nucleons (six protons and six neutrons).
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| The actual [[Invariant mass|mass of an atom at rest]] is often expressed using the [[Atomic mass unit|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 {{val|1.66|e=-27|u=kg}}.<ref name=iupac/> [[hydrogen atom|Hydrogen-1]], the lightest isotope of hydrogen and the atom with the lowest mass, has an atomic weight of 1.007825 u.<ref name=chieh2001/> The value of this number is called the [[atomic mass]]. A given atom has an atomic mass approximately equal (within 1%) to its mass number times the mass of the atomic mass unit. However, this number will not be an exact whole number except in the case of carbon-12 (see below).<ref name=nist_wc/> The heaviest [[stable atom]] is lead-208,{{sfn|Sills|2003|pp=131–134}} with a mass of {{val|207.9766521|u=u}}.<ref name=audi2003/>
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| As even the most massive atoms are far too light to work with directly, chemists instead use the unit of [[Mole (unit)|moles]]. One mole of atoms of any element always has the same number of atoms (about [[Avogadro constant|{{val|6.022|e=23}}]]). 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 [[Atomic mass unit|unified atomic mass unit]], each carbon-12 atom has an atomic mass of exactly 12 u, and so a mole of carbon-12 atoms weighs exactly 0.012 kg.<ref name=iupac>Mills ''et al.'' (1993).</ref>{{Page needed|date=April 2011}}<!--the reference given is available online at http://old.iupac.org/publications/books/gbook/green_book_2ed.pdf for anyone who wants to check whether this paragraph constitutes original research-->
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| ===Shape and size===
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| {{Main|Atomic radius}}
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| Atoms lack a well-defined outer boundary, so their dimensions are usually described in terms of an [[atomic radius]]. This is a measure of the distance out to which the electron cloud extends from the nucleus.{{citation needed|date=January 2014}} However, 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 [[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 mechanics|quantum mechanical]] property known as [[Spin (physics)|spin]].<ref name=aca32_5_751/> 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).<ref name=dong1998/> Consequently, the smallest atom is helium with a radius of 32 [[Picometre|pm]], while one of the largest is [[caesium]] at 225 pm.<ref>Zumdahl (2002).</ref>
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| When subjected to external fields, like an [[electrical field]], the shape of an atom may deviate from [[spherical symmetry]]. The deformation depends on the field magnitude and the orbital type of outer shell electrons, as shown by [[group theory|group-theoretical]] considerations. Aspherical deviations might be elicited for instance in [[crystal]]s, where large crystal-electrical fields may occur at [[crystal symmetry|low-symmetry]] lattice sites.<ref name=adp5f_3_133/> Significant [[ellipsoid]]al deformations have recently been shown to occur for sulfur ions in [[pyrite]]-type compounds.<ref name=pssb245_9_1858/>
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| Atomic dimensions are thousands of times smaller than the wavelengths of [[light]] (400–700 [[nanometre|nm]]) so they can not be viewed using an [[optical microscope]]. However, individual atoms can be observed using a [[scanning tunneling microscope]]. To visualize the minuteness of the atom, consider that a typical human hair is about 1 million carbon atoms in width.<ref name=osu2007/> A single drop of water contains about 2 [[sextillion]] ({{val|2|e=21}}) atoms of oxygen, and twice the number of hydrogen atoms.<ref>Padilla ''et al.'' (2002:32)—"There are 2,000,000,000,000,000,000,000 (that's 2 sextillion) atoms of oxygen in one drop of water—and twice as many atoms of hydrogen."</ref> A single [[Carat (unit)|carat]] [[diamond]] with a mass of {{val|2|e=-4|u=kg}} contains about 10 sextillion (10<sup>22</sup>) atoms of [[carbon]].<ref group=note>A carat is 200 milligrams. [[Atomic mass unit|By definition]], carbon-12 has 0.012 kg per mole. The [[Avogadro constant]] defines {{val|6|e=23}} atoms per mole.</ref> 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.{{sfn|Feynman|1995|p=5}}
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| ===Radioactive decay===
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| {{Main|Radioactive decay}}
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| [[File:Isotopes and half-life.svg|right|300px|thumb|This diagram shows the [[half-life]] (T<sub>½</sub>) of various isotopes with Z protons and N neutrons.]]
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| 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.<ref name=splung/>
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| The most common forms of radioactive decay are:{{sfn|L'Annunziata|2003|pp=3–56}}<ref name=firestone20000522/>
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| * [[Alpha decay]]: this process 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]].
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| * [[Beta decay]] (and [[electron capture]]): these processes are regulated by the [[weak force]], and result from a transformation of a neutron into a proton, or a proton into a neutron. The neutron to proton transition is accompanied by the emission of an electron and an [[antineutrino]], while proton to neutron transition (except in electron capture) 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. Electron capture is more common than positron emission, because it requires less energy. In this type of decay, an electron is absorbed by the nucleus, rather than a positron emitted from the nucleus. A [[neutrino]] is still emitted in this process, and a proton changes to a neutron.
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| * [[Gamma decay]]: this process results from a change in the energy level of the nucleus to a lower state, resulting in the emission of electromagnetic radiation. The excited state of a nucleus which results in gamma emission usually occurs following the emission of an alpha or a beta particle. Thus, gamma decay usually follows alpha or beta decay.
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| Other more rare types of [[radioactive decay]] include ejection of neutrons or protons or clusters of [[nucleon]]s from a nucleus, or more than one [[beta particle]]. An analog of gamma emission which allows excited nuclei to lose energy in a different way, is [[internal conversion]]— 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 [[nuclear fission]].
