View source for Atom

{{Short description|Smallest unit of a chemical element}}
{{Other uses}}
{{pp-semi-indef|small=yes}}
{{Use dmy dates|date=August 2024}}
{{Infobox
| above = Atom
| abovestyle = background-color:#B5B5CC;
| headerstyle = background-color:#B5B5CC;
| image = [[File:Helium atom QM.svg|300px|Helium atom ground state]]
| caption = An illustration of the [[helium]] atom, depicting the [[atomic nucleus|nucleus]] (pink) and the [[electron cloud]] distribution (black). The nucleus (upper right) in helium-4 is in reality spherically symmetric and closely resembles the electron cloud, although for more complicated nuclei this is not always the case. The black bar is one [[angstrom]] ({{val|e=-10|u=m}} or {{val|100|ul=pm}}).
| header1 = Classification
| data2 = Smallest recognized division of a chemical element
| header3 = Properties
| label4 = [[atomic mass|Mass range]]
| data4  = {{val|1.67|e=-27}} to {{val|4.52|e=-25|u=kg}}
| label5 = [[Electric charge]]
| data5  = zero (neutral), or [[ion]] charge
| label6 = [[Diameter]] range
| data6  = 62&nbsp;pm ([[Helium|He]]) to 520&nbsp;pm ([[Caesium|Cs]]) ([[Atomic radii of the elements (data page)|data&nbsp;page]])
| label7 = [[Subatomic particle|Components]]
| data7  = [[Electron]]s and a compact [[atomic nucleus|nucleus]] of [[proton]]s and [[neutron]]s
}}

'''Atoms''' are the basic particles of the [[chemical element]]s. An atom consists of a [[atomic nucleus|nucleus]] of [[proton]]s and generally [[neutron]]s, surrounded by an electromagnetically bound swarm of [[electron]]s.  The chemical elements are distinguished from each other by the number of protons that are in their atoms. For example, any atom that contains 11 protons is [[sodium]], and any atom that contains 29 protons is [[copper]]. Atoms with the same number of protons but a different number of neutrons are called [[isotope]]s of the same element.

Atoms are extremely small, typically around 100&nbsp;[[picometer]]s across. A human hair is about a million carbon atoms wide. Atoms are smaller than the shortest wavelength of visible light, which means humans cannot see atoms with conventional microscopes. They are so small that accurately predicting their behavior using [[classical physics]] is not possible due to [[quantum mechanics|quantum effects]].

More than 99.9994%<ref>{{cite web |title=DOE Explains...Nuclei |url=https://www.energy.gov/science/doe-explainsnuclei |access-date=2024-11-05 |website=Energy.gov |language=en}}</ref> of an atom's [[mass]] is in the nucleus. Protons have a positive [[electric charge]] and neutrons have no charge, so the nucleus is positively charged. The electrons are negatively charged, and this opposing charge is what binds them to the nucleus. If the numbers of [[proton]]s and electrons are equal, as they normally are, then the atom is electrically neutral as a whole. If an atom has more electrons than protons, then it has an overall negative charge and is called a negative [[ion]] (or anion). Conversely, if it has more protons than electrons, it has a positive charge and is called a positive ion (or cation).

The electrons of an atom are attracted to the protons in an atomic nucleus by the [[electromagnetic force]]. The protons and neutrons in the nucleus are attracted to each other by the [[nuclear force]]. This force is usually stronger than the electromagnetic force that repels the positively charged protons from one another. Under certain circumstances, the repelling electromagnetic force becomes stronger than the nuclear force. In this case, the nucleus [[Nuclear fission|splits]] and leaves behind different elements. This is a form of [[nuclear decay]].

Atoms can attach to one or more other atoms by [[chemical bond]]s to form [[chemical compound]]s such as [[molecule]]s or [[crystal]]s. The ability of atoms to attach and detach from each other is responsible for most of the physical changes observed in nature. [[Chemistry]] is the science that studies these changes.

== History of atomic theory ==
{{Main|History of atomic theory}}

=== In philosophy ===
{{Main|Atomism}}
The basic idea that matter is made up of tiny indivisible particles is an old idea that appeared in many ancient cultures. The word ''atom'' is derived from the [[ancient Greek]] word ''atomos'',{{efn|a combination of the negative term "a-" and "τομή," the term for "cut"}} which means "uncuttable". But this ancient idea was based in philosophical reasoning rather than scientific reasoning. Modern atomic theory is not based on these old concepts.<ref>{{cite book|last1=Pullman|first1=Bernard|title=The Atom in the History of Human Thought|date=1998|publisher=Oxford University Press|location=Oxford, England|isbn=978-0-19-515040-7|pages=31–33|url=https://books.google.com/books?id=IQs5hur-BpgC&q=Leucippus+Democritus+atom&pg=PA56|access-date=25 October 2020|archive-date=5 February 2021|archive-url=https://web.archive.org/web/20210205165029/https://books.google.com/books?id=IQs5hur-BpgC&q=Leucippus+Democritus+atom&pg=PA56|url-status=live}}</ref><ref>[[#refMelsen1952|Melsen (1952). ''From Atomos to Atom'', pp. 18–19]]</ref> In the early 19th century, the scientist [[John Dalton]] found evidence that matter really is composed of discrete units, and so applied the word ''atom'' to those units.<ref>[[#refPullman1998|Pullman (1998). ''The Atom in the History of Human Thought'', p. 201]]</ref>

=== Dalton's law of multiple proportions ===
[[File:Daltons symbols.gif|thumb|right|Various atoms and molecules from ''A New System of Chemical Philosophy'' (John Dalton 1808).]]
In the early 1800s, John Dalton compiled experimental data gathered by him and other scientists and discovered a pattern now known as the "[[law of multiple proportions]]". He noticed that in any group of chemical compounds which all contain two particular chemical elements, the amount of Element A per measure of Element B will differ across these compounds by ratios of small whole numbers. This pattern suggested that each element combines with other elements in multiples of a basic unit of weight, with each element having a unit of unique weight. Dalton decided to call these units "atoms".<ref>Pullman (1998). ''The Atom in the History of Human Thought'', p. 199: "The constant ratios, expressible in terms of integers, of the weights of the constituents in composite bodies could be construed as evidence on a macroscopic scale of interactions at the microscopic level between basic units with fixed weights. For Dalton, this agreement strongly suggested a corpuscular structure of matter, even though it did not constitute definite proof."</ref>

For example, there are two types of [[tin oxide (disambiguation)|tin oxide]]: one is a grey powder that is 88.1% tin and 11.9% oxygen, and the other is a white powder that is 78.7% tin and 21.3% oxygen. Adjusting these figures, in the grey powder there is about 13.5&nbsp;g of oxygen for every 100&nbsp;g of tin, and in the white powder there is about 27&nbsp;g of oxygen for every 100&nbsp;g of tin. 13.5 and 27 form a ratio of 1:2. Dalton concluded that in the grey oxide there is one atom of oxygen for every atom of tin, and in the white oxide there are two atoms of oxygen for every atom of tin ([[tin(II) oxide|SnO]] and [[tin dioxide|SnO<sub>2</sub>]]).<ref>[[#refDalton1817|Dalton (1817). ''A New System of Chemical Philosophy'' vol. 2, p. 36]]</ref><ref>[[#refMelsen1952|Melsen (1952). ''From Atomos to Atom'', p. 137]]</ref>

Dalton also analyzed [[iron oxide]]s. There is one type of iron oxide that is a black powder which is 78.1% iron and 21.9% oxygen; and there is another iron oxide that is a red powder which is 70.4% iron and 29.6% oxygen. Adjusting these figures, in the black powder there is about 28&nbsp;g of oxygen for every 100&nbsp;g of iron, and in the red powder there is about 42&nbsp;g of oxygen for every 100&nbsp;g of iron. 28 and 42 form a ratio of 2:3. Dalton concluded that in these oxides, for every two atoms of iron, there are two or three atoms of oxygen respectively ([[iron(II) oxide|Fe<sub>2</sub>O<sub>2</sub>]] and [[iron(III) oxide|Fe<sub>2</sub>O<sub>3</sub>]]).{{efn|Iron(II) oxide's formula is written here as "Fe<sub>2</sub>O<sub>2</sub>" rather than the more conventional "FeO" because this better illustrates the explanation.}}<ref>[[#refDalton1817|Dalton (1817). ''A New System of Chemical Philosophy'' vol. 2, p. 28]]</ref><ref>[[#refMillington1906|Millington (1906). ''John Dalton'', p. 113]]</ref>

