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{{Infobox atom}}
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, forming a [[molecule]]. An atom containing an equal number of protons and electrons is electrically neutral, otherwise it has a positive or negative charge and is an [[ion]]. An atom is 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/>

The name atom comes from the [[Greek language|Greek]] "ἄτομος"&mdash;''átomos'' (from α-, "un-" + τέμνω - ''temno'', "to 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|India]]n and [[Greek philosophy|Greek]] philosophers. In the 17th and 18th centuries, [[chemist]]s provided a physical basis for this idea by showing that certain substances could not be further broken down by chemical methods. During the late 19th and early 20th centuries, [[physicist]]s discovered subatomic components and structure inside the atom, thereby demonstrating that the 'atom' was divisible. The principles of [[quantum mechanics]] were used to successfully [[Scientific modelling|model]] the atom.<ref name=haubold_mathai1998/><ref>Harrison (2003:123–139).</ref>

Relative to everyday experience, atoms are minuscule objects with proportionately tiny masses. Atoms can only be observed individually using special instruments such as the [[scanning tunneling microscope]]. Over 99.9% of an atom's mass is concentrated in the nucleus,<ref group=note>Most isotopes have more nucleons than electrons. In the case of hydrogen-1, with a single electron and nucleon, the proton is <math>\begin{smallmatrix}\frac{1836}{1837} \approx 0.9995\end{smallmatrix}</math>, or 99.95% of the total atomic mass.</ref> with protons and neutrons having roughly equal mass. Each element has at least one isotope with unstable nuclei 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.

==History==
{{Main|Atomic theory}}

===Atomism===
{{Main|Atomism}}
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.<ref name=Ponomarev>Ponomarev (1993:14–15).</ref>

The earliest references to the concept of atoms date back to [[History of India|ancient India]] in the 6th century [[BCE]],<ref>Gangopadhyaya (1981).</ref> appearing first in [[Jainism]].<ref>Iannone (2001:62).</ref> The [[Nyaya]] and [[Vaisheshika]] schools developed elaborate [[Vaisheshika#The atomic theory|theories]] of how atoms combined into more complex objects.<ref>Teresi (2003:213–214).</ref> In the West, the references to atoms emerged a century later from [[Leucippus]], whose student, [[Democritus]], systematized his views. In approximately 450&nbsp;BCE, Democritus coined the term ''átomos'' ({{lang-el|ἄτομος}}), which means "uncuttable" or "the smallest indivisible particle of matter". Although the Indian and Greek concepts of the atom were based purely on philosophy, modern science has retained the name coined by Democritus.<ref name=Ponomarev/>

[[Corpuscularianism]] is the postulate, expounded
in the 13th-century by the [[alchemy|alchemist]] [[Pseudo-Geber]] (Geber),<ref>Moran (2005:146).</ref> that all physical bodies possess an inner and outer layer of minute particles or corpuscles.<ref name="Levere" >Levere (2001)</ref>{{Failed verification|date=June 2010}} Corpuscularianism is similar to the theory 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>
{{cite web
|last=Pratt|first=Vernon|date=28 September 2007
|title=The Mechanical Philosophy|url=http://www.vernonpratt.com/conceptualisations/d06bl2_1mechanical.htm
|work=Reason, nature and the human being in the West
|accessdate=2009-06-28
}}</ref> Corpuscularianism stayed a dominant theory over the next several hundred years. 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.<ref>Siegfried (2002:42–55).</ref> During the 1670s was used by [[Isaac Newton]] in his development of the [[corpuscular theory of light]].<ref name= "Levere" /><ref>
{{cite web
|last=Kemerling|first=Garth|date=8 August 2002
|title=Corpuscularianism
|url=http://www.philosophypages.com/dy/c9.htm
|work=Philosophical Dictionary
|accessdate=2009-06-17
}}</ref>

===Origin of scientific theory===
[[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.]]
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>
{{cite web
|date=
|title=Lavoisier's Elements of Chemistry
|url=http://web.lemoyne.edu/~GIUNTA/EA/LAVPREFann.HTML
|work=Elements and Atoms
|publisher=[[Le Moyne College]], Department of Chemistry
|accessdate=2007-12-18
}}</ref>

In 1803, English instructor and natural philosopher [[John Dalton]] used the concept of atoms to explain why elements always react in a ratio of small [[Natural number|whole number]]s—the
[[law of multiple proportions]]—and why certain gases dissolve 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.<ref>Wurtz (1881:1–2).</ref><ref>Dalton (1808).</ref> Dalton is considered the originator of modern [[atomic theory]].<ref>{{cite book|last = Patterson|first = Elizabeth C.|title = John Dalton and the Atomic Theory|publisher = Anchor |location = Garden City, New York|year = 1970}}</ref>

Additional validation of particle theory (and by extension [[atomic theory]]) occurred 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>
{{cite journal
|last=Einstein|first=Albert|year=1905
|title=Über die von der molekularkinetischen Theorie der Wärme geforderte Bewegung von in ruhenden Flüssigkeiten suspendierten Teilchen
|journal=[[Annalen der Physik]]
|volume=322|issue=8|pages=549–560|language=German
|url=http://www.zbp.univie.ac.at/dokumente/einstein2.pdf
|format=PDF|doi=10.1002/andp.19053220806
|accessdate=2007-02-04
}}</ref><ref>Mazo (2002:1–7).</ref><ref>
{{cite web
|last=Lee|first=Y.K.|year=1995
|coauthors=Hoon, K.
|title=Brownian Motion
|url=http://www.doc.ic.ac.uk/~nd/surprise_95/journal/vol4/ykl/report.html
|publisher=[[Imperial College]]
|accessdate=2007-12-18
}}</ref> 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>
{{cite journal
|last=Patterson|first=G.
|year=2007
|title=Jean Perrin and the triumph of the atomic doctrine
|url=http://www.ncbi.nlm.nih.gov/pubmed/17602746
|journal=[[Endeavour (journal)|Endeavour]]
|volume=31|issue=2|pages=50–53
|doi=10.1016/j.endeavour.2007.05.003
}}</ref>

[[File:Mendeleev's 1869 periodic table.png|thumb|left|[[Dmitri Mendeleev|Mendeleev's]] first periodic table (1869).]]
In 1869, building upon earlier discoveries by such scientists as Lavoisier, [[Dmitri Mendeleev]] published the first functional [[periodic table]].<ref>{{cite web
|url=http://old.iupac.org/reports/periodic_table/
|title=Periodic Table of the Elements|date=November 1, 2007
|publisher=The International Union of Pure and Applied Chemistry|accessdate=2010-05-14}}</ref> The table itself is a visual representation of the periodic law, which states that certain chemical properties of [[chemical element|element]]s repeat ''periodically'' when arranged by [[atomic number]].<ref>Scerri (2007:10–17).</ref>

===Subcomponents and quantum theory===

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">{{cite web
|year=1906
|title=J.J. Thomson
|url=http://nobelprize.org/nobel_prizes/physics/laureates/1906/thomson-bio.html
|publisher=[[Nobel Foundation]]
|accessdate=2007-12-20
}}</ref> 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]].