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| 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 is present, and so forth.<ref name=splung/>
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| ===Magnetic moment===
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| {{Main|Electron magnetic dipole moment|Nuclear magnetic moment}}
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| Elementary particles possess an intrinsic quantum mechanical property known as [[Spin (physics)|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]] (ħ), with electrons, protons and neutrons all having spin ½ ħ, or "spin-½". In an atom, electrons in motion around the [[Atomic nucleus|nucleus]] possess orbital [[angular momentum]] in addition to their spin, while the nucleus itself possesses angular momentum due to its nuclear spin.<ref name=hornak2006/>
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| 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.<ref name=schroeder2/>
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| In [[Ferromagnetism|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 known as an [[exchange interaction]]. When the magnetic moments of ferromagnetic atoms are lined up, the material can produce a measurable macroscopic field. [[Paramagnetism|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.<ref name=schroeder2/><ref name=goebel20070901/>
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| 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|xenon-129]]) it is possible to [[spin polarization|polarize]] a significant proportion of the nuclear spin states so that they are aligned in the same direction—a condition called [[hyperpolarization (physics)|hyperpolarization]]. This has important applications in [[magnetic resonance imaging]].<ref name=yarris1997/>{{sfn|Liang|Haacke|1999|pp=412–426}}
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| ===Energy levels===
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| [[Image:Atomic orbital energy levels.svg|thumb|right|These electron's energy levels (not to scale) are sufficient for ground states of atoms up to [[cadmium]] (5s<sup>2</sup> 4d<sup>10</sup>) inclusively. Do not forget that even the top of the diagram is lower than an unbound electron state.]]
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| The [[potential energy]] of an electron in an atom is [[negative number|negative]], its dependence of its [[position (vector)|position]] reaches the [[minimum]] (the most [[absolute value]]) inside the nucleus, and vanishes when the [[distance]] from the nucleus [[limit at infinity|goes to infinity]], roughly in an [[inverse proportion]] to the distance. In the quantum-mechanical model, a bound electron can only occupy a set of [[quantum state|states]] centered on the nucleus, and each state corresponds to a specific [[energy level]]; see [[time-independent Schrödinger equation]] for theoretical explanation. An energy level can be measured by the [[ionization potential|amount of energy needed to unbind]] the electron from the atom, and is usually given in units of [[electronvolt]]s (eV). The lowest energy state of a bound electron is called the ground state,{{clarify|date=February 2013}} while an electron transition to a higher level results in an excited state.<ref name=zeghbroeck1998/> The electron's energy raises when ''[[principal quantum number|n]]'' increases because the (average) distance to the nucleus increases. Dependence of the energy on [[azimuthal quantum number|{{ell}}]] is caused not by [[electrostatic potential]] of the nucleus, but by interaction between electrons.
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| For an electron to [[atomic electron transition|transition between two different states]], it must{{citation needed|date=February 2013}} 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]].<ref>Fowles (1989:227–233).</ref> 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.<ref name=martin2007/>
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| [[File:Fraunhofer lines.svg|right|thumb|300px|An example of absorption lines in a spectrum]]
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| When a continuous [[electromagnetic spectrum|spectrum of energy]] is passed through a gas or [[plasma (physics)|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 [[absorption band]]s 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 [[emission line]]s from the photons emitted by the atoms.) [[Spectroscopy|Spectroscopic]] measurements of the strength and width of [[atomic spectral line]]s allow the composition and physical properties of a substance to be determined.<ref name=avogadro/>
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| 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.<ref name=fitzpatrick20070216/> 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 configuration]]s 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.<ref name=weiss2001/> 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]].{{sfn|Beyer|2003|pp=232–236}}
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| 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 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 [[laser]]s, which can emit a coherent beam of light energy in a narrow frequency band.<ref name=watkins_sjsu/>
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| ===Valence and bonding behavior===
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| {{Main|Valence (chemistry)|Chemical bond}}
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| The outermost electron shell of an atom in its uncombined state is known as the [[valence shell]], and the electrons in
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| that shell are called [[valence electron]]s. The number of valence electrons determines the [[chemical bond|bonding]]
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| behavior with other atoms. Atoms tend to [[Chemical reaction|chemically react]] with each other in a manner that fills (or empties) their outer valence shells.<ref name=reusch20070716/> 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 [[sodium chloride]] and other chemical ionic salts. However, many elements display multiple valences, or tendencies to share differing numbers of electrons in different compounds. Thus, [[chemical bond]]ing between these elements takes many forms of electron-sharing that are more than simple electron transfers. Examples include the element carbon and the [[organic compounds]].<ref name=chemguide/>
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| The [[chemical element]]s 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 gas]]es.<ref name=husted20031211/><ref name=baum2003/>
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| ===States===
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| {{Main|State of matter|Phase (matter)}}
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| [[File:Bose Einstein condensate.png|left|thumb|Snapshots illustrating the formation of a [[Bose–Einstein condensate]]]]
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| 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 [[solid]]s, [[liquid]]s, [[gas]]es and plasmas.
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| {{sfn|Goodstein|2002|pp=436–438}} Within a state, a material can also exist in different [[allotropes]]. An example of this is solid carbon, which can exist as [[graphite]] or [[diamond]].<ref name=pu49_7_719/> Gaseous allotropes exist as well, such as [[dioxygen]] and [[ozone]].
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| 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.{{sfn|Myers|2003|p=85}}<ref name=nist_bec/> This super-cooled collection of atoms
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| then behaves as a single [[super atom]], which may allow fundamental checks of quantum mechanical behavior.<ref name=colton_fyffe1999/>
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| {{Clear}}
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| ==Identification==
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| [[File:Atomic resolution Au100.JPG|right|thumb|[[Scanning tunneling microscope]] image showing the individual atoms making up this [[gold]] ([[Miller index|100]]) surface. [[Surface reconstruction|Reconstruction]] causes the surface atoms to deviate from the bulk [[crystal structure]] and arrange in columns several atoms wide with pits between them.]]