As a final example: [[nitrous oxide]] is 63.3% nitrogen and 36.7% oxygen, [[nitric oxide]] is 44.05% nitrogen and 55.95% oxygen, and [[nitrogen dioxide]] is 29.5% nitrogen and 70.5% oxygen. Adjusting these figures, in nitrous oxide there is 80&nbsp;g of oxygen for every 140&nbsp;g of nitrogen, in nitric oxide there is about 160&nbsp;g of oxygen for every 140&nbsp;g of nitrogen, and in nitrogen dioxide there is 320&nbsp;g of oxygen for every 140&nbsp;g of nitrogen. 80, 160, and 320 form a ratio of 1:2:4. The respective formulas for these oxides are [[nitrous oxide|N<sub>2</sub>O]], [[nitric oxide|NO]], and [[nitrogen dioxide|NO<sub>2</sub>]].<ref>[[#refDalton1808|Dalton (1808). ''A New System of Chemical Philosophy'' vol. 1, pp. 316–319]]</ref><ref>[[#refHolbrowEtAl2010|Holbrow et al. (2010). ''Modern Introductory Physics'', pp. 65–66]]</ref>

=== Discovery of the electron ===
In 1897, [[J. J. Thomson]] discovered that [[cathode ray]]s can be deflected by electric and magnetic fields, which meant that cathode rays are not a form of light but made of electrically charged particles, and their charge was negative given the direction the particles were deflected in.<ref>{{cite journal |author=J. J. Thomson |url=http://web.lemoyne.edu/~GIUNTA/thomson1897.html |title=Cathode rays |journal=Philosophical Magazine |volume=44 |issue=269 |pages=293–316 |year=1897}}</ref> He measured these particles to be 1,700 times lighter than [[hydrogen]] (the lightest atom).<ref>In his book ''The Corpuscular Theory of Matter'' (1907), Thomson estimates electrons to be 1/1700 the mass of hydrogen.</ref> He called these new particles ''corpuscles'' but they were later renamed ''[[electron]]s'' since these are the particles that carry electricity.<ref>[http://library.thinkquest.org/C0111709/English/DC-Circuts/mechanism.html "The Mechanism Of Conduction In Metals"] {{Webarchive|url=https://web.archive.org/web/20121025004809/http://library.thinkquest.org/ |date=25 October 2012 }}, Think  Quest.</ref> Thomson also showed that electrons were identical to particles given off by [[Photoelectric effect|photoelectric]] and radioactive materials.<ref name="Thomson">{{cite journal|last=Thomson|first=J.J.|title=On bodies smaller than atoms|journal=The Popular Science Monthly|pages=323–335|date=August 1901|url=https://books.google.com/books?id=3CMDAAAAMBAJ&pg=PA323|access-date=21 June 2009|archive-date=1 December 2016|archive-url=https://web.archive.org/web/20161201152039/https://books.google.com/books?id=3CMDAAAAMBAJ&pg=PA323|url-status=live}}</ref> Thomson explained that an electric current is the passing of electrons from one atom to the next, and when there was no current the electrons embedded themselves in the atoms. This in turn meant that atoms were not indivisible as scientists thought. The atom was composed of electrons whose negative charge was balanced out by some source of positive charge to create an electrically neutral atom. Ions, Thomson explained, must be atoms which have an excess or shortage of electrons.<ref>J. J. Thomson (1907). ''On the Corpuscular Theory of Matter'', p. 26: "The simplest interpretation of these results is that the positive ions are the atoms or groups of atoms of various elements from which one or more corpuscles have been removed [...] while the negative electrified body is one with more corpuscles than the unelectrified one."</ref>

=== Discovery of the nucleus ===
[[File:Geiger-Marsden experiment expectation and result.svg|thumb|right|The [[Rutherford scattering experiments]]: The extreme scattering of some alpha particles suggested the existence of a nucleus of concentrated charge.]]
{{Main|Rutherford scattering experiments}}
The electrons in the atom logically had to be balanced out by a commensurate amount of positive charge, but Thomson had no idea where this positive charge came from, so he tentatively proposed that it was everywhere in the atom, the atom being in the shape of a sphere. This was the mathematically simplest hypothesis to fit the available evidence, or lack thereof. Following from this, Thomson imagined that the balance of electrostatic forces would distribute the electrons throughout the sphere in a more or less even manner.<ref>J. J. Thomson (1907). ''The Corpuscular Theory of Matter'', p. 103: "In default of exact knowledge of the nature of the way in which positive electricity occurs in the atom, we shall consider a case in which the positive electricity is distributed in the way most amenable to mathematical calculation, i.e., when it occurs as a sphere of uniform density, throughout which the corpuscles are distributed."</ref> Thomson's model is popularly known as the [[plum pudding model]], though neither Thomson nor his colleagues used this analogy.<ref name=HonGoldstein2013>{{cite journal |author1=Giora Hon |author2=Bernard R. Goldstein |date=6 September 2013 |title=J. J. Thomson's plum-pudding atomic model: The making of a scientific myth |journal=Annalen der Physik |volume=525 |issue=8–9 |pages=A129–A133 |doi= 10.1002/andp.201300732 |bibcode=2013AnP...525A.129H |url=https://onlinelibrary.wiley.com/doi/10.1002/andp.201300732 | issn=0003-3804}}</ref> Thomson's model was incomplete, it was unable to predict any other properties of the elements such as [[emission spectra]] and [[valency (chemistry)|valencies]]. It was soon rendered obsolete by the discovery of the [[atomic nucleus]].

Between 1908 and 1913, [[Ernest Rutherford]] and his colleagues [[Hans Geiger]] and [[Ernest Marsden]] performed a series of experiments in which they bombarded thin foils of metal with a beam of [[alpha particles]]. They did this to measure the scattering patterns of the alpha particles. They spotted a small number of alpha particles being deflected by angles greater than 90°. This shouldn't have been possible according to the Thomson model of the atom, whose charges were too diffuse to produce a sufficiently strong electric field. The deflections should have all been negligible. Rutherford proposed that the positive charge of the atom is concentrated in a tiny volume at the center of the atom and that the electrons surround this nucleus in a diffuse cloud. This nucleus carried almost all of the atom's mass, the electrons being so very light. Only such an intense concentration of charge, anchored by its high mass, could produce an electric field that could deflect the alpha particles so strongly.<ref name=Heilbron2003p64-68>[[#refHeilbron2003|Heilbron (2003). ''Ernest Rutherford and the Explosion of Atoms'', pp. 64–68]]</ref>

=== Bohr model ===
{{Main|Bohr model}}
[[File:Bohr atom animation 2.gif|right|thumb|The Bohr model of the atom, with an electron making instantaneous "quantum leaps" from one orbit to another with gain or loss of energy. This model of electrons in orbits is obsolete.]]
A problem in classical mechanics is that an accelerating charged particle radiates electromagnetic radiation, causing the particle to lose kinetic energy. Circular motion counts as acceleration, which means that an electron orbiting a central charge should spiral down into that nucleus as it loses speed. In 1913, the physicist [[Niels Bohr]] proposed a new model in which the electrons of an atom were assumed to orbit the nucleus but could only do so in a finite set of orbits, and could jump between these orbits only in discrete changes of energy corresponding to absorption or radiation of a photon.<ref name=stern20050516 /> This quantization was used to explain why the electrons' orbits are stable and why elements absorb and emit electromagnetic radiation in discrete spectra.<ref name=bohr19221211 /> Bohr's model could only predict the emission spectra of hydrogen, not atoms with more than one electron.