In 1909, [[Hans Geiger]] and [[Ernest Marsden]], under the direction of physicist [[Ernest Rutherford]], bombarded a sheet of gold foil with 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>
{{cite journal
|last=Rutherford|first=E.
|title=The Scattering of α and β Particles by Matter and the Structure of the Atom|url=http://ion.elte.hu/~akos/orak/atfsz/atom/rutherford_atom11.pdf
|journal=[[Philosophical Magazine]]
|year=1911|volume=21|pages=669–88}}</ref>

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>
{{cite web
|url=http://nobelprize.org/nobel_prizes/chemistry/laureates/1921/soddy-bio.html
|title=Frederick Soddy, The Nobel Prize in Chemistry 1921
|publisher=[[Nobel Foundation]]
|accessdate=2008-01-18
}}</ref> 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>
{{cite journal
|last=Thomson|first=Joseph John
|title=Rays of positive electricity
|url=http://web.lemoyne.edu/~giunta/canal.html
|journal=[[Proceedings of the Royal Society]]
|year=1913|volume=A 89|pages=1–20
|doi=
}}</ref>

[[File:Bohr Model.svg|right|thumb|200px|A [[Bohr model]] of the
hydrogen atom, showing an electron jumping between fixed orbits and emitting a [[photon]]
of energy with a specific frequency.]]

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>
{{cite web
|last=Stern|first=David P.|date=16 May 2005
|title=The Atomic Nucleus and Bohr's Early Model of the Atom
|url=http://www-spof.gsfc.nasa.gov/stargaze/Q5.htm
|publisher=[[NASA]]/[[Goddard Space Flight Center]]
|accessdate=2007-12-20
}}</ref> 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>
{{cite web
|last=Bohr|first=Neils|date=11 December 1922
|title=Niels Bohr, The Nobel Prize in Physics 1922, Nobel Lecture
|url=http://nobelprize.org/nobel_prizes/physics/laureates/1922/bohr-lecture.html
|publisher=[[Nobel Foundation]]
|accessdate=2008-02-16
}}</ref>

[[Chemical bond]]s between atoms were now explained, by [[Gilbert Newton Lewis]] in 1916, as the interactions between their constituent electrons.<ref>{{cite journal
|last=Lewis|first=Gilbert N.|year=1916
|title=The Atom and the Molecule
|journal=[[Journal of the American Chemical Society]]
|volume=38|issue=4|pages=762–786
|doi=10.1021/ja02261a002
}}</ref> As the [[Chemical property|chemical properties]] of the elements were known to largely repeat themselves according to the [[periodic law]],<ref>Scerri (2007:205–226)</ref> 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>
{{cite journal
|last=Langmuir|first=Irving|year=1919
|title=The Arrangement of Electrons in Atoms and Molecules
|journal=[[Journal of the American Chemical Society]]
|volume=41|issue=6|pages=868–934
|doi=10.1021/ja02227a002
}}</ref>

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>{{cite journal
|last=Scully|first=Marlan O.|year=1987
|title=On the theory of the Stern-Gerlach apparatus
|journal=[[Foundations of Physics]]
|volume=17|issue=6|pages=575–583
|doi=10.1007/BF01882788
|last2=Lamb
|first2=Willis E.
|last3=Barut
|first3=Asim
}}</ref>

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>
{{cite web
|last=Brown|first=Kevin|year=2007
|url=http://www.mathpages.com/home/kmath538/kmath538.htm
|title=The Hydrogen Atom
|publisher=MathPages
|accessdate=2007-12-21
}}</ref><ref>
{{cite web
|last=Harrison|first=David M.|year=2000
|title=The Development of Quantum Mechanics|url=http://www.upscale.utoronto.ca/GeneralInterest/Harrison/DevelQM/DevelQM.html
|publisher=[[University of Toronto]]
|accessdate=2007-12-21
}}</ref>

[[File:Mass Spectrometer Schematic.svg|left|thumb|280px|Schematic diagram of a simple mass spectrometer.]]
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>
{{cite journal
|last=Aston|first=Francis W.|year=1920
|title=The constitution of atmospheric neon
|journal=[[Philosophical Magazine]]
|volume=39|issue=6|pages=449–55
|doi=
}}</ref> 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>
{{cite web
|last=Chadwick|first=James|date=12 December 1935
|title=Nobel Lecture: The Neutron and Its Properties
|url=http://nobelprize.org/nobel_prizes/physics/laureates/1935/chadwick-lecture.html
|publisher=[[Nobel Foundation]]
|accessdate=2007-12-21
}}</ref>

===Fission, high energy physics and condensed matter===

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>{{cite web
|title=Otto Hahn, Lise Meitner and Fritz Strassmann
|publisher=Chemical Heritage Foundation
|work=Chemical Achievers: The Human Face of the Chemical Sciences
|url=http://www.chemheritage.org/classroom/chemach/atomic/hahn-meitner.html|accessdate=2009-09-15
}}</ref> A year later, [[Lise Meitner]] and her nephew [[Otto Frisch]] verified that Hahn's result were the first experimental ''nuclear fission''.<ref>
{{cite journal
|last=Meitner|first=Lise
|last2=Frisch|first2=Otto Robert|year=1939
|title=Disintegration of uranium by neutrons: a new type of nuclear reaction
|journal=[[Nature (journal)|Nature]]
|volume=143 |pages=239
|doi=10.1038/143239a0
}}</ref><ref>
{{cite web
|last=Schroeder |first=M.
|date=
|title=Lise Meitner - Zur 125. Wiederkehr Ihres Geburtstages
|url=http://www.physik3.gwdg.de/~mrs/Vortraege/Lise_Meitner-Vortrag-20031106/
|language=German
|accessdate=2009-06-04
}}</ref> In 1944, Hahn received the [[Nobel prize]] in chemistry. Despite Hahn's efforts, the contributions of Meitner and Frisch were not recognized.<ref>
{{cite journal
|last=Crawford |first=E.| year=1997
|title=A Nobel tale of postwar injustice
|url=http://md1.csa.com/partners/viewrecord.php?requester=gs&collection=TRD&recid=63212AN&q=A+Nobel+tale+of+postwar+injustice&uid=787269344&setcookie=yes
|journal=[[Physics Today]]
|volume=50|issue=9|pages=26–32
|doi=10.1063/1.881933
|last2=Sime
|first2=Ruth Lewin
|last3=Walker
|first3=Mark
}}</ref>