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| 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 [[adsorb]]ed 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]].<ref name=jacox1997/><ref name=nf_physics1986/>
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| An atom can be [[ion]]ized 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 spectrometry|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.<ref name=sab53_13_1739/>
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| 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|atom-probe tomograph]] has sub-nanometer resolution in 3-D and can chemically identify individual atoms using time-of-flight mass spectrometry.<ref name=rsi39_1_83/>
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| Spectra of [[excited state]]s can be used to analyze the atomic composition of distant [[star]]s. Specific light [[wavelength]]s 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.<ref name=lochner2007/> [[Helium]] was discovered in this way in the spectrum of the Sun 23 years before it was found on Earth.<ref name=winter2007/>
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| ==Origin and current state==
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| Atoms form about 4% of the total energy density of the [[observable Universe]], with an average density of about 0.25 atoms/m<sup>3</sup>.<ref name=hinshaw20060210/> 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 10<sup>5</sup> to 10<sup>9</sup> atoms/m<sup>3</sup>.{{sfn|Choppin|Liljenzin|Rydberg|2001|p=441}} 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 10<sup>3</sup> atoms/m<sup>3</sup>.<ref name=science259_5093_327/> 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.{{sfn|Lequeux|2005|p=4}} (The remainder of the mass is an unknown [[dark matter]].)<ref name=nigel2000/>
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| ===Formation===
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| Electrons are thought to exist in the Universe since early stages of the [[Big Bang]]. Atomic nuclei forms in [[nucleosynthesis]] reactions. In about three minutes [[Big Bang nucleosynthesis]] produced most of the [[helium]], [[lithium]], and [[deuterium]] in the Universe, and perhaps some of the [[beryllium]] and [[boron]].<ref name=ns1794_42/><ref name=science267_5195_192/><ref name=hinshaw20051215/>
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| Ubiquitousness and stability of atoms relies on their [[binding energy]], which means that an atom has a lower energy than an unbound system of the nucleus and electrons. Where the [[temperature]] is much higher than [[ionization potential]], the matter exists in the form of [[plasma (physics)|plasma]] – a gas of positively-charged ions (possibly, bare nuclei) and electrons. When the temperature drops below the ionization potential, atoms become [[statistical physics|statistically]] favorable. Atoms (complete with bound electrons) became to dominate over [[electric charge|charged]] [[particle]]s 380,000 years after the Big Bang—an epoch called [[recombination (cosmology)|recombination]], when the expanding Universe cooled enough to allow electrons to become attached to nuclei.<ref name=abbott20070530/>
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| Since the Big Bang, which produced no [[carbon]] or [[atomic number|heavier elements]], atomic nuclei have been combined in [[star]]s through the process of [[nuclear fusion]] to produce more of the element [[helium]], and (via the [[triple alpha process]]) the sequence of elements from carbon up to [[iron]];<ref name=mnras106_343/> see [[stellar nucleosynthesis]] for details.
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| Isotopes such as lithium-6, as well as some beryllium and boron are generated in space through [[cosmic ray spallation]].<ref name=nature405_656/> This occurs when a high-energy proton strikes an atomic nucleus, causing large numbers of nucleons to be ejected.
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| Elements heavier than iron were produced in [[supernova]]e through the [[r-process]] and in [[Asymptotic giant branch|AGB stars]] through the [[s-process]], both of which involve the capture of neutrons by atomic nuclei.<ref name=mashnik2000/> Elements such as [[lead]] formed largely through the radioactive decay of heavier elements.<ref name=kgs20050504/>
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| ===Earth===
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| 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]].{{sfn|Manuel|2001|pp=407–430, 511–519}}<ref name=gs190_1_205/> 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]].<ref name=anderson_foulger_meibom2006/>
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| 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.<ref name=pennicott2001/> Some atoms on Earth have been artificially generated either deliberately or as by-products of nuclear reactors or explosions.<ref name=yarris2001/><ref name=pr119_6_2000/> Of the [[Transuranium element|transuranic elements]]—those with atomic numbers greater than 92—only [[plutonium]] and [[neptunium]] occur naturally on Earth.<ref name=poston1998/><ref name=cz97_10_522/> Transuranic elements have radioactive lifetimes shorter than the current age of the Earth{{sfn|Zaider|Rossi|2001|p=17}} and thus identifiable quantities of these elements have long since decayed, with the exception of traces of [[plutonium-244]] possibly deposited by cosmic dust.{{sfn|Manuel|2001|pp=407–430,511–519}} Natural deposits of plutonium and neptunium are produced by [[neutron capture]] in uranium ore.<ref name=ofr_cut/>