=== Discovery of protons and neutrons===
{{Main|Atomic nucleus|Discovery of the neutron}}
Back in 1815, [[William Prout]] observed that the atomic weights of many elements were multiples of hydrogen's atomic weight, which is in fact true for all of them if one takes [[isotopes]] into account. In 1898, [[J. J. Thomson]] found that the positive charge of a hydrogen ion is equal to the negative charge of an electron, and these were then the smallest known charged particles.<ref>{{cite journal |last=J. J. Thomson |date=1898 |title=On the Charge of Electricity carried by the Ions produced by Röntgen Rays |url=https://archive.org/details/londonedinburgh5461898lon/page/528/mode/2up |journal=The London, Edinburgh and Dublin Philosophical Magazine and Journal of Science |series=5 |volume=46 |issue=283 |pages=528–545 |doi=10.1080/14786449808621229}}</ref> Thomson later found that the positive charge in an atom is a positive multiple of an electron's negative charge.<ref>J. J. Thomson (1907). ''The Corpuscular Theory of Matter''. p. 26–27: "In an unelectrified atom there are as many units of positive electricity as there are of negative; an atom with a unit of positive charge is a neutral atom which has lost one corpuscle, while an atom with a unit of negative charge is a neutral atom to which an additional corpuscle has been attached."</ref> In 1913, [[Henry Moseley]] discovered that the frequencies of X-ray emissions from an [[excited state|excited]] atom were a mathematical function of its [[atomic number]] and hydrogen's nuclear charge. In 1919, [[Ernest Rutherford|Rutherford]] bombarded [[nitrogen]] gas with [[alpha particle]]s and detected [[hydrogen]] ions being emitted from the gas, and concluded that they were produced by alpha particles hitting and splitting the nuclei of the nitrogen atoms.<ref>{{cite journal|author=Rutherford, Ernest|url=http://web.lemoyne.edu/~GIUNTA/rutherford.html |title=Collisions of alpha Particles with Light Atoms. IV. An Anomalous Effect in Nitrogen|journal=Philosophical Magazine|year=1919|volume=37|page=581|doi=10.1080/14786440608635919|issue=222}}</ref>

These observations led Rutherford to conclude that the hydrogen nucleus is a singular particle with a positive charge equal to the electron's negative charge.<ref>''The Development of the Theory of Atomic Structure'' (Rutherford 1936). Reprinted in ''Background to Modern Science: Ten Lectures at Cambridge arranged by the History of Science Committee 1936'':<br />"In 1919 I showed that when light atoms were bombarded by α-particles they could be broken up with the emission of a proton, or hydrogen nucleus. We therefore presumed that a proton must be one of the units of which the nuclei of other atoms were composed..."</ref> He named this particle "[[proton]]" in 1920.<ref>{{cite journal |author=Orme Masson |date=1921 |title=The Constitution of Atoms |journal=The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science |volume=41 |issue=242 |pages=281–285 |doi=10.1080/14786442108636219 |url=https://archive.org/details/londonedinburg6411921lond/page/280/mode/2up }}<br />Footnote by Ernest Rutherford: 'At the time of writing this paper in Australia, Professor Orme Masson was not aware that the name "proton" had already been suggested as a suitable name for the unit of mass nearly 1, in terms of oxygen 16, that appears to enter into the nuclear structure of atoms. The question of a suitable name for this unit was discussed at an informal meeting of a number of members of Section A of the British Association at Cardiff this year. The name "baron" suggested by Professor Masson was mentioned, but was considered unsuitable on account of the existing variety of meanings. Finally the name "proton" met with general approval, particularly as it suggests the original term "protyle " given by Prout in his well-known hypothesis that all atoms are built up of hydrogen. The need of a special name for the nuclear unit of mass 1 was drawn attention to by Sir Oliver Lodge at the Sectional meeting, and the writer then suggested the name "proton."'</ref> The number of protons in an atom (which Rutherford called the "[[atomic number]]"<ref>Eric Scerri (2020). ''The Periodic Table: Its Story and Its Significance'', p. 185</ref><ref>Helge Kragh (2012). ''Niels Bohr and the Quantum Atom'', p. 33</ref>) was found to be equal to the element's ordinal number on the [[periodic table]] and therefore provided a simple and clear-cut way of distinguishing the elements from each other. The atomic weight of each element is higher than its proton number, so Rutherford hypothesized that the surplus weight was carried by unknown particles with no electric charge and a mass equal to that of the proton.

In 1928, [[Walter Bothe]] observed that [[beryllium]] emitted a highly penetrating, electrically neutral radiation when bombarded with alpha particles. It was later discovered that this radiation could knock hydrogen atoms out of [[paraffin wax]]. Initially it was thought to be high-energy [[gamma radiation]], since gamma radiation had a similar effect on electrons in metals, but [[James Chadwick]] found that the [[ionization]] effect was too strong for it to be due to electromagnetic radiation, so long as energy and momentum were conserved in the interaction. In 1932, Chadwick exposed various elements, such as hydrogen and nitrogen, to the mysterious "beryllium radiation", and by measuring the energies of the recoiling charged particles, he deduced that the radiation was actually composed of electrically neutral particles which could not be massless like the gamma ray, but instead were required to have a mass similar to that of a proton. Chadwick now claimed these particles as Rutherford's neutrons.<ref>{{cite journal|author=James Chadwick |year=1932|url=http://web.mit.edu/22.54/resources/Chadwick.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://web.mit.edu/22.54/resources/Chadwick.pdf |archive-date=9 October 2022 |url-status=live |title=Possible Existence of a Neutron|doi=10.1038/129312a0|journal=Nature|page=312|volume=129|bibcode = 1932Natur.129Q.312C|issue=3252|s2cid=4076465|doi-access=free}}</ref>

=== The current consensus model ===
[[File:S-p-Orbitals.svg|thumb|right|The modern model of atomic orbitals draws zones where an electron is most likely to be found at any moment.]]
In 1925, [[Werner Heisenberg]] published the first consistent mathematical formulation of quantum mechanics ([[matrix mechanics]]).<ref name="Pais">{{cite book|last=Pais|first=Abraham|year=1986|location=New York|title=Inward Bound: Of Matter and Forces in the Physical World|publisher=Oxford University Press|isbn=978-0-19-851971-3|pages=[https://archive.org/details/inwardboundofmat00pais_0/page/228 228–230]|url=https://archive.org/details/inwardboundofmat00pais_0/page/228}}</ref> One year earlier, [[Louis de Broglie]] had proposed that all particles behave like waves to some extent,<ref>{{cite book |title=Introducing Quantum Theory |author1=McEvoy, J. P. |author2=Zarate, Oscar  |publisher=Totem Books |year=2004 |isbn=978-1-84046-577-8 |pages=110–114}}</ref> and in 1926 [[Erwin Schrödinger]] used this idea to develop the [[Schrödinger equation]], which describes electrons as three-dimensional [[waveform]]s rather than points in space.<ref>{{cite web |last=Kozłowski |first=Miroslaw |year=2019 |title=The Schrödinger equation A History |url=https://www.researchgate.net/publication/332241721}}</ref> 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 a given point in time. This became known as the [[uncertainty principle]], formulated by Werner Heisenberg in 1927.<ref name="Pais" /> 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.<ref>{{cite web|author=Chad Orzel|url=https://www.youtube.com/watch?v=TQKELOE9eY4|title=What is the Heisenberg Uncertainty Principle?|website=TED-Ed|date=16 September 2014|via=YouTube|archive-url=https://web.archive.org/web/20150913185956/https://www.youtube.com/watch?v=TQKELOE9eY4|archive-date=13 September 2015|url-status=live}}</ref> 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 found.<ref name=brown2007 /><ref name=harrison2000 /> 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.

== Structure ==
=== 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]].

The electron is the least massive of these particles by four orders of magnitude 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.<ref>{{cite book|last=Demtröder|first=Wolfgang|year=2002|title=Atoms, Molecules and Photons: An Introduction to Atomic- Molecular- and Quantum Physics|url=https://archive.org/details/atomsmoleculesph00demt_277|url-access=limited|publisher=Springer|edition=1st|isbn=978-3-540-20631-6|oclc=181435713|pages=[https://archive.org/details/atomsmoleculesph00demt_277/page/n51 39]–42}}</ref> It was the lightest particle with a positive rest mass measured, until the discovery of [[neutrino]] mass. Under ordinary conditions, electrons are bound to the positively charged nucleus by the attraction created from opposite electric charges. If an atom has more or fewer electrons than its atomic number, then it becomes respectively negatively or positively charged as a whole; a charged atom is called an [[ion]]. Electrons have been known since the late 19th century, mostly thanks to [[J.J. Thomson]]; see [[history of subatomic physics]] for details.

Protons have a positive charge and a mass of {{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]].