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>
{{cite web
|last=Kullander|first=Sven|date=28 August 2001
|title=Accelerators and Nobel Laureates
|url=http://nobelprize.org/nobel_prizes/physics/articles/kullander/
|publisher=[[Nobel Foundation]]
|accessdate=2008-01-31
}}</ref> Neutrons and protons were found to be [[hadron]]s, or composites of smaller particles called [[quark]]s. Standard models of nuclear physics were developed that successfully explained the properties of the nucleus in terms of these sub-atomic particles and the forces that govern their interactions.<ref>
{{cite web
|date=17 October 1990
|title=The Nobel Prize in Physics 1990
|url=http://nobelprize.org/nobel_prizes/physics/laureates/1990/press.html
|publisher=[[Nobel Foundation]]
|accessdate=2008-01-31
}}</ref>

==Components==
===Subatomic particles===
{{Main|Subatomic particle}}
Though the word ''atom'' originally denoted a particle that cannot be cut into smaller particles, in modern scientific usage the atom is composed of various [[subatomic particle]]s. The constituent particles of an atom are the [[electron]], the [[proton]] and the [[neutron]]. However, the [[hydrogen|hydrogen-1]] atom has no neutrons and a positive [[hydrogen ion]] has no electrons.

The electron is by far the least massive of these particles at 9.11{{Esp|−31}}&nbsp;kg, with a negative [[Electric charge|electrical charge]] and a size that is too small to be measured using available techniques.<ref>Demtröder (2002:39–42).</ref> Protons have a positive charge and a mass 1,836 times that of the electron, at 1.6726{{Esp|−27}}&nbsp;kg, although this can be reduced by changes to the [[binding energy|energy binding]] the proton into an atom. Neutrons have no electrical charge and have a free mass of 1,839 times the mass of electrons,<ref>Woan (2000:8).</ref> or 1.6929{{Esp|−27}}&nbsp;kg. Neutrons and protons have comparable dimensions—on the order of 2.5{{Esp|−15}}&nbsp;m—although the 'surface' of these particles is not sharply defined.<ref>MacGregor (1992:33–37).</ref>

In the [[Standard Model]] of physics, both protons and neutrons are composed of [[elementary particle]]s called [[quark]]s. The quark belongs to the [[fermion]] group of particles, and is one of the two basic constituents of matter—the other being the [[lepton]], of which the electron is an example. There are six types of quarks, each having a fractional electric charge of either +2/3 or −1/3. Protons are composed of two [[up quark]]s and one [[down quark]], while a neutron consists of one up quark and two down quarks. This distinction accounts for the difference in mass and charge between the two particles. The quarks are held together by the [[strong nuclear force]], which is mediated by [[gluon]]s. The gluon is a member of the family of [[gauge boson|gauge]] [[boson]]s, which are elementary particles that mediate physical [[force]]s.<ref>{{cite web
|author=Particle Data Group|year=2002
|url=http://www.particleadventure.org/
|title=The Particle Adventure
|publisher=Lawrence Berkeley Laboratory
|accessdate=2007-01-03
}}</ref><ref>{{cite web
|first=James|last=Schombert
|date=April 18, 2006
|url=http://abyss.uoregon.edu/~js/ast123/lectures/lec07.html
|title=Elementary Particles
|publisher=University of Oregon
|accessdate=2007-01-03
}}</ref>

===Nucleus===
{{Main|Atomic nucleus}}
[[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. -->

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
<math>\begin{smallmatrix}1.07 \sqrt[3]{A}\end{smallmatrix}</math>&nbsp;&nbsp;[[femtometre|fm]],
where ''A'' is the total number of nucleons.<ref>Jevremovic (2005:63).</ref> This is much smaller than the radius of the atom, which is on the order of 10<sup>5</sup>&nbsp;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.<ref name=pfeffer>Pfeffer (2000:330–336).</ref>

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>{{cite web
|last=Wenner|first=Jennifer M.|date=October 10, 2007
|url=http://serc.carleton.edu/quantskills/methods/quantlit/RadDecay.html
|title=How Does Radioactive Decay Work?
|publisher=Carleton College|accessdate=2008-01-09}}</ref>

The neutron and the proton are different types of [[fermion]]s. The [[Pauli exclusion principle]] is a [[quantum mechanics|quantum mechanical]] effect that prohibits ''identical'' fermions, such as multiple protons, from occupying the same quantum physical state at the same time. Thus every proton in the nucleus must occupy a different state, with its own energy level, and the same rule applies to all of the neutrons. This prohibition does not apply to a proton and neutron occupying the same quantum state.<ref name="raymond"/>

For atoms with low atomic numbers, a nucleus that has a different number of protons than neutrons can potentially drop to a lower energy state through a radioactive decay that causes the number of protons and neutrons to more closely match. As a result, atoms with roughly matching numbers of protons and neutrons are more stable against decay. However, with increasing atomic number, the mutual repulsion of the protons requires an increasing proportion of neutrons to maintain the stability of the nucleus, which modifies this trend. Thus, there are no stable nuclei with equal proton and neutron numbers above atomic number Z = 20 (calcium); and as Z increases toward the heaviest nuclei, the ratio of neutrons per proton required for stability increases to about 1.5.<ref name="raymond"/>

[[File:Wpdms physics proton proton chain 1.svg|right|thumb|200px|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]].]]