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| The Earth contains approximately {{val|1.33|e=50}} atoms.<ref name=weisenberger/> Although small numbers of independent atoms of [[noble gas]]es exist, such as [[argon]], [[neon]], and [[helium]]<!-- note that noble gases exist not only in the atmosphere -->, 99% of [[Earth's atmosphere|the atmosphere]] is bound in the form of molecules, including [[carbon dioxide]] and [[Diatomic molecule|diatomic]] [[oxygen]] and [[nitrogen]]. At the surface of the Earth, an overwhelming majority of atoms combine to form various compounds, including [[water]], [[salt]], [[silicate]]s and [[oxide]]s. Atoms can also combine to create materials that do not consist of discrete molecules, including [[crystal]]s and liquid or solid [[metal]]s.<ref name=pidwirnyf/><ref name=pnas99_22_13966/> This atomic matter forms networked arrangements that lack the particular type of small-scale interrupted order associated with molecular matter.{{sfn|Pauling|1960|pp=5–10}}
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| ===Rare and theoretical forms===
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| ====Superheavy elements====
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| {{Main|Transuranium element}}
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| 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 element]]s may have a nucleus that is relatively stable against radioactive decay.<ref name=cern28509/> The most likely candidate for a stable superheavy atom, [[unbihexium]], has 126 protons and 184 neutrons.<ref name=cen84_10_19/>
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| | |
| ==== Exotic matter ====
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| {{Main|1=Exotic matter}}
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| 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]]. 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 [[baryogenesis]] may offer an explanation. As a result, no antimatter atoms have been discovered in nature.<ref name=koppes1999/><ref name=cromie20010816/> However, in 1996 the antimatter counterpart of the hydrogen atom ([[antihydrogen]]) was synthesized at the [[CERN]] laboratory in [[Geneva]].<ref name=nature419_6906_439/><ref name=BBC20021030/>
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| Other [[exotic atom]]s 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.<ref name=ns1728_77/><ref name=psT112_1_20/><ref name=ripin1998/>
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| {{Category see also|Exotic atoms|LABEL=See here for a list of particles under the}}
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| ==See also==
| |
| {{col-start}}
| |
| {{col-break}}
| |
| * [[History of quantum mechanics]]
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| * [[Infinite divisibility]]
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| * [[List of basic chemistry topics]]
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| * [[Timeline of atomic and subatomic physics]]
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| {{col-break}}
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| * [[Vector model of the atom]]
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| * [[Nuclear model]]
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| * [[Radioactive isotope]]
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| {{col-end}}
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| ==Notes==
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| {{reflist|group="note"|colwidth=30em}}
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| ==References==
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| [[Encyclopedia Britannica]]
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| <!-- this 'empty' section displays references defined elsewhere -->
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| |url=http://oregonstate.edu/terra/2007/02/small-miracles/
| |
| |title=Small Miracles: Harnessing nanotechnology
| |
| |publisher=Oregon State University |accessdate=2007-01-07}}—describes the width of a human hair as {{val|e=5|u=nm}} and 10 carbon atoms as spanning 1 nm.</ref>
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| <ref name=splung>{{cite web |url=http://www.splung.com/content/sid/5/page/radioactivity |title=Radioactivity |publisher=Splung.com |accessdate=2007-12-19| archiveurl= http://web.archive.org/web/20071204135150/http://www.splung.com/content/sid/5/page/radioactivity| archivedate= 4 December 2007 <!--DASHBot-->| deadurl= no}}</ref>
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| <ref name=firestone20000522>{{cite web |last=Firestone|first=Richard B.|date=May 22, 2000 |url=http://isotopes.lbl.gov/education/decmode.html |title=Radioactive Decay Modes |publisher=Berkeley Laboratory |accessdate=2007-01-07}}</ref>
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| <ref name=hornak2006>{{cite web |last=Hornak|first=J. P.|year=2006 |url=http://www.cis.rit.edu/htbooks/nmr/chap-3/chap-3.htm |title=Chapter 3: Spin Physics|work=The Basics of NMR |publisher=Rochester Institute of Technology |accessdate=2007-01-07| archiveurl= http://web.archive.org/web/20070203044312/http://www.cis.rit.edu/htbooks/nmr/chap-3/chap-3.htm| archivedate= 3 February 2007 <!--DASHBot-->| deadurl= no}}</ref>
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| <ref name=goebel20070901>{{cite web |last=Goebel|first=Greg |date=September 1, 2007 |url=http://www.vectorsite.net/tpqm_04.html |title=<nowiki>[4.3]</nowiki> Magnetic Properties of the Atom |work=Elementary Quantum Physics |publisher=In The Public Domain website |accessdate=2007-01-07 }}</ref>
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| <ref name=yarris1997>{{cite journal |last=Yarris|first=Lynn|title=Talking Pictures |journal=Berkeley Lab Research Review |date=Spring 1997 |url=http://www.lbl.gov/Science-Articles/Research-Review/Magazine/1997/story1.html |accessdate=2008-01-09 | archiveurl= http://web.archive.org/web/20080113104939/http://www.lbl.gov/Science-Articles/Research-Review/Magazine/1997/story1.html| archivedate= 13 January 2008 <!--DASHBot-->| deadurl= no}}</ref>
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| <ref name=zeghbroeck1998>{{cite web |last=Zeghbroeck|first=Bart J. Van|year=1998