Neutrons have no electrical charge and have a mass of {{val|1.6749|e=-27|u=kg}}.<ref>{{cite book|last=Woan|first=Graham|year=2000|title=The Cambridge Handbook of Physics|publisher=Cambridge University Press|isbn=978-0-521-57507-2|oclc=224032426|page=[https://archive.org/details/cambridgehandboo0000woan/page/8 8]|url=https://archive.org/details/cambridgehandboo0000woan/page/8}}</ref><ref name="2014 CODATA">Mohr, P.J.; Taylor, B.N. and Newell, D.B. (2014), [http://physics.nist.gov/constants "The 2014 CODATA Recommended Values of the Fundamental Physical Constants"] {{Webarchive|url=https://web.archive.org/web/20120211083747/http://physics.nist.gov/cuu/Constants/index.html |date=11 February 2012 }} (Web Version 7.0). The database was developed by J. Baker, M. Douma, and [[Svetlana Kotochigova|S. Kotochigova]]. (2014). National Institute of Standards and Technology, Gaithersburg, Maryland 20899.</ref> Neutrons are the heaviest of the three constituent particles, but their mass 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.<ref>{{cite book|last=MacGregor|first=Malcolm H.|year=1992|title=The Enigmatic Electron|publisher=Oxford University Press|isbn=978-0-19-521833-6|oclc=223372888|pages=[https://archive.org/details/astronomyencyclo0000unse/page/33 33–37]|url=https://archive.org/details/astronomyencyclo0000unse/page/33}}</ref> The neutron was discovered in 1932 by the English physicist [[James Chadwick]].

In the [[Standard Model]] of physics, electrons are truly elementary particles with no internal structure, whereas 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 +{{sfrac|2|3}}) and one [[down quark]] (with a charge of −{{sfrac|1|3}}). Neutrons consist of one up quark and two down quarks. This distinction accounts for the difference in mass and charge between the two particles.<ref name=pdg2002 /><ref name=schombert2006 />

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 />

=== Nucleus ===
{{Main|Atomic nucleus}}
[[File:Binding energy curve - common isotopes.svg|thumb|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 section. -->

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>1.07 \sqrt[3]{A}</math>&nbsp;[[femtometre]]s, where <math>A</math> is the total number of nucleons.<ref>{{cite book|last=Jevremovic|first=Tatjana|year=2005|title=Nuclear Principles in Engineering|url=https://archive.org/details/nuclearprinciple00jevr_450|url-access=limited|publisher=Springer|isbn=978-0-387-23284-3|oclc=228384008|page=[https://archive.org/details/nuclearprinciple00jevr_450/page/n83 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&nbsp;fm this force is much more powerful than the [[electrostatic force]] that causes positively charged protons to repel each other.<ref>{{cite book|last1=Pfeffer|first1=Jeremy I.|last2=Nir|first2=Shlomo|year=2000|title=Modern Physics: An Introductory Text|publisher=Imperial College Press|isbn=978-1-86094-250-1|oclc=45900880|pages=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 name=wenner2007 />

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.<ref name="raymond" />

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

[[File:Wpdms physics proton proton chain 1.svg|right|thumb|upright|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 to 10&nbsp;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 />

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.<ref>{{cite book|last1=Shultis|first1=J. Kenneth|last2=Faw|first2=Richard E.|title=Fundamentals of Nuclear Science and Engineering|year=2002|publisher=CRC Press|isbn=978-0-8247-0834-4|oclc=123346507|pages=10–17}}</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 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]] begins to decrease. That means that a fusion process producing a nucleus that has an atomic number higher than about 26, and a [[mass number]] higher than about 60, is an [[endothermic reaction|endothermic process]]. Thus, more massive nuclei cannot undergo an energy-producing fusion reaction that can sustain the [[hydrostatic equilibrium]] of a star.<ref name="raymond" />

=== Electron cloud ===
{{Main|Electron configuration|Electron shell|Atomic orbital}}{{See also|Electronegativity}}[[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>.]]
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 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 differ from each other in size, shape and orientation.<ref name=manthey2001 />

[[File:Atomic-orbital-clouds spdf m0.png|thumb|upright=1.5|3D views of some [[Hydrogen-like atom|hydrogen-like]] atomic orbitals showing probability density and phase ('''g''' orbitals and higher are not shown)]]

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 />

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 name=herter_8 /> compared to 2.23&nbsp;''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]].<ref>{{cite book|last=Smirnov|first=Boris M.|year=2003|title=Physics of Atoms and Ions|url=https://archive.org/details/physicsatomsions00smir|url-access=limited|publisher=Springer|isbn=978-0-387-95550-6|pages=[https://archive.org/details/physicsatomsions00smir/page/n262 249]–272}}</ref>

== Properties ==
=== Nuclear properties ===
{{Main|Isotope|Stable isotope|List of nuclides|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-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 [[oganesson]].<ref name=weiss20061017 /> All known isotopes of elements with atomic numbers greater than 82 are radioactive, although the radioactivity of element 83 ([[bismuth]]) is so slight as to be practically negligible.<ref name=s131>{{cite book|last=Sills|first=Alan D.|year=2003|title=Earth Science the Easy Way|publisher=Barron's Educational Series|isbn=978-0-7641-2146-3|oclc=51543743|pages=[https://archive.org/details/earthscienceeasy00alan/page/131 131–134]|url=https://archive.org/details/earthscienceeasy00alan/page/131}}</ref><ref name=dume20030423 />

About 339 nuclides occur naturally on [[Earth]],<ref name=lidsay20000730 /> of which 251 (about 74%) have not been observed to decay, and are referred to as "[[stable isotope]]s". Only 90 nuclides are stable [[list of nuclides|theoretically]], while another 161 (bringing the total to 251) have not been observed to decay, even though in theory it is energetically possible. These are also formally classified as "stable". An additional 35 radioactive nuclides have half-lives longer than 100 million years, and are long-lived enough to have been present since the birth of the [[Solar System]]. This collection of 286 nuclides are known as [[primordial nuclide]]s. Finally, an additional 53 short-lived nuclides are known to occur naturally, as daughter products of primordial nuclide decay (such as [[radium]] from [[uranium]]), or 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 [[Brookhaven National Laboratory]]'s [http://www.nndc.bnl.gov/chart Interactive Chart of Nuclides] ] {{Webarchive|url=https://web.archive.org/web/20200725182342/https://www.nndc.bnl.gov/nudat2/ |date=25 July 2020 }}.</ref><!-- See article [[list of nuclides]]. The numbers are derived by [[WP:CALC]] (counting the table), which is not [[WP:OR]]-->

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 3.1 stable isotopes per element. Twenty-six "[[monoisotopic element]]s" 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>{{rp|1–12}}

Stability of isotopes is affected by the ratio of protons to neutrons, and also by the presence of certain "magic numbers" of neutrons or protons that represent closed and filled quantum shells. These quantum shells correspond to a set of energy levels within the [[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 251 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]]. ([[Tantalum-180m]] is odd-odd and observationally stable, but is predicted to decay with a very long half-life.) Also, only four naturally occurring, radioactive odd-odd nuclides have a half-life over a billion years: [[potassium-40]], [[vanadium-50]], [[lanthanum-138]], and [[lutetium-176]]. 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>{{cite book |last=Krane |first=K. |year=1988 |title=Introductory Nuclear Physics |url=https://archive.org/details/introductorynucl00kran |url-access=limited |publisher=[[John Wiley & Sons]] |isbn=978-0-471-85914-7 |pages=[https://archive.org/details/introductorynucl00kran/page/n90 68]}}</ref>

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

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

The actual [[Invariant mass|mass of an atom at rest]] is often expressed in [[dalton (unit)|daltons]] (Da), also called the unified atomic mass unit (u). This unit is defined as a twelfth of the mass of a free neutral atom of [[carbon-12]], which is approximately {{val|1.66|e=-27|u=kg}}.<ref name=iupac /> [[hydrogen atom|Hydrogen-1]] (the lightest isotope of hydrogen which is also the nuclide with the lowest mass) has an atomic weight of 1.007825&nbsp;Da.<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 atomic mass unit (for example the mass of a nitrogen-14 is roughly 14&nbsp;Da), but this number will not be exactly an integer except (by definition) in the case of carbon-12.<ref name=nist_wc /> The heaviest [[stable atom]] is lead-208,<ref name=s131 /> with a mass of {{val|207.9766521|u=Da}}.<ref name=audi2003 />