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>{{cite web
|last=Mihos|first=Chris|date=July 23, 2002
|url=http://burro.cwru.edu/Academics/Astr221/StarPhys/coulomb.html
|title=Overcoming the Coulomb Barrier
|publisher=Case Western Reserve University
|accessdate=2008-02-13}}</ref> [[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>{{cite web
|author=Staff|date=March 30, 2007
|url=http://www.lbl.gov/abc/Basic.html
|title=ABC's of Nuclear Science
|publisher=Lawrence Berkeley National Laboratory
|accessdate=2007-01-03
}}</ref><ref>{{cite web
|first=Arjun|last=Makhijani|coauthors=Saleska, Scott
|date=March 2, 2001
|url=http://www.ieer.org/reports/n-basics.html
|title=Basics of Nuclear Physics and Fission
|publisher=Institute for Energy and Environmental Research
|accessdate=2007-01-03
}}</ref>

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''&nbsp;=&nbsp;''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.<ref>Shultis ''et al.'' (2002:72–6).</ref>

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>{{cite journal
|last = Fewell|first = M. P.
|title=The atomic nuclide with the highest mean binding energy
|journal=[[American Journal of Physics]]
|year=1995|volume=63|issue=7|pages=653–58
|url=http://adsabs.harvard.edu/abs/1995AmJPh..63..653F
|accessdate = 2007-02-01
|doi=10.1119/1.17828}}</ref> 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">{{cite web
|last=Raymond|first=David|date=April 7, 2006
|url=http://physics.nmt.edu/~raymond/classes/ph13xbook/node216.html
|title=Nuclear Binding Energies
|publisher=New Mexico Tech
|accessdate=2007-01-03}}</ref>

===Electron cloud===
{{Main|Atomic orbital|Electron configuration}}

[[File:Potential energy well.svg|200px|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>.]]
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.

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>{{cite journal
|last=Mulliken|first=Robert S.
|title=Spectroscopy, Molecular Orbitals, and Chemical Bonding
|journal=[[Science (journal)|Science]] |year=1967
|volume=157|issue=3784|pages=13–24
|doi=10.1126/science.157.3784.13
|pmid=5338306}}</ref> Only a discrete (or [[wikt:quantize|quantize]]d) set of these orbitals exist around the nucleus, as other possible wave patterns rapidly decay into a more stable form.<ref name=Brucat>{{cite web
|last=Brucat|first=Philip J.|year=2008
|url=http://www.chem.ufl.edu/~itl/2045/lectures/lec_10.html
|title=The Quantum Atom|publisher=University of Florida
|accessdate=2007-01-04}}</ref> Orbitals can have one or more ring or node structures, and they differ from each other in size, shape and orientation.<ref>{{cite web
|last=Manthey|first=David|year=2001
|url=http://www.orbitals.com/orb/
|title=Atomic Orbitals|publisher=Orbital Central
|accessdate=2008-01-21}}</ref>

[[File:AOs-1s-2pz.png|left|250px|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.]]

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=Brucat/>

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&nbsp;eV to strip a [[Stationary state|ground-state]] electron from a hydrogen atom,<ref>{{cite web
|last=Herter|first=Terry|year=2006
|url=http://astrosun2.astro.cornell.edu/academics/courses/astro101/herter/lectures/lec08.htm
|title=Lecture 8: The Hydrogen Atom
|publisher=Cornell University|accessdate=2008-02-14}}</ref> compared to 2.23&nbsp;''million'' eV for splitting a [[deuterium]] nucleus.<ref>{{cite journal
|last=Bell|first=R. E.
|title=Gamma-Rays from the Reaction H<sup>1</sup>(n,γ)D<sup>2</sup> and the Binding Energy of the Deuteron
|journal=[[Physical Review]]|year=1950
|volume=79|issue=2|pages=282–285
|doi=10.1103/PhysRev.79.282
|last2=Elliott
|first2=L. G.}}</ref> 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]].<ref>Smirnov (2003:249–72).</ref>

==Properties==
===Nuclear properties===
{{Main|Isotope|Stable isotope|List of elements by nuclear stability|List of elements by stability of isotopes}}

By definition, any two atoms with an identical number of ''protons'' in their nuclei belong to the same [[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 atom|hydrogen-1]], one neutron ([[deuterium]]), two neutrons ([[tritium]]) and [[isotopes of hydrogen|more than two neutrons]]. The hydrogen-1 is by far the most common form, and is sometimes called protium.<ref>{{cite web
|last=Matis|first=Howard S.|date=August 9, 2000
|url=http://www.lbl.gov/abc/wallchart/chapters/02/3.html
|title=The Isotopes of Hydrogen
|work=Guide to the Nuclear Wall Chart
|publisher=Lawrence Berkeley National Lab
|accessdate=2007-12-21}}</ref> The known elements form a set of atomic numbers from hydrogen with a single proton up to the 118-proton element [[ununoctium]].<ref>{{cite news
|last=Weiss|first=Rick|date=October 17, 2006
|title=Scientists Announce Creation of Atomic Element, the Heaviest Yet
|publisher=Washington Post
|url=http://www.washingtonpost.com/wp-dyn/content/article/2006/10/16/AR2006101601083.html
|accessdate=2007-12-21}}</ref> All known isotopes of elements with atomic numbers greater than 82 are radioactive.<ref name=sills>Sills (2003:131–134).</ref><ref name=dume>{{cite news
|last=Dumé|first=Belle|date=April 23, 2003
|title=Bismuth breaks half-life record for alpha decay
|publisher=Physics World
|url=http://physicsworld.com/cws/article/news/17319
|accessdate=2007-12-21}}</ref>

About 339 nuclides occur naturally on [[Earth]],<ref>{{cite web
|last=Lindsay|first=Don|date=July 30, 2000
|url=http://www.don-lindsay-archive.org/creation/isotope_list.html
|title=Radioactives Missing From The Earth
|publisher=Don Lindsay Archive
|accessdate=2007-05-23}}</ref> of which 256 (about 76%) have not been observed to decay, and are referred to as "[[stable isotope]]s". For 80 of the chemical elements, there is at least one stable isotope. Elements [[technitium|43]], [[promethium|61]], and all elements numbered [[bismuth|83]] or higher have no stable isotopes. As a rule, there is, for each element, only a handful of stable isotopes, the average being 3.1 stable isotopes per element among those that have stable isotopes. Twenty-seven elements have only a single stable isotope, while the largest number of stable isotopes observed for any element is ten, for the element [[tin]].<ref name=CRC>CRC Handbook (2002).</ref>