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| |url=http://physics.ship.edu/~mrc/pfs/308/semicon_book/eband2.htm
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| |title=Energy levels|publisher=Shippensburg University
| |
| |accessdate=2007-12-23| archiveurl = http://web.archive.org/web/20050115030639/http://physics.ship.edu/~mrc/pfs/308/semicon_book/eband2.htm| archivedate = January 15, 2005}}</ref>
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| <ref name=martin2007>{{cite web |last=Martin|first=W. C. |coauthors=Wiese, W. L.|date=May 2007 |url=http://physics.nist.gov/Pubs/AtSpec/ |title=Atomic Spectroscopy: A Compendium of Basic Ideas, Notation, Data, and Formulas |publisher=National Institute of Standards and Technology |accessdate=2007-01-08| archiveurl= http://web.archive.org/web/20070208113156/http://physics.nist.gov/Pubs/AtSpec/| archivedate= 8 February 2007 <!--DASHBot-->| deadurl= no}}</ref>
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| <ref name=avogadro>{{cite web |url=http://www.avogadro.co.uk/light/bohr/spectra.htm |title=Atomic Emission Spectra — Origin of Spectral Lines |publisher=Avogadro Web Site |accessdate=2006-08-10 }}</ref>
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| <ref name=fitzpatrick20070216>{{cite web |last=Fitzpatrick|first=Richard |date=February 16, 2007 |url=http://farside.ph.utexas.edu/teaching/qm/lectures/node55.html |title=Fine structure |publisher=University of Texas at Austin |accessdate=2008-02-14}}</ref>
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| <ref name=weiss2001>{{cite web |last=Weiss|first=Michael|year=2001 |url=http://math.ucr.edu/home/baez/spin/node8.html |title=The Zeeman Effect |publisher=University of California-Riverside |accessdate=2008-02-06 | archiveurl= http://web.archive.org/web/20080202143147/http://math.ucr.edu/home/baez/spin/node8.html| archivedate= 2 February 2008 <!--DASHBot-->| deadurl= no}}</ref>
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| <ref name=watkins_sjsu>{{cite web |last=Watkins |first=Thayer |url=http://www.sjsu.edu/faculty/watkins/stimem.htm |title=Coherence in Stimulated Emission |publisher=San José State University |accessdate=2007-12-23 | archiveurl= http://web.archive.org/web/20080112234014/http://www.sjsu.edu/faculty/watkins/stimem.htm| archivedate= 12 January 2008 <!--DASHBot-->| deadurl= no}}</ref>
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| <ref name=reusch20070716>{{cite web |last=Reusch|first=William|date=July 16, 2007 |url=http://www.cem.msu.edu/~reusch/VirtualText/intro1.htm |title=Virtual Textbook of Organic Chemistry |publisher=Michigan State University |accessdate=2008-01-11}}</ref>
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| <ref name=chemguide>{{cite web |url=http://www.chemguide.co.uk/atoms/bonding/covalent.html |title=Covalent bonding – Single bonds |publisher=chemguide |year=2000}}</ref>
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| <ref name=husted20031211>{{cite web |last=Husted | first=Robert | coauthors=''et al''|date=December 11, 2003 |url=http://periodic.lanl.gov/default.htm |title=Periodic Table of the Elements |publisher=Los Alamos National Laboratory |accessdate=2008-01-11 | archiveurl= http://web.archive.org/web/20080110103232/http://periodic.lanl.gov/default.htm| archivedate= 10 January 2008 <!--DASHBot-->| deadurl= no}}</ref>
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| <ref name=baum2003>{{cite web |first=Rudy|last=Baum|year=2003 |url=http://pubs.acs.org/cen/80th/elements.html |title=It's Elemental: The Periodic Table |publisher=Chemical & Engineering News |accessdate=2008-01-11}}</ref>
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| <ref name=pu49_7_719>{{cite journal |last=Brazhkin|first=Vadim V. |title=Metastable phases, phase transformations, and phase diagrams in physics and chemistry |journal=Physics-Uspekhi |year=2006|volume=49 |issue=7|pages=719–24 |doi=10.1070/PU2006v049n07ABEH006013 |bibcode = 2006PhyU...49..719B }}</ref>
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| <ref name=nist_bec>{{cite news |author=Staff|date=October 9, 2001 |title=Bose-Einstein Condensate: A New Form of Matter |publisher=National Institute of Standards and Technology |url=http://www.nist.gov/public_affairs/releases/BEC_background.htm |accessdate=2008-01-16| archiveurl= http://web.archive.org/web/20080103192918/http://www.nist.gov/public_affairs/releases/BEC_background.htm| archivedate= 3 January 2008 <!--DASHBot-->| deadurl= no}}</ref>
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| <ref name=colton_fyffe1999>{{cite web |last=Colton|first=Imogen|coauthors=Fyffe, Jeanette
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| |date=February 3, 1999 |url=http://www.ph.unimelb.edu.au/~ywong/poster/articles/bec.html |title=Super Atoms from Bose-Einstein Condensation |publisher=The University of Melbourne |accessdate=2008-02-06| archiveurl = http://web.archive.org/web/20070829200820/http://www.ph.unimelb.edu.au/~ywong/poster/articles/bec.html| archivedate = August 29, 2007}}</ref>
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| <ref name=jacox1997>{{cite web |last=Jacox|first=Marilyn|coauthors=Gadzuk, J. William |url=http://physics.nist.gov/GenInt/STM/stm.html |title=Scanning Tunneling Microscope |publisher=National Institute of Standards and Technology |date=November 1997|accessdate=2008-01-11 | archiveurl= http://web.archive.org/web/20080107133132/http://physics.nist.gov/GenInt/STM/stm.html| archivedate= 7 January 2008 <!--DASHBot-->| deadurl= no}}</ref>
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| <ref name=nf_physics1986>{{cite web |url=http://nobelprize.org/nobel_prizes/physics/laureates/1986/index.html |title=The Nobel Prize in Physics 1986 |publisher=The Nobel Foundation |accessdate=2008-01-11}}—in particular, see the Nobel lecture by G. Binnig and H. Rohrer.</ref>