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&nbsp;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&nbsp;Da, and so a mole of carbon-12 atoms weighs exactly 0.012&nbsp;kg.<ref name=iupac>{{cite book
|last=Mills
|first=Ian
|author2=Cvitaš, Tomislav
|author3=Homann, Klaus
|author4=Kallay, Nikola
|author5=Kuchitsu, Kozo
|title=Quantities, Units and Symbols in Physical Chemistry
|publisher=[[International Union of Pure and Applied Chemistry]], Commission on Physiochemical Symbols Terminology and Units, Blackwell Scientific Publications
|location=Oxford
|edition=2nd
|year=1993
|isbn=978-0-632-03583-0
|oclc=27011505
|url=https://archive.org/details/quantitiesunitss0000unse/page/70
|page=[https://archive.org/details/quantitiesunitss0000unse/page/70 70]
}}</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.<ref name=Ghosh02>{{cite journal | author = Ghosh, D.C. |author2= Biswas, R. | title = Theoretical calculation of Absolute Radii of Atoms and Ions. Part 1. The Atomic Radii | journal = Int. J. Mol. Sci. | volume = 3 |issue= 11 | pages = 87–113 | year = 2002 | doi=10.3390/i3020087| doi-access = free }}</ref> 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&nbsp;[[Picometre|pm]], while one of the largest is [[caesium]] at 225&nbsp;pm.<ref>{{cite book
|last=Zumdahl|first=Steven S.|year=2002
|title=Introductory Chemistry: A Foundation
|edition=5th|publisher=Houghton Mifflin
|url=http://college.hmco.com/chemistry/intro/zumdahl/intro_chemistry/5e/students/protected/periodictables/pt/pt/pt_ar5.html
|isbn=978-0-618-34342-3
|oclc=173081482| archive-url= https://web.archive.org/web/20080304155935/http://college.hmco.com/chemistry/intro/zumdahl/intro_chemistry/5e/students/protected/periodictables/pt/pt/pt_ar5.html| archive-date= 4 March 2008 | url-status= live}}</ref>

When subjected to external forces, like [[electrical field]]s, 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= Bethe1929>{{cite journal|author = Bethe, Hans|title = Termaufspaltung in Kristallen|journal = Annalen der Physik|volume = 3|issue = 2|pages = 133–208|year = 1929|doi = 10.1002/andp.19293950202|bibcode = 1929AnP...395..133B }}</ref><ref name= ZPB1995a>{{cite journal | author = Birkholz, Mario | title = Crystal-field induced dipoles in heteropolar crystals – I. concept | journal = Z. Phys. B | volume = 96 | issue = 3 | pages = 325–332 | year = 1995 | doi = 10.1007/BF01313054 |bibcode = 1995ZPhyB..96..325B | url=https://www.researchgate.net/publication/227050494| citeseerx = 10.1.1.424.5632 | s2cid = 122527743 }}</ref> Significant [[ellipsoid]]al deformations have been shown to occur for sulfur ions<ref name=pssb2008>{{cite journal | author = Birkholz, M. | author2 = Rudert, R. | title = Interatomic distances in pyrite-structure disulfides – a case for ellipsoidal modeling of sulfur ions | journal = Physica Status Solidi B | volume = 245 | issue = 9 | pages = 1858–1864 | year = 2008 | url = https://www.mariobirkholz.de/pssb2008.pdf | doi = 10.1002/pssb.200879532 | bibcode = 2008PSSBR.245.1858B | s2cid = 97824066 | access-date = 2 May 2021 | archive-date = 2 May 2021 | archive-url = https://web.archive.org/web/20210502151542/https://www.mariobirkholz.de/pssb2008.pdf | url-status = live }}</ref> and [[chalcogen]] ions<ref name=mdpi2014>{{cite journal | author = Birkholz, M. | title = Modeling the Shape of Ions in Pyrite-Type Crystals| journal = Crystals | volume = 4 | issue = 3| pages = 390–403 | year = 2014 | doi = 10.3390/cryst4030390| doi-access = free| bibcode = 2014Cryst...4..390B}}</ref> in [[pyrite]]-type compounds.

Atomic dimensions are thousands of times smaller than the wavelengths of [[light]] (400–700&nbsp;[[nanometre|nm]]) so they cannot be viewed using an [[optical microscope]], although 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 name=osu2007 /> A single drop of water contains about 2&nbsp;[[sextillion]] ({{val|2|e=21}}) atoms of oxygen, and twice the number of hydrogen atoms.<ref>{{cite book
|last=Padilla|first=Michael J.
|author2=Miaoulis, Ioannis|author3= Cyr, Martha|year = 2002
|title = Prentice Hall Science Explorer: Chemical Building Blocks
|publisher = Prentice-Hall, Inc.
|location = Upper Saddle River, New Jersey
|isbn = 978-0-13-054091-1
|oclc=47925884|page=32
|quote=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 {{val|2|e=-4|u=kg}} contains about 10&nbsp;sextillion (10<sup>22</sup>) atoms of [[carbon]].<ref group=note>A carat is 200&nbsp;milligrams. [[Atomic mass unit|By definition]], carbon-12 has 0.012&nbsp;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.<ref>{{cite web |url=https://feynmanlectures.caltech.edu/I_01.html#Ch1-S2-p3 |title=The Feynman Lectures on Physics Vol. I Ch. 1: Atoms in Motion |access-date=3 May 2022 |archive-date=30 July 2022 |archive-url=https://web.archive.org/web/20220730092955/https://www.feynmanlectures.caltech.edu/I_01.html#Ch1-S2-p3 |url-status=live }}</ref>

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

[[File:Isotopes and half-life.svg|right|thumb|This diagram shows the [[half-life]] (T<sub>{{frac|1|2}}</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 />

The most common forms of radioactive decay are:<ref>{{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
|url=https://archive.org/details/handbookradioact00lann|url-access=limited|publisher=Academic Press|isbn=978-0-12-436603-9
|oclc=16212955|pages=[https://archive.org/details/handbookradioact00lann/page/n22 3]–56}}</ref><ref name=firestone20000522 />
* [[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]].
* [[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.
* [[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.

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 fission|spontaneous nuclear fission]].

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 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 {{frac|1|2}}&nbsp;ħ, or "spin-{{frac|1|2}}". 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 />

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, but the most dominant contribution comes from electron 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 />

In [[Ferromagnetism|ferromagnetic]] elements such as iron, cobalt and nickel, an odd number of electrons leads to an unpaired electron and a net overall magnetic moment. The orbitals of neighboring atoms overlap and a lower energy state is achieved when the spins of unpaired electrons are aligned with each other, a spontaneous process known as an [[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 />

The nucleus of an atom will have no spin when it has even numbers of both neutrons and protons, but for other cases of odd numbers, the nucleus may have a spin. Normally nuclei with spin are aligned in random directions because of [[thermal equilibrium]], but 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 /><ref>{{cite book
|last1=Liang|first1=Z.-P.|last2=Haacke|first2=E.M.
|editor=Webster, J.G.|year=1999
|volume=2
|title=Encyclopedia of Electrical and Electronics Engineering: Magnetic Resonance Imaging
|publisher=John Wiley & Sons
|isbn=978-0-471-13946-1|pages=412–426}}</ref>

=== Energy levels ===
[[File: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. The top of the diagram is lower than an unbound electron state.]]
The [[potential energy]] of an electron in an atom is [[negative number|negative]] relative to when the [[distance]] from the nucleus [[limit at infinity|goes to infinity]]; its dependence on the electron's [[position (vector)|position]] reaches the [[minimum]] inside the nucleus, roughly in [[inverse proportion]] to the distance. In the quantum-mechanical model, a bound electron can occupy only 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 a 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, i.e., [[stationary state]], while an electron transition to a higher level results in an excited state.<ref name=zeghbroeck1998 /> The electron's energy increases along with [[principal quantum number|''n'']] because the (average) distance to the nucleus increases. Dependence of the energy on [[azimuthal quantum number|{{ell}}]] is caused not by the [[electrostatic potential]] of the nucleus, but by interaction between electrons.

For an electron to [[atomic electron transition|transition between two different states]], e.g. [[ground state]] to first [[excited state]], it must absorb or emit a [[photon]] at an energy matching the difference in the potential energy of those levels, according to the [[Niels Bohr]] model, what can be precisely calculated by the [[Schrödinger equation]]. Electrons jump between orbitals in a particle-like fashion. For example, if a single photon strikes the electrons, only a single electron changes states in response to the photon; see [[Atomic orbital|Electron properties]].