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 256 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/>

===Mass===
{{Main|Atomic mass}}

Because the large majority of an atom's mass comes from the protons and neutrons, the total number of these particles in an atom is called the [[mass number]]. The [[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 1.66{{Esp|−27}}&nbsp;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&nbsp;u.<ref>{{cite web
|last=Chieh|first=Chung
|date=January 22, 2001
|url=http://www.science.uwaterloo.ca/~cchieh/cact/nuctek/nuclideunstable.html
|title=Nuclide Stability
|publisher=University of Waterloo
|accessdate=2007-01-04}}</ref> An atom has a mass approximately equal to the mass number times the atomic mass unit.<ref>{{cite web
|url=http://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ele=&ascii=html&isotype=some
|title=Atomic Weights and Isotopic Compositions for All Elements
|publisher=National Institute of Standards and Technology
|accessdate=2007-01-04}}</ref> The heaviest [[stable atom]] is lead-208,<ref name=sills/> with a mass of 207.9766521&nbsp;u.<ref>{{cite journal
|last=Audi|first=G.
|title=The Ame2003 atomic mass evaluation (II)
|journal=[[Nuclear Physics A]]
|year=2003|volume=729|pages=337–676
|url=http://www.nndc.bnl.gov/amdc/web/masseval.html
|accessdate=2008-02-07
|doi=10.1016/j.nuclphysa.2003.11.003}}</ref>

As even the most massive atoms are far too light to work with directly, chemists instead use the unit of [[Mole (unit)|mole]]s. The mole is defined such that one mole of any element always has the same number of atoms (about [[Avogadro constant|6.022{{Esp|23}}]]). This number was chosen so that if an element has an atomic mass of 1&nbsp;u, a mole of atoms of that element has a mass close to 0.001&nbsp;kg, or 1&nbsp;gram. Because of the definition of the [[Atomic mass unit|unified atomic mass unit]], [[carbon]] has an atomic mass of ''exactly'' 12&nbsp;u, and so a mole of carbon atoms weighs exactly 0.012&nbsp;kg.<ref name=iupac>Mills ''et al.'' (1993).</ref>

===Shape and size===
{{Main|Atomic radius}}
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. 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>{{cite journal
|last = Shannon|first = R. D.
|title=Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides
|journal=[[Acta Crystallographica]], Section a
|year=1976|volume=32 |pages=751
|url=http://journals.iucr.org/a/issues/1976/05/00/issconts.html
|accessdate=2007-01-03
|doi=10.1107/S0567739476001551}}</ref> 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>{{cite web
|last=Dong|first=Judy|year=1998
|url=http://hypertextbook.com/facts/MichaelPhillip.shtml
|title=Diameter of an Atom
|publisher=The Physics Factbook
|accessdate=2007-11-19}}</ref> Consequently, the smallest atom is helium with a radius of 32&nbsp;[[Picometre|pm]], while one of the largest is [[caesium]] at 225&nbsp;pm.<ref>Zumdahl (2002).</ref>

When subjected to external fields, like an [[electrical field]], the shape of an atom may deviate from that a sphere. 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>{{cite journal
|last = [[Bethe]]|first = H.
|title=Termaufspaltung in Kristallen
|journal=[[Annalen der Physik]], 5. Folge
|year=1929|volume=3 |pages=133}}</ref>. Significant [[ellipsoid]]al deformations have recently been shown to occur for sulfur ions in [[pyrite]]-type compounds <ref>{{cite journal
|last = Birkholz|first = M.|coauthors=Rudert, R.
|title=Interatomic distances in pyrite-structure disulfides – a case for ellipsoidal modeling of sulfur ions
|journal=physica status solidi b
|year=2008|volume=245 |pages=1858
|url=http://www.mariobirkholz.de/pssb2008.pdf
|doi=10.1002/pssb.200879532}}</ref>

Atomic dimensions are thousands of times smaller than the wavelengths of [[light]] (400–700&nbsp;[[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&nbsp;million carbon atoms in width.<ref>{{cite web
|author=Staff|year=2007
|url=http://oregonstate.edu/terra/2007winter/features/nanotech.php
|title=Small Miracles: Harnessing nanotechnology
|publisher=Oregon State University
|accessdate=2007-01-07}}—describes the width of a human hair as 10<sup>5</sup>&nbsp;nm and 10 carbon atoms as spanning 1&nbsp;nm.</ref> A single drop of water contains about 2&nbsp;[[sextillion]] (2{{Esp|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&nbsp;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 2{{Esp|-4}}&nbsp;kg contains about 10&nbsp;sextillion (10<sup>22</sup>) atoms of [[carbon]].<ref group=note>A carat is 200&nbsp;milligrams. [[Atomic mass|By definition]], carbon-12 has 0.012&nbsp;kg per mole. The [[Avogadro constant]] defines 6{{Esp|23}}&nbsp;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.<ref>Feynman (1995).</ref>

===Radioactive decay===
{{Main|Radioactive decay}}

[[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.]]

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&nbsp;fm.<ref name=splung>{{cite web
|url=http://www.splung.com/content/sid/5/page/radioactivity
|title=Radioactivity|publisher=Splung.com
|accessdate=2007-12-19}}</ref>

The most common forms of radioactive decay are:<ref>L'Annunziata (2003:3–56).</ref><ref>{{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>
*[[Alpha decay]] is caused when the nucleus emits an alpha particle, which is a helium nucleus consisting of two protons and two neutrons. The result of the emission is a new element with a lower [[atomic number]].
*[[Beta decay]] is regulated by the [[weak force]], and results from a transformation of a neutron into a proton, or a proton into a neutron. The first is accompanied by the emission of an electron and an [[antineutrino]], while the second causes the emission of a [[positron]] and a [[neutrino]]. The electron or positron emissions are called beta particles. Beta decay either increases or decreases the atomic number of the nucleus by one.
*[[Gamma decay]] results from a change in the energy level of the nucleus to a lower state, resulting in the emission of electromagnetic radiation. This can occur following the emission of an alpha or a beta particle from radioactive decay.

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]], or result (through [[internal conversion]]) in production of high-speed electrons that are not beta rays, and high-energy photons that are not gamma rays.