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| <ref name=sab53_13_1739>{{cite journal |first=N.|last=Jakubowski |title = Sector field mass spectrometers in ICP-MS |journal = Spectrochimica Acta Part B: Atomic Spectroscopy |volume = 53|issue = 13|year = 1998 |doi=10.1016/S0584-8547(98)00222-5|pages = 1739–63|bibcode = 1998AcSpe..53.1739J |last2=Moens |first2=Luc |last3=Vanhaecke |first3=Frank }}</ref>
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| <ref name=rsi39_1_83>{{cite journal |last1=Müller |first1=Erwin W. |authorlink1=Erwin Wilhelm Müller |last2=Panitz |first2=John A. |authorlink2=J. A. Panitz |last3=McLane |first3=S. Brooks |authorlink3=S. Brooks McLane |year=1968 |title=The Atom-Probe Field Ion Microscope |journal=[[Review of Scientific Instruments]] |volume=39 |issue=1 |pages=83–86 |doi=10.1063/1.1683116
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| |bibcode = 1968RScI...39...83M }}</ref>
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| <ref name=lochner2007>{{cite web |last=Lochner|first=Jim |coauthors=Gibb, Meredith; Newman, Phil |date=April 30, 2007 |url=http://imagine.gsfc.nasa.gov/docs/science/how_l1/spectral_what.html |title=What Do Spectra Tell Us? |publisher=NASA/Goddard Space Flight Center |accessdate=2008-01-03| archiveurl= http://web.archive.org/web/20080116035542/http://imagine.gsfc.nasa.gov/docs/science/how_l1/spectral_what.html| archivedate= 16 January 2008 <!--DASHBot-->| deadurl= no}}</ref>
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| <ref name=winter2007>{{cite web |last=Winter|first=Mark|year=2007 |url=http://www.webelements.com/webelements/elements/text/He/hist.html |title=Helium|publisher=WebElements |accessdate=2008-01-03| archiveurl= http://web.archive.org/web/20071230182148/http://www.webelements.com/webelements/elements/text/He/hist.html| archivedate= 30 December 2007 <!--DASHBot-->| deadurl= no}}</ref>
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| <ref name=hinshaw20060210>{{cite web |last=Hinshaw|first=Gary |date=February 10, 2006 |url=http://map.gsfc.nasa.gov/m_uni/uni_101matter.html |title=What is the Universe Made Of? |publisher=NASA/WMAP|accessdate=2008-01-07| archiveurl= http://web.archive.org/web/20071231143948/http://map.gsfc.nasa.gov/m_uni/uni_101matter.html| archivedate= 31 December 2007 <!--DASHBot-->| deadurl= no}}</ref>
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| <ref name=science259_5093_327>{{cite journal |last=Davidsen|first=Arthur F. |title=Far-Ultraviolet Astronomy on the Astro-1 Space Shuttle Mission |journal=[[Science (journal)|Science]] |year=1993|volume=259 |issue=5093|pages=327–34 |doi=10.1126/science.259.5093.327 |pmid=17832344|bibcode = 1993Sci...259..327D }}</ref>
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| <ref name=nigel2000>{{cite web |first=Nigel|last=Smith|date=January 6, 2000 |url=http://physicsworld.com/cws/article/print/809 |title=The search for dark matter |publisher=Physics World|accessdate = 2008-02-14| archiveurl= http://web.archive.org/web/20080216185952/http://physicsworld.com/cws/article/print/809| archivedate= 16 February 2008 <!--DASHBot-->| deadurl= no}}</ref>
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| <ref name=science267_5195_192>{{cite journal |last=Copi|first=Craig J. |last2=Schramm|first2=DN |last3=Turner|first3=MS |year=1995 |title=Big-Bang Nucleosynthesis and the Baryon Density of the Universe |journal=[[Science (journal)|Science]] |volume=267|issue=5195 |pages=192–99 |doi = 10.1126/science.7809624 |pmid=7809624 |arxiv = astro-ph/9407006 |bibcode = 1995Sci...267..192C }}</ref>
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| <ref name=hinshaw20051215>{{cite web |last=Hinshaw|first=Gary|date=December 15, 2005 |url=http://map.gsfc.nasa.gov/m_uni/uni_101bbtest2.html |title=Tests of the Big Bang: The Light Elements |publisher=NASA/WMAP|accessdate=2008-01-13 | archiveurl= http://web.archive.org/web/20080117021252/http://map.gsfc.nasa.gov/m_uni/uni_101bbtest2.html| archivedate= 17 January 2008 <!--DASHBot-->| deadurl= no}}</ref>
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| <ref name=abbott20070530>{{cite web |last=Abbott|first=Brian|date=May 30, 2007 |url=http://www.haydenplanetarium.org/universe/duguide/exgg_wmap.php |title=Microwave (WMAP) All-Sky Survey |publisher=Hayden Planetarium|accessdate=2008-01-13 }}</ref>
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| <ref name=mnras106_343>{{cite journal |title=The synthesis of the elements from hydrogen |first=F. |last=Hoyle |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=106|pages=343–83|year=1946 |bibcode=1946MNRAS.106..343H}}</ref>
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| <ref name=mashnik2000>{{cite arXiv |last=Mashnik|first=Stepan G. |year=2000 |title=On Solar System and Cosmic Rays Nucleosynthesis and Spallation Processes |class=astro-ph |eprint=astro-ph/0008382 }}</ref>
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| <ref name=kgs20050504>{{cite web |author=Kansas Geological Survey |date=May 4, 2005 |title=Age of the Earth |url=http://www.kgs.ku.edu/Extension/geotopics/earth_age.html |publisher=University of Kansas |accessdate=2008-01-14 }}</ref>
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| <ref name=yarris2001>{{cite news |last=Yarris |first=Lynn |date=July 27, 2001 |title=New Superheavy Elements 118 and 116 Discovered at Berkeley Lab |publisher=Berkeley Lab |url=http://enews.lbl.gov/Science-Articles/Archive/elements-116-118.html |accessdate=2008-01-14 | archiveurl= http://web.archive.org/web/20080109103538/http://enews.lbl.gov/Science-Articles/Archive/elements-116-118.html| archivedate= 9 January 2008 <!--DASHBot-->| deadurl= no}}</ref>
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| <ref name=pr119_6_2000>{{cite journal |author=Diamond, H |coauthors=''et al.'' |year=1960 |title=Heavy Isotope Abundances in Mike Thermonuclear Device |journal=[[Physical Review]] |volume=119 |issue=6 |pages=2000–04 |doi=10.1103/PhysRev.119.2000 |bibcode = 1960PhRv..119.2000D }}</ref>