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>{{cite book
|last=Fowles|first=Grant R.|year=1989
|title=Introduction to Modern Optics
|url=https://archive.org/details/introductiontomo00fowl_441|url-access=limited|publisher=Courier Dover Publications
|isbn=978-0-486-65957-2
|oclc=18834711|pages=[https://archive.org/details/introductiontomo00fowl_441/page/n233 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 />

[[File:Fraunhofer lines.svg|right|thumb|upright=1.5|An example of absorption lines in a spectrum]]

When a continuous [[electromagnetic spectrum|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 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 />

Close examination of the spectral lines reveals that some display a [[fine structure]] splitting. This occurs because of [[spin–orbit interaction|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]].<ref>{{cite book
|last1=Beyer|first1=H.F.
|last2=Shevelko|first2=V.P.
|year=2003
|title=Introduction to the Physics of Highly Charged Ions
|publisher=CRC Press|isbn=978-0-7503-0481-8
|oclc=47150433|pages=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 name=watkins_sjsu />

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

Valency is the combining power of an element. It is determined by the number of bonds it can form to other atoms or groups.<ref>{{GoldBookRef|title=valence|file=V06588}}</ref> 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 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. 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 />

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 />

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

[[File:Bose Einstein condensate.png|right|thumb|Graphic 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>
{{cite book
 |last=Goodstein|first=David L.|year=2002
 |title=States of Matter|url=https://archive.org/details/statesmatter00good_082|url-access=limited|publisher=Courier Dover Publications
 |isbn=978-0-13-843557-8|pages=[https://archive.org/details/statesmatter00good_082/page/n445 436]–438
}}</ref> 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]].

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>
{{cite book
 |last=Myers|first=Richard|year=2003
 |title=The Basics of Chemistry|url=https://archive.org/details/basicschemistry00myer|url-access=limited|publisher=Greenwood Press
 |isbn=978-0-313-31664-7
 |oclc=50164580|page=[https://archive.org/details/basicschemistry00myer/page/n98 85]
}}</ref><ref name=nist_bec /> This super-cooled collection of atoms then behaves as a single [[super atom]], which may allow fundamental checks of quantum mechanical behavior.<ref name=colton_fyffe1999 />
{{clear}}

== Identification ==
[[File:Atomic resolution Au100.JPG|right|thumb|[[Scanning tunneling microscope]] [[surface reconstruction]] image showing the individual atoms making up this [[gold]] ([[Miller index|100]]) surface. The surface atoms deviate from the bulk [[crystal structure]] and arrange in columns several atoms wide with pits between them.]]
While atoms are too small to be seen, devices such as the [[scanning tunneling microscope]] (STM) enable their visualization at the surfaces of solids. The microscope uses the [[quantum tunneling]] phenomenon, which allows particles to pass through a barrier that would be insurmountable in the classical perspective. Electrons tunnel through the vacuum between two [[Biasing|biased]] electrodes, providing a tunneling current that is exponentially dependent on their separation. One electrode is a sharp tip ideally ending with a single atom. At each point of the scan of the surface the tip's height is adjusted so as to keep the tunneling current at a set value. How much the tip moves to and away from the surface is interpreted as the height profile. For low bias, the microscope images the averaged electron orbitals across closely packed energy levels—the local [[density of states|density of the electronic states]] near the [[Fermi level]].<ref name=jacox1997 /><ref name=nf_physics1986 /> Because of the distances involved, both electrodes need to be extremely stable; only then periodicities can be observed that correspond to individual atoms. The method alone is not chemically specific, and cannot identify the atomic species present at the surface.

Atoms can be easily identified by their mass. If an atom is [[ion]]ized by removing one of its electrons, its trajectory when it passes through a [[magnetic field]] will bend. The radius by which the trajectory of a moving ion is turned by the magnetic field is determined by the mass of the atom. The [[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 />

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 />

Electron emission techniques such as [[X-ray photoelectron spectroscopy]] (XPS) and [[Auger electron spectroscopy]] (AES), which measure the binding energies of the [[core electron]]s, are used to identify the atomic species present in a sample in a non-destructive way. With proper focusing both can be made area-specific. Another such method is [[electron energy loss spectroscopy]] (EELS), which measures the energy loss of an [[electron beam]] within a [[transmission electron microscope]] when it interacts with a portion of a sample.

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&nbsp;years before it was found on Earth.<ref name=winter2007 />

== Origin and current state ==
[[Baryonic matter]] forms about 4% of the total energy density of the [[observable universe]], with an average density of about 0.25&nbsp;particles/m<sup>3</sup> (mostly [[proton]]s and electrons).<ref name=hinshaw20060210 /> Within a galaxy such as the [[Milky Way]], particles 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>
{{cite book
 |last1=Choppin|first1=Gregory R.
 |last2=Liljenzin|first2=Jan-Olov|last3=Rydberg|first3=Jan
 |year=2001|title=Radiochemistry and Nuclear Chemistry
 |publisher=Elsevier|isbn=978-0-7506-7463-8
 |oclc=162592180|page=441
}}</ref> The Sun is believed to be inside the [[Local Bubble]], 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 baryonic matter are concentrated inside stars, where conditions are unfavorable for atomic matter. The total baryonic mass is about 10% of the mass of the galaxy;<ref>
{{cite book
 |last=Lequeux|first=James|year=2005
 |title=The Interstellar Medium
 |url=https://archive.org/details/interstellarmedi00ryte|url-access=limited|publisher=Springer|isbn=978-3-540-21326-0
 |oclc=133157789|page=[https://archive.org/details/interstellarmedi00ryte/page/n411 4]
}}</ref> the remainder of the mass is an unknown [[dark matter]].<ref name=nigel2000 /> High [[temperature]] inside stars makes most "atoms" fully ionized, that is, separates ''all'' electrons from the nuclei. In [[stellar remnant]]s—with exception of their surface layers—an immense [[pressure]] make electron shells impossible.

=== Formation ===
{{Main|Nucleosynthesis}}
[[File:Nucleosynthesis periodic table.svg|thumb|600px|Periodic table showing the origin of each element. Elements from carbon up to sulfur may be made in small stars by the [[alpha process]]. Elements beyond iron are made in large stars with slow neutron capture ([[s-process]]). Elements heavier than iron may be made in neutron star mergers or supernovae after the [[r-process]].]]

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 />

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 [[charged particle]]s 380,000&nbsp;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 />

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.

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.

Elements heavier than iron were produced in [[supernova]]e and colliding [[neutron star]]s 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 />

=== 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=Manuel2001pp511-519>[[#refManuel2001|Manuel (2001). ''Origin of Elements in the Solar System'', pp. 40–430, 511–519]]</ref><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 />

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<ref>{{cite book|last1=Zaider|first1=Marco|last2=Rossi|first2=Harald H.|year=2001|title=Radiation Science for Physicians and Public Health Workers|publisher=Springer|isbn=978-0-306-46403-4|oclc=44110319|page=[https://archive.org/details/radiationscience0000zaid/page/17 17]|url=https://archive.org/details/radiationscience0000zaid/page/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=Manuel2001pp511-519 /> Natural deposits of plutonium and neptunium are produced by [[neutron capture]] in uranium ore.<ref name=ofr_cut />

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.<ref>{{cite book
|last=Pauling|first=Linus|year=1960
|title=The Nature of the Chemical Bond
|publisher=Cornell University Press
|isbn=978-0-8014-0333-0
|oclc=17518275|pages=5–10}}</ref>