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/>

===Magnetic moment===
{{Main|Electron magnetic dipole moment|Nuclear magnetic moment}}

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&nbsp;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>{{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}}</ref>

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=schroeder>{{cite web
|last=Schroeder|first=Paul A.
|date=February 25, 2000
|url=http://www.gly.uga.edu/schroeder/geol3010/magnetics.html
|title=Magnetic Properties
|publisher=University of Georgia
|accessdate=2007-01-07
}}</ref>

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=schroeder/><ref>{{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>

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>{{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
}}</ref><ref>Liang and Haacke (1999:412–26).</ref>

===Energy levels===
{{Main|Energy level|Atomic spectral line}}

When an electron is bound to an atom, it has a [[potential energy]] that is inversely proportional to its distance from the nucleus. This is measured by the amount of energy needed to unbind the electron from the atom, and is usually given in units of [[electronvolt]]s (eV). In the quantum mechanical model, a bound electron can only occupy a set of states centered on the nucleus, and each state corresponds to a specific energy level. The lowest energy state of a bound electron is called the ground state, while an electron at a higher energy level is in an excited state.<ref>{{cite web
|last=Zeghbroeck|first=Bart J. Van|year=1998
|url=http://physics.ship.edu/~mrc/pfs/308/semicon_book/eband2.htm
|title=Energy levels|publisher=Shippensburg University
|accessdate=2007-12-23}}</ref>

For an electron to transition between two different states, it must absorb or emit a [[photon]] at an energy matching the difference in the potential energy of those levels. The energy of an emitted photon is proportional to its [[frequency]], so these specific energy levels appear as distinct bands in the [[electromagnetic spectrum]].<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>{{cite web
|last=Martin|first=W. C.
|coauthors=Wiese, W. L.|month=May|year=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}}</ref>

[[File:Fraunhofer lines.svg|right|thumb|300px|An example of absorption lines in a spectrum.]]

When a continuous spectrum of energy is passed through a gas or plasma, some of the photons are absorbed by atoms, causing electrons to change their energy level. Those excited electrons that remain bound to their atom 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 doesn't include the continuous spectrum in the background, instead sees a series of [[Spectral line|emission lines]] from the photons emitted by the atoms.) [[Spectroscopy|Spectroscopic]] measurements of the strength and width of [[spectral line]]s allow the composition and physical properties of a substance to be determined.<ref name=>{{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>

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>{{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> 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>{{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
}}</ref> 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]].<ref>Beyer (2003:232–236).</ref>

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>{{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
}}</ref>

===Valence and bonding behavior===
{{Main|Valence (chemistry)|Chemical bond}}

The outermost electron shell of an atom in its uncombined state is known as the valence shell, and the electrons in
that shell are called [[valence electron]]s. The number of valence electrons determines the [[chemical bond|bonding]]
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>{{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> 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>{{cite web |url=http://www.chemguide.co.uk/atoms/bonding/covalent.html |title=Covalent bonding - Single bonds |publisher=chemguide |year=2000}}</ref>

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>{{cite web
|author=Husted, Robert 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}}
</ref><ref>{{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>

===States===
{{Main|State of matter|Phase (matter)}}

[[File:Bose Einstein condensate.png|left|250px|thumb|Snapshots illustrating the formation of a [[Bose–Einstein condensate]].]]
Quantities of atoms are found in different states of matter that depend on the physical conditions, such as [[temperature]] and [[pressure]]. By varying the conditions, materials can transition between [[solid]]s, [[liquid]]s, [[gas]]es and [[plasma (physics)|plasmas]].<ref>Goodstein (2002:436–438).</ref> Within a state, a material can also exist in different phases. An example of this is solid carbon, which can exist as [[graphite]] or [[diamond]].<ref>{{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|pages=719–24
|doi=10.1070/PU2006v049n07ABEH006013}}</ref>

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.<ref>Myers (2003:85).</ref><ref>{{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}}</ref> This super-cooled collection of atoms
then behaves as a single [[super atom]], which may allow fundamental checks of quantum mechanical behavior.<ref>{{cite web
|last=Colton|first=Imogen|coauthors=Fyffe, Jeanette
|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}}</ref>

{{Clear}}

==Identification==
[[File:Atomic resolution Au100.JPG|right|250px|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.]]
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>{{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
|month=November|year=1997|accessdate=2008-01-11
}}</ref><ref>{{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>

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>{{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}}</ref>

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>{{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
|issn=0034-6748|doi=10.1063/1.1683116}}</ref>

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>{{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}}</ref> [[Helium]] was discovered in this way in the spectrum of the Sun 23&nbsp;years before it was found on Earth.<ref>{{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}}</ref>

==Origin and current state==
Atoms form about 4% of the total energy density of the observable [[universe]], with an average density of about 0.25&nbsp;atoms/m<sup>3</sup>.<ref>{{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}}</ref> 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>.<ref>Choppin ''et al.'' (2001).</ref> 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>{{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
|url=http://www.sciencemag.org/cgi/content/abstract/259/5093/327
|accessdate=2008-01-07
|doi=10.1126/science.259.5093.327
|pmid=17832344}}</ref> 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.<ref>Lequeux (2005:4).</ref> (The remainder of the mass is an unknown [[dark matter]].<ref>{{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}}</ref>)

===Nucleosynthesis===
{{Main|Nucleosynthesis}}
Stable protons and electrons appeared one second after the [[Big Bang]]. During the following three minutes, [[Big Bang nucleosynthesis]] produced most of the [[helium]], [[lithium]], and [[deuterium]] in the universe, and perhaps some of the [[beryllium]] and [[boron]].<ref>{{cite journal
|last=Croswell|first=Ken
|title=Boron, bumps and the Big Bang: Was matter spread evenly when the Universe began? Perhaps not; the clues lie in the creation of the lighter elements such as boron and beryllium
|journal=New Scientist|year=1991|issue=1794|pages=42
|url=http://space.newscientist.com/article/mg13217944.700-boron-bumps-and-the-big-bang-was-matter-spread-evenly-whenthe-universe-began-perhaps-not-the-clues-lie-in-the-creation-of-thelighter-elements-such-as-boron-and-beryllium.html
|accessdate=2008-01-14}}</ref><ref>{{cite journal
|last=Copi|first=Craig J.
|title=Big-Bang Nucleosynthesis and the Baryon Density of the Universe
|journal=[[Science (journal)|Science]] |year=1995|volume=267|pages=192–99
|url=http://www.sciencemag.org/cgi/reprint/267/5195/192.pdf
|doi = 10.1126/science.7809624
|format=PDF|accessdate=2008-01-13 |pmid=7809624
|last2=Schramm
|first2=DN
|last3=Turner
|first3=MS
|issue=5195
}}</ref><ref>{{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
}}</ref> The first atoms (complete with bound electrons) were theoretically created 380,000&nbsp;years after the Big Bang—an epoch called [[Timeline of the Big Bang#Recombination: ca 377,000 years|recombination]], when the expanding universe cooled enough to allow electrons to become attached to nuclei.<ref>{{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> Since then, atomic nuclei have been combined in [[star]]s through the process of [[nuclear fusion]] to produce elements up to iron.<ref>{{cite journal
|title=The synthesis of the elements from hydrogen
|author = F. Hoyle
|journal = [[Monthly Notices of the Royal Astronomical Society]]
|volume = 106|pages = 343–83|year=1946
|url=http://adsabs.harvard.edu/abs/1946MNRAS.106..343H
|accessdate=2008-01-13}}</ref>