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| <ref name=poston1998>{{cite web |author=Poston Sr., John W.|date=March 23, 1998 |title=Do transuranic elements such as plutonium ever occur naturally? |publisher=Scientific American |url=http://www.sciam.com/chemistry/article/id/do-transuranic-elements-s/topicID/4/catID/3 |accessdate=2008-01-15 }}</ref>
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| <ref name=cz97_10_522>{{cite journal |last=Keller|first=C. |title=Natural occurrence of lanthanides, actinides, and superheavy elements |journal=Chemiker Zeitung |year=1973|volume=97|issue=10|pages=522–30 |osti=4353086 }}</ref>
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| <ref name=ofr_cut>{{cite web |url=http://www.oklo.curtin.edu.au/index.cfm |title=Oklo Fossil Reactors |publisher=Curtin University of Technology |accessdate=2008-01-15| archiveurl= http://web.archive.org/web/20071218194159/http://www.oklo.curtin.edu.au/index.cfm| archivedate= 18 December 2007 <!--DASHBot-->| deadurl= no}}</ref>
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| <ref name=weisenberger>{{cite web |last=Weisenberger|first=Drew |url=http://education.jlab.org/qa/mathatom_05.html |title=How many atoms are there in the world? |publisher=Jefferson Lab |accessdate=2008-01-16}}</ref>
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| <ref name=pidwirnyf>{{cite web |last=Pidwirny|first=Michael |url=http://www.physicalgeography.net/fundamentals/contents.html |title=Fundamentals of Physical Geography |publisher=University of British Columbia Okanagan |accessdate=2008-01-16 | archiveurl= http://web.archive.org/web/20080121080709/http://www.physicalgeography.net/fundamentals/contents.html| archivedate= 21 January 2008 <!--DASHBot-->| deadurl= no}}</ref>
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| <ref name=pnas99_22_13966>{{cite journal |last=Anderson|first=Don L. |title=The inner inner core of Earth |journal=[[Proceedings of the National Academy of Sciences]] |year=2002|volume=99|issue=22|pages=13966–68 |doi=10.1073/pnas.232565899 |pmid=12391308 |pmc=137819|bibcode = 2002PNAS...9913966A }}</ref>
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| <ref name=cern28509>{{cite journal |title=Second postcard from the island of stability |author=Anonymous|journal=CERN Courier |date=October 2, 2001 |url=http://cerncourier.com/cws/article/cern/28509 |accessdate=2008-01-14| archiveurl= http://web.archive.org/web/20080203031237/http://cerncourier.com/cws/article/cern/28509| archivedate= 3 February 2008 <!--DASHBot-->| deadurl= no}}</ref>
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| <ref name=cen84_10_19>{{cite journal |last=Jacoby|first=Mitch |title=As-yet-unsynthesized superheavy atom should form a stable diatomic molecule with fluorine |journal=[[Chemical & Engineering News]] |year=2006|volume=84|issue=10|page=19 |doi=10.1021/cen-v084n010.p019a }}</ref>
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| <ref name=koppes1999>{{cite news |last=Koppes|first=Steve|date=March 1, 1999 |title=Fermilab Physicists Find New Matter-Antimatter Asymmetry |publisher=University of Chicago |url=http://www-news.uchicago.edu/releases/99/990301.ktev.shtml |accessdate=2008-01-14 }}</ref>
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| <ref name=cromie20010816>{{cite news |last=Cromie|first=William J.|date=August 16, 2001 |title=A lifetime of trillionths of a second: Scientists explore antimatter |publisher=Harvard University Gazette |url=http://news.harvard.edu/gazette/2001/08.16/antimatter.html |accessdate=2008-01-14 }}</ref>
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| <ref name=nature419_6906_439>{{cite journal |last=Hijmans|first=Tom W. |title=Particle physics: Cold antihydrogen |journal=[[Nature (journal)|Nature]] |year=2002|volume=419 |pages=439–40|doi=10.1038/419439a
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| |pmid=12368837 |issue=6906 |bibcode = 2002Natur.419..439H }}</ref>
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| <ref name=BBC20021030>{{cite news |author=Staff|date=October 30, 2002 |title=Researchers 'look inside' antimatter|publisher=BBC News |url=http://news.bbc.co.uk/2/hi/science/nature/2375717.stm |accessdate=2008-01-14}}</ref>
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| <ref name=ns1728_77>{{cite journal |last=Barrett|first=Roger |title=The Strange World of the Exotic Atom |journal=New Scientist |year=1990|issue=1728|pages=77–115 |url=http://media.newscientist.com/article/mg12717284.600-the-strange-world-of-the-exotic-atom-physicists-can-nowmake-atoms-and-molecules-containing-negative-particles-other-than-electronsand-use-them-not-just-to-test-theories-but-also-to-fight-cancer-.html |accessdate=2008-01-04 | archiveurl= http://web.archive.org/web/20071221164440/http://media.newscientist.com/article/mg12717284.600-the-strange-world-of-the-exotic-atom-physicists-can-nowmake-atoms-and-molecules-containing-negative-particles-other-than-electronsand-use-them-not-just-to-test-theories-but-also-to-fight-cancer-.html| archivedate= 21 December 2007 <!--DASHBot-->| deadurl= no}}</ref>
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| <ref name=psT112_1_20>{{cite journal |last=Indelicato|first=Paul |title=Exotic Atoms|journal=[[Physica Scripta]] |year=2004|volume=T112 |issue=1|pages=20–26 |doi=10.1238/Physica.Topical.112a00020 |arxiv = physics/0409058 |bibcode = 2004PhST..112...20I }}
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| </ref>
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| <ref name=ripin1998>{{cite web |last=Ripin|first=Barrett H.|date=July 1998 |url=http://www.aps.org/publications/apsnews/199807/experiment.cfm.html |title=Recent Experiments on Exotic Atoms |publisher=American Physical Society |accessdate=2008-02-15}}</ref>
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| }}
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| ===Book references===
| |
| {{refbegin|colwidth=30em}}
| |
| * {{cite book
| |
| |last=L'Annunziata<!-- Note: the single quote mark before the name is correct. -->
| |
| |first=Michael F.