=== Rare and theoretical forms ===

==== Superheavy elements ====
{{Main|Superheavy element}}

All nuclides with atomic numbers higher than 82 ([[lead]]) are known to be radioactive. No nuclide with an atomic number exceeding 92 ([[uranium]]) exists on Earth as a [[primordial nuclide]], and heavier elements generally have shorter half-lives. Nevertheless, an "[[island of stability]]" encompassing relatively long-lived isotopes of superheavy elements<ref name=cern28509 /> with atomic numbers [[darmstadtium|110]] to [[flerovium|114]] might exist.<ref name=KarpovSHE>{{cite journal|last1=Karpov|first1=A. V.|last2=Zagrebaev|first2=V. I.|last3=Palenzuela|first3=Y. M.|last4=Ruiz|first4=L. F.|last5=Greiner|first5=W.|title=Decay properties and stability of the heaviest elements|journal=International Journal of Modern Physics E|date=2012|volume=21|issue=2|pages=1250013-1–1250013-20<!-- Deny Citation Bot-->|doi=10.1142/S0218301312500139|url=http://nrv.jinr.ru/karpov/publications/Karpov12_IJMPE.pdf|bibcode=2012IJMPE..2150013K|display-authors=3|access-date=24 March 2020|archive-date=3 December 2016|archive-url=https://web.archive.org/web/20161203230540/http://nrv.jinr.ru/karpov/publications/Karpov12_IJMPE.pdf|url-status=live}}</ref> Predictions for the half-life of the most stable nuclide on the island range from a few minutes to millions of years.<ref name=physorg>{{cite web |url=http://newscenter.lbl.gov/2009/09/24/114-confirmed/ |title=Superheavy Element&nbsp;114 Confirmed: A Stepping Stone to the Island of Stability |date=2009 |publisher=[[Lawrence Berkeley National Laboratory|Berkeley Lab]] |access-date=24 March 2020 |archive-date=20 July 2019 |archive-url=https://web.archive.org/web/20190720200414/https://newscenter.lbl.gov/2009/09/24/114-confirmed/ |url-status=live }}</ref> In any case, superheavy elements (with ''Z''&nbsp;>&nbsp;104) would not exist due to increasing [[Coulomb]] repulsion (which results in [[spontaneous fission]] with increasingly short half-lives) in the absence of any stabilizing effects.<ref name=liquiddrop>{{cite journal |last=Möller |first=P. |date=2016 |title=The limits of the nuclear chart set by fission and alpha decay |journal=EPJ Web of Conferences |volume=131 |pages=03002-1–03002-8<!-- Deny Citation Bot--> |url=http://inspirehep.net/record/1502715/files/epjconf-NS160-03002.pdf |doi=10.1051/epjconf/201613103002 |bibcode=2016EPJWC.13103002M |doi-access=free |access-date=24 March 2020 |archive-date=11 March 2020 |archive-url=https://web.archive.org/web/20200311130852/http://inspirehep.net/record/1502715/files/epjconf-NS160-03002.pdf |url-status=live }}</ref>

==== Exotic matter ====
{{Main|1=Exotic matter}}

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 /> 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 />

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 fundamental predictions of physics.<ref name=ns1728_77 /><ref name=psT112_1_20 /><ref name=ripin1998 />

== See also ==
{{Portal|Physics|Chemistry}}
{{cmn|colwidth=21em|
* [[History of quantum mechanics]]
* [[Infinite divisibility]]
* [[Outline of chemistry]]
* [[Motion]]
* [[Timeline of atomic and subatomic physics]]
* [[Nuclear model]]
* [[Radionuclide]]
}}

== Notes ==
{{reflist|group="note"}}
{{notelist}}

== References ==
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{{reflist|30em|refs=

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<ref name=harrison2000>{{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]]| archive-url= https://web.archive.org/web/20071225095938/http://www.upscale.utoronto.ca/GeneralInterest/Harrison/DevelQM/DevelQM.html| archive-date= 25 December 2007 | url-status= live}}</ref>

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<!-- UNUSED REF <ref name=schroeder>{{cite web|last=Schroeder|first=M.|title=Lise Meitner – Zur 125. Wiederkehr Ihres Geburtstages|url=http://www.physik3.gwdg.de/~mrs/Vortraege/Lise_Meitner-Vortrag-20031106/|language=de|access-date=4 June 2009|url-status=dead|archive-url=https://web.archive.org/web/20110719034227/http://www.physik3.gwdg.de/~mrs/Vortraege/Lise_Meitner-Vortrag-20031106/|archive-date=19 July 2011}}</ref> -->

<ref name=schroeder2>{{cite web |last=Schroeder|first=Paul A.
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<ref name=pdg2002>{{cite web|author=Particle Data Group|year=2002|url=http://www.particleadventure.org/|title=The Particle Adventure|publisher=Lawrence Berkeley Laboratory| archive-url= https://web.archive.org/web/20070104075936/http://www.particleadventure.org/| archive-date= 4 January 2007 | url-status= live}}</ref>

<ref name=schombert2006>{{cite web|first=James|last=Schombert|date=18 April 2006|url=http://abyss.uoregon.edu/~js/ast123/lectures/lec07.html|title=Elementary Particles|publisher=University of Oregon|archive-url=https://web.archive.org/web/20110830212645/http://abyss.uoregon.edu/~js/ast123/lectures/lec07.html|archive-date=30 August 2011|url-status=live}}</ref>

<ref name=wenner2007>{{cite web|last=Wenner|first=Jennifer M.|date=10 October 2007|url=http://serc.carleton.edu/quantskills/methods/quantlit/RadDecay.html|title=How Does Radioactive Decay Work?|publisher=Carleton College|archive-url=https://web.archive.org/web/20080511173156/http://serc.carleton.edu/quantskills/methods/quantlit/RadDecay.html|archive-date=11 May 2008|url-status=live}}</ref>

<ref name=mihos2002>{{cite web|last=Mihos|first=Chris|date=23 July 2002|url=http://burro.cwru.edu/Academics/Astr221/StarPhys/coulomb.html|title=Overcoming the Coulomb Barrier|publisher=Case Western Reserve University|archive-url=https://web.archive.org/web/20060912013620/http://burro.cwru.edu/Academics/Astr221/StarPhys/coulomb.html|archive-date=12 September 2006|url-status=live}}</ref>

<ref name=lbnl20070330>{{cite web|author=Staff|date=30 March 2007|url=http://www.lbl.gov/abc/Basic.html|title=ABC's of Nuclear Science|publisher=Lawrence Berkeley National Laboratory| archive-url= https://web.archive.org/web/20061205215708/http://www.lbl.gov/abc/Basic.html| archive-date= 5 December 2006 | url-status= live}}</ref>

<ref name=makhijani_saleska2001>{{cite web|first=Arjun|last=Makhijani|author2=Saleska, Scott|date=2 March 2001|url=http://www.ieer.org/reports/n-basics.html|title=Basics of Nuclear Physics and Fission|publisher=Institute for Energy and Environmental Research| archive-url= https://web.archive.org/web/20070116045217/http://www.ieer.org/reports/n-basics.html| archive-date= 16 January 2007 | url-status= live}}</ref>

<ref name=ajp63_7_653>{{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=653–658|bibcode=1995AmJPh..63..653F|doi=10.1119/1.17828}}</ref>

<ref name="raymond">{{cite web|last=Raymond |first=David |date=7 April 2006 |url=http://physics.nmt.edu/~raymond/classes/ph13xbook/node216.html |archive-url=https://web.archive.org/web/20021201030437/http://physics.nmt.edu/~raymond/classes/ph13xbook/node216.html |url-status=dead |archive-date=1 December 2002 |title=Nuclear Binding Energies |publisher=New Mexico Tech}}</ref>

<ref name=science157_3784_13>{{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|bibcode = 1967Sci...157...13M }}</ref>

<ref name=Brucat2008>{{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|archive-url=https://web.archive.org/web/20061207032136/http://www.chem.ufl.edu/~itl/2045/lectures/lec_10.html |archive-date=7 December 2006 |url-status=dead }}</ref>

<ref name=herter_8>{{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|url-status=dead |archive-url=https://web.archive.org/web/20120222062433/http://astrosun2.astro.cornell.edu/academics/courses/astro101/herter/lectures/lec08.htm |archive-date=22 February 2012 }}</ref>

<ref name=pr79_2_282>{{cite journal|last1=Bell|first1=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.|bibcode = 1950PhRv...79..282B }}</ref>

<ref name=matis2000>{{cite web|last=Matis|first=Howard S.|date=9 August 2000|url=http://www.lbl.gov/abc/wallchart/chapters/02/3.html|title=The Isotopes of Hydrogen|website=Guide to the Nuclear Wall Chart|publisher=Lawrence Berkeley National Lab| archive-url= https://web.archive.org/web/20071218153548/http://www.lbl.gov/abc/wallchart/chapters/02/3.html| archive-date= 18 December 2007 | url-status= live}}</ref>

<ref name=weiss20061017>{{cite news|last=Weiss|first=Rick|date=17 October 2006|title=Scientists Announce Creation of Atomic Element, the Heaviest Yet|newspaper=Washington Post|url=https://www.washingtonpost.com/wp-dyn/content/article/2006/10/16/AR2006101601083.html|archive-url=https://web.archive.org/web/20110820082130/http://www.washingtonpost.com/wp-dyn/content/article/2006/10/16/AR2006101601083.html|archive-date=20 August 2011|url-status=live}}</ref>