Isotopes such as lithium-6 are generated in space through [[cosmic ray spallation]].<ref>{{cite journal
|last=Knauth|first=D. C.
|title=Newly synthesized lithium in the interstellar medium
|journal=[[Nature (journal)|Nature]] |year=2000|volume=405|pages=656–58
|doi=10.1038/35015028
|last2=Knauth
|first2=D. C.
|last3=Lambert
|first3=David L.
|last4=Crane
|first4=P.}}</ref> This occurs when a high-energy proton strikes an atomic nucleus, causing large numbers of nucleons to be ejected. Elements heavier than iron were produced in [[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>{{cite web
|last=Mashnik|first=Stepan G.
|title=On Solar System and Cosmic Rays Nucleosynthesis and Spallation Processes
|url=http://arxiv.org/abs/astro-ph/0008382
|month=August|year=2000|publisher=Cornell University
|accessdate=2008-01-14}}</ref> Elements such as [[lead]] formed largely through the radioactive decay of heavier elements.<ref>{{cite web
|author=Kansas Geological Survey|date=May 4, 2005
|url=http://www.kgs.ku.edu/Extension/geotopics/earth_age.html
|title=Age of the Earth|publisher=University of Kansas
|accessdate=2008-01-14}}</ref>

===Earth===

Most of the atoms that make up the [[Earth]] and its inhabitants were present in their current form in the [[nebula]] that collapsed out of a [[molecular cloud]] to form the [[Solar System]]. The rest are the result of radioactive decay, and their relative proportion can be used to determine the [[age of the Earth]] through [[radiometric dating]].<ref name = "Manuel_2001">Manuel (2001:407–430,511–519).</ref><ref>{{cite journal
|last=Dalrymple|first=G. Brent
|title=The age of the Earth in the twentieth century: a problem (mostly) solved
|journal=Geological Society, London, Special Publications
|year=2001|volume=190|pages=205–21
|doi=10.1144/GSL.SP.2001.190.01.14
|url=http://sp.lyellcollection.org/cgi/content/abstract/190/1/205
|accessdate=2008-01-14}}</ref> 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>{{cite web
|last=Anderson|first=Don L.
|authorlink=Don L. Anderson
|coauthors=Foulger, G. R.; Meibom, Anders
|date=September 2, 2006
|url=http://www.mantleplumes.org/HeliumFundamentals.html
|title=Helium: Fundamental models
|publisher=MantlePlumes.org|accessdate=2007-01-14}}</ref>

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>{{cite news
|last=Pennicott|first=Katie|date=May 10, 2001
|title=Carbon clock could show the wrong time
|publisher=PhysicsWeb
|url=http://physicsworld.com/cws/article/news/2676
|accessdate=2008-01-14}}</ref> Some atoms on Earth have been artificially generated either deliberately or as by-products of nuclear reactors or explosions.<ref>{{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
}}</ref><ref>{{cite journal
|author=Diamond, H
|title=Heavy Isotope Abundances in Mike Thermonuclear Device
|journal=[[Physical Review]] |year=1960
|volume=119
|pages=2000–04
|url=http://prola.aps.org/abstract/PR/v119/i6/p2000_1
|accessdate=2008-01-14
|doi=10.1103/PhysRev.119.2000
|format=subscription required
|last2=Fields
|first2=P. R.
|last3=Stevens
|first3=C. S.
|last4=Studier
|first4=M. H.
|last5=Fried
|first5=S. M.
|last6=Inghram
|first6=M. G.
|last7=Hess
|first7=D. C.
|last8=Pyle
|first8=G. L.
|last9=Mech
|first9=J. F.}}</ref> Of the [[Transuranium element|transuranic elements]]—those with atomic numbers greater than 92—only [[plutonium]] and [[neptunium]] occur naturally on Earth.<ref>{{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><ref>{{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
|url=http://www.osti.gov/energycitations/product.biblio.jsp?osti_id=4353086
|accessdate=2008-01-15}}</ref> Transuranic elements have radioactive lifetimes shorter than the current age of the Earth<ref>Marco (2001:17).</ref> and thus identifiable quantities of these elements have long since decayed, with the exception of traces of [[plutonium-244]] possibly deposited by cosmic dust.<ref name = "Manuel_2001"/> Natural deposits of plutonium and neptunium are produced by [[neutron capture]] in uranium ore.<ref>{{cite web
|url=http://www.oklo.curtin.edu.au/index.cfm
|title=Oklo Fossil Reactors
|publisher=Curtin University of Technology
|accessdate=2008-01-15}}</ref>

The Earth contains approximately 1.33{{Esp|50}} atoms.<ref>{{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> In the planet's atmosphere, small numbers of independent atoms of [[noble gas]]es exist, such as [[argon]] and [[neon]]. The remaining 99% of 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, 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>{{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
}}</ref><ref>{{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
|url=http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=137819
|accessdate=2008-01-16|doi=10.1073/pnas.232565899
|pmid=12391308
|pmc=137819}}</ref> This atomic matter forms networked arrangements that lack the particular type of small-scale interrupted order associated with molecular matter.<ref>Pauling (1960:5–10).</ref>