| |
| |year=2003|title=Handbook of Radioactivity Analysis
| |
| |publisher=Academic Press|isbn=0-12-436603-1
| |
| |oclc=16212955|ref=harv}}
| |
| * {{cite book
| |
| |last=Beyer|first=H. F.|coauthors=Shevelko, V. P.
| |
| |year=2003
| |
| |title=Introduction to the Physics of Highly Charged Ions
| |
| |publisher=CRC Press|isbn=0-7503-0481-2
| |
| |oclc=47150433|ref=harv}}
| |
| * {{cite book
| |
| |last=Choppin|first=Gregory R.
| |
| |last2=Liljenzin|first2=Jan-Olov|last3=Rydberg|first3=Jan
| |
| |year=2001|title=Radiochemistry and Nuclear Chemistry
| |
| |publisher=Elsevier|isbn=0-7506-7463-6
| |
| |oclc=162592180|ref=harv}}
| |
| * {{cite book
| |
| |last=Dalton|first=J. |authorlink=John Dalton|year=1808
| |
| |title=A New System of Chemical Philosophy, Part 1
| |
| |publisher=S. Russell
| |
| |location=London and Manchester|ref=harv}}
| |
| * {{cite book
| |
| |last=Demtröder|first=Wolfgang|year=2002
| |
| |title=Atoms, Molecules and Photons: An Introduction to Atomic- Molecular- and Quantum Physics
| |
| |publisher=Springer|edition=1st
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| |
| {{refend}}
| |
| | |
| ==External links==
| |
| {{Sister project links|voy=no|wikt=atom|v=The Atom|n=no|q=no|s=The New Student's Reference Work}}
| |
| * {{cite web
| |
| |last=Francis|first=Eden|year=2002|url=http://dl.clackamas.cc.or.us/ch104-07/atomic_size.htm
| |
| |title=Atomic Size|publisher=Clackamas Community College
| |
| |accessdate=2007-01-09| archiveurl= http://web.archive.org/web/20070204073653/http://dl.clackamas.cc.or.us/ch104-07/atomic_size.htm| archivedate= 4 February 2007 <!--DASHBot-->| deadurl= no}}
| |
| * {{cite web
| |
| |last=Freudenrich|first=Craig C.|url=http://www.howstuffworks.com/atom.htm
| |
| |title=How Atoms Work|publisher=How Stuff Works
| |
| |accessdate=2007-01-09| archiveurl= http://web.archive.org/web/20070108023359/http://www.howstuffworks.com/atom.htm| archivedate= 8 January 2007 <!--DASHBot-->| deadurl= no}}
| |
| * {{cite web
| |
| |url=http://en.wikibooks.org/wiki/FHSST_Physics/Atom
| |
| |work=Free High School Science Texts: Physics
| |
| |title=The Atom|publisher=Wikibooks
| |
| |accessdate=2010-07-10}}
| |
| * {{cite web
| |
| |author=Anonymous|year=2007|url=http://www.scienceaid.co.uk/chemistry/fundamental/atom.html
| |
| |title=The atom|publisher=Science aid+
| |
| |accessdate=2010-07-10}}—a guide to the atom for teens.
| |
| * {{cite web
| |
| |author=Anonymous|date=2006-01-03|url=http://www.bbc.co.uk/dna/h2g2/A6672963
| |
| |title=Atoms and Atomic Structure
| |
| |publisher=BBC|accessdate=2007-01-11| archiveurl= http://web.archive.org/web/20070102114833/http://www.bbc.co.uk/dna/h2g2/A6672963| archivedate= 2 January 2007 <!--DASHBot-->| deadurl= no}}
| |
| * {{cite web
| |
| |author=Various|date=2006-01-03|url=http://www.colorado.edu/physics/2000/index.pl?Type=TOC
| |
| |title=Physics 2000, Table of Contents
| |
| |publisher=University of Colorado|accessdate=2008-01-11| archiveurl= http://web.archive.org/web/20080114002007/http://www.colorado.edu/physics/2000/index.pl?Type=TOC| archivedate= 14 January 2008 <!--DASHBot-->| deadurl= no}}
| |
| * {{cite web
| |
| |author=Various|date=2006-02-03|url=http://www.hydrogenlab.de/elektronium/HTML/einleitung_hauptseite_uk.html
| |
| |title=What does an atom look like?
| |
| |publisher=University of Karlsruhe|accessdate=2008-05-12}}
| |
| | |
| {{particles}}
| |
| {{composition}}
| |
| {{biological organisation}}
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| {{featured article}}
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| | |
| [[Category:Atoms| ]]
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| [[Category:Concepts in physics]]
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| [[Category:Chemistry]]
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| | |
| {{Link GA|no}}
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| {{Link GA|th}}
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| {{Link GA|uk}}
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| {{Link FA|bg}}
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| {{Link FA|id}}
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| {{Link FA|pl}}
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| {{Link FA|zh}}
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| {{Link FA|vi}}
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| {{Link GA|frr}}
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