===Rare and theoretical forms===
While isotopes with atomic numbers higher than [[lead]] (82) are known to be radioactive, an "[[island of stability]]" has been proposed for some elements with atomic numbers above 103. These [[superheavy element]]s may have a nucleus that is relatively stable against radioactive decay.<ref>{{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}}</ref> The most likely candidate for a stable superheavy atom, [[unbihexium]], has 126&nbsp;protons and 184&nbsp;neutrons.<ref>{{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|pages=19
|url=http://pubs.acs.org/cen/news/84/i10/8410notw9.html
|accessdate=2008-01-14}}</ref>

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 the [[baryogenesis]] theories may offer an explanation.) As a result, no antimatter atoms have been discovered in nature.<ref>{{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><ref>{{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://www.hno.harvard.edu/gazette/2001/08.16/antimatter.html
|accessdate=2008-01-14
}}</ref> However, in 1996, [[antihydrogen]], the antimatter counterpart of hydrogen, was synthesized at the [[CERN]] laboratory in [[Geneva]].<ref>{{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
}}</ref><ref>{{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>

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>{{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}}
</ref><ref>{{cite journal
|last=Indelicato|first=Paul
|title=Exotic Atoms|journal=[[Physica Scripta]]
|year=2004|volume=T112|pages=20–26
|doi=10.1238/Physica.Topical.112a00020}}
</ref><ref>{{cite web
|last=Ripin|first=Barrett H.|month=July|year=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>

==See also==
{{col-start}}
{{col-break}}
*[[Atomic mirror]]
*[[History of quantum mechanics]]
*[[Infinite divisibility]]
*[[Introduction to quantum mechanics]]
*[[List of basic chemistry topics]]
{{col-break}}
*[[Subatomic particle]]
*[[Molecule]]
*[[Nuclear model]]
*[[Radioactive isotope]]
*[[Transuranium element]]

{{col-end}}

==Notes==
<references group=note/>

==References==
<!-- this 'empty' section displays references defined elsewhere -->
{{Reflist|colwidth=35em|refs=

<ref name=leigh1990>{{cite book | editor=Leigh, G. J. | year=1990 | title=International Union of Pure and Applied Chemistry, Commission on the Nomenclature of Inorganic Chemistry, [[Nomenclature of Organic Chemistry]] - Recommendations 1990 | publisher=Blackwell Scientific Publications | isbn=0-08-022369-9 | page=35 | location=Oxford | quote=An atom is the smallest unit quantity of an element that is capable of existence whether alone or in chemical combination with other atoms of the same or other elements. }}</ref>

<ref name=liddell_scott_to_cut>{{cite web | author=Liddell, Henry George; Scott, Robert | title=τέμνω | title=A Greek-English Lexicon | publisher=Perseus Digital Library | url=http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3Dte%2Fmnw1 }}</ref>

<ref name=liddell_scott_uncuttable>{{cite web | author=Liddell, Henry George; Scott, Robert | title=ἄτομος | work=A Greek-English Lexicon | publisher=Perseus Digital Library | url=http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3Da%29%2Ftomos | accessdate=2010-06-21 }}</ref>

<ref name=haubold_mathai1998>{{cite web | last=Haubold | first=Hans | last2=Mathai | first2=A.M. | year=1998 | title=Microcosmos: From Leucippus to Yukawa|url=http://www.columbia.edu/~ah297/unesa/universe/universe-chapter3.html | work=Structure of the Universe | accessdate=2008-01-17 }}</ref>

<ref name=slac_20090615>{{cite web | date=15 June 2009 | title=Radioactive Decays | url=http://www2.slac.stanford.edu/vvc/theory/nuclearstability.html | publisher=[[Stanford Linear Accelerator Center]] | accessdate=2009-07-04 }}</ref>

}}

===Book references===
*{{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=0124366031
|oclc=162129551}}
*{{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=0750304812
|oclc=47150433}}
*{{cite book
|last=Choppin|first=Gregory R.
|coauthors=Liljenzin, Jan-Olov; Rydberg, Jan
|year=2001|title=Radiochemistry and Nuclear Chemistry
|publisher=Elsevier|isbn=0750674636
|oclc=162592180}}
*{{cite book
|first=J.|last=Dalton
|authorlink=John Dalton|year=1808
|title=A New System of Chemical Philosophy, Part 1
|publisher=S. Russell
|location=London and Manchester}}
*{{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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|oclc=181435713}}
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|title=Indian Atomism: History and Sources
|publisher=Humanities Press|year=1981
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|oclc=10916778}}
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|title=Masks of the Universe: Changing Ideas on the Nature of the Cosmos
|publisher=Cambridge University Press
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|title= Dictionary of World Philosophy
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|last = Levere|first = Trevor, H.|year=2001
|title= Transforming Matter – A History of Chemistry for Alchemy to the Buckyball
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|volume=vol. 2|pages=412–26
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|publisher=John Wiley & Sons
|url=http://ieeexplore.ieee.org/iel5/8734/27658/01233976.pdf?arnumber=1233976
|format=PDF|accessdate=2008-01-09|isbn=0471139467}}
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|oclc=173081482}}

==External links==
{{Wikisource1914NSRW|Atom}}
{{Commons|Atom}}
*{{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}}
*{{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}}
*{{cite web
|url=http://en.wikibooks.org/wiki/FHSST_Physics_Atom:The_Atom
|work=Free High School Science Texts: Physics
|title=Atom:The Atom|publisher=Wikibooks
|accessdate=2007-01-09}}
*{{cite web
|author=Anonymous|year=2007|url=http://www.scienceaid.co.uk/chemistry/basics/theatom.html
|title=The atom|publisher=Science aid+
|accessdate=2007-01-09}}—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}}
*{{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}}
*{{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}}
{{featured article}}

[[Category:Atoms| ]]
[[Category:Fundamental physics concepts]]
[[Category:Chemistry]]
[[Category:Greek loanwords]]

{{Link GA|no}}
{{Link GA|uk}}
{{Link FA|id}}
{{Link FA|zh}}

[[af:Atoom]]
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[[ru:Атом]]
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[[scn:Àtumu]]
[[si:පරමාණු]]
[[simple:Atom]]
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[[szl:Atům]]
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[[ug:ئاتوم]]
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[[fiu-vro:Aadom]]
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[[wo:Xarefulwoon]]
[[yi:אטאם]]
[[yo:Átọ̀mù]]
[[zh-yue:原子]]
[[bat-smg:Atuoms]]
[[zh:原子]]

Revision as of 09:04, 30 June 2010

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