<ref name=dume20030423>{{cite news|last=Dumé|first=Belle|date=23 April 2003|title=Bismuth breaks half-life record for alpha decay|publisher=Physics World|url=http://physicsworld.com/cws/article/news/17319| archive-url= https://web.archive.org/web/20071214151450/http://physicsworld.com/cws/article/news/17319| archive-date= 14 December 2007 | url-status= live}}</ref>

<ref name=lidsay20000730>{{cite web|last=Lindsay|first=Don|date=30 July 2000|url=http://www.don-lindsay-archive.org/creation/isotope_list.html|title=Radioactives Missing From The Earth|publisher=Don Lindsay Archive| archive-url= https://web.archive.org/web/20070428225550/http://www.don-lindsay-archive.org/creation/isotope_list.html| archive-date= 28 April 2007 | url-status= live}}</ref>

<ref name=manthey2001>{{cite web|last=Manthey|first=David|year=2001|url=http://www.orbitals.com/orb/|title=Atomic Orbitals|publisher=Orbital Central| archive-url= https://web.archive.org/web/20080110102801/http://www.orbitals.com/orb/| archive-date= 10 January 2008 | url-status= live}}</ref>

<ref name=tuli2005>{{cite web|first=Jagdish K.|last=Tuli|date=April 2005|title=Nuclear Wallet Cards|publisher=National Nuclear Data Center, Brookhaven National Laboratory|url=http://nucleus.iaea.org/CIR/CIR/NuclearWalletCards.html|archive-url=https://web.archive.org/web/20111003185243/http://nucleus.iaea.org/CIR/CIR/NuclearWalletCards.html|archive-date=3 October 2011|url-status=live}}</ref>

<ref name=chieh2001>{{cite web|last=Chieh|first=Chung|date=22 January 2001|url=http://www.science.uwaterloo.ca/~cchieh/cact/nuctek/nuclideunstable.html|title=Nuclide Stability|publisher=University of Waterloo|archive-url=https://web.archive.org/web/20070830110015/http://www.science.uwaterloo.ca/~cchieh/cact/nuctek/nuclideunstable.html|archive-date=30 August 2007|url-status=dead}}</ref>

<ref name=nist_wc>{{cite web
|url=http://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ele=&ascii=html&isotype=some
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|access-date=4 January 2007| archive-url= https://web.archive.org/web/20061231212733/http://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ele=&ascii=html&isotype=some| archive-date= 31 December 2006 | url-status= live}}</ref>

<ref name=audi2003>{{cite journal |last1=Audi |first1=G. |title=The Ame2003 atomic mass evaluation (II) |journal=[[Nuclear Physics A]] |year=2003 |volume=729 |issue=1 |pages=337–676 |url=http://amdc.in2p3.fr/masstables/Ame2003/Ame2003b.pdf |doi=10.1016/j.nuclphysa.2003.11.003 |bibcode=2003NuPhA.729..337A |last2=Wapstra |first2=A.H. |last3=Thibault |first3=C.|archive-url=https://web.archive.org/web/20051016185841/http://amdc.in2p3.fr/masstables/Ame2003/Ame2003b.pdf |archive-date=16 October 2005 |url-status=live }}</ref>

<ref name=aca32_5_751>{{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 A]]|year=1976|volume=32|issue=5|pages=751–767|doi=10.1107/S0567739476001551|bibcode=1976AcCrA..32..751S|url=http://journals.iucr.org/a/issues/1976/05/00/a12967/a12967.pdf|access-date=25 August 2019|archive-date=14 August 2020|archive-url=https://web.archive.org/web/20200814154832/https://journals.iucr.org/a/issues/1976/05/00/a12967/a12967.pdf|url-status=live}}</ref>

<ref name=dong1998>{{cite web|last=Dong|first=Judy|year=1998 |url=http://hypertextbook.com/facts/MichaelPhillip.shtml|title=Diameter of an Atom|publisher=The Physics Factbook| archive-url= https://web.archive.org/web/20071104160920/http://hypertextbook.com/facts/MichaelPhillip.shtml| archive-date= 4 November 2007 | url-status= live}}</ref>

<ref name=osu2007>{{cite web
|author=Staff
|year=2007
|url=http://oregonstate.edu/terra/2007/02/small-miracles/
|title=Small Miracles: Harnessing nanotechnology
|publisher=Oregon State University
|archive-url=https://web.archive.org/web/20110521145107/http://oregonstate.edu/terra/2007/02/small-miracles/
|archive-date=21 May 2011
|url-status=live
}} – describes the width of a human hair as {{val|e=5|u=nm}} and 10 carbon atoms as spanning 1&nbsp;nm.</ref>

<ref name=splung>{{cite web |url=http://www.splung.com/content/sid/5/page/radioactivity |title=Radioactivity |publisher=Splung.com |access-date=19 December 2007| archive-url= https://web.archive.org/web/20071204135150/http://www.splung.com/content/sid/5/page/radioactivity| archive-date= 4 December 2007 | url-status= live}}</ref>

<ref name=firestone20000522>{{cite web|last=Firestone |first=Richard B. |date=22 May 2000 |url=http://isotopes.lbl.gov/education/decmode.html |title=Radioactive Decay Modes |publisher=Berkeley Laboratory|url-status=dead |archive-url=https://web.archive.org/web/20060929111801/http://isotopes.lbl.gov/education/decmode.html |archive-date=29 September 2006 }}</ref>

<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|website=The Basics of NMR |publisher=Rochester Institute of Technology| archive-url= https://web.archive.org/web/20070203044312/http://www.cis.rit.edu/htbooks/nmr/chap-3/chap-3.htm| archive-date= 3 February 2007 | url-status= live}}</ref>

<ref name=goebel20070901>{{cite web |last=Goebel |first=Greg |date=1 September 2007 |url=http://www.vectorsite.net/tpqm_04.html |title=<nowiki>[4.3]</nowiki> Magnetic Properties of the Atom |website=Elementary Quantum Physics |publisher=In The Public Domain website|archive-url=https://web.archive.org/web/20110629143026/http://www.vectorsite.net/tpqm_04.html |archive-date=29 June 2011 |url-status=usurped}}</ref>

<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| archive-url= https://web.archive.org/web/20080113104939/http://www.lbl.gov/Science-Articles/Research-Review/Magazine/1997/story1.html| archive-date= 13 January 2008 | url-status= live}}</ref>

<ref name=zeghbroeck1998>{{cite web
 |last=Zeghbroeck
 |first=Bart J. Van
 |year=1998
 |url=http://physics.ship.edu/~mrc/pfs/308/semicon_book/eband2.htm
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 |url-status=dead
 |archive-date=15 January 2005
|title=Energy levels
 |publisher=Shippensburg University}}</ref>

<ref name=martin2007>{{cite web |last=Martin|first=W.C. |author2=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| archive-url= https://web.archive.org/web/20070208113156/http://physics.nist.gov/Pubs/AtSpec/| archive-date= 8 February 2007 | url-status= live}}</ref>

<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 |access-date=10 August 2006 |url-status=dead |archive-url=https://web.archive.org/web/20060228231025/http://www.avogadro.co.uk/light/bohr/spectra.htm |archive-date=28 February 2006 }}</ref>

<ref name=fitzpatrick20070216>{{cite web |last=Fitzpatrick |first=Richard |date=16 February 2007 |url=http://farside.ph.utexas.edu/teaching/qm/lectures/node55.html |title=Fine structure |publisher=University of Texas at Austin|archive-url=https://web.archive.org/web/20110927021402/http://farside.ph.utexas.edu/teaching/qm/lectures/node55.html |archive-date=27 September 2011 |url-status=live }}</ref>

<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| archive-url= https://web.archive.org/web/20080202143147/http://math.ucr.edu/home/baez/spin/node8.html| archive-date= 2 February 2008 | url-status= live}}</ref>

<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 |access-date=23 December 2007 | archive-url= https://web.archive.org/web/20080112234014/http://www.sjsu.edu/faculty/watkins/stimem.htm| archive-date= 12 January 2008 | url-status= live}}</ref>

<ref name=reusch20070716>{{cite web|last=Reusch |first=William |date=16 July 2007 |url=http://www.cem.msu.edu/~reusch/VirtualText/intro1.htm |title=Virtual Textbook of Organic Chemistry |publisher=Michigan State University|url-status=dead |archive-url=https://web.archive.org/web/20071029211245/http://www.cem.msu.edu/~reusch/VirtualText/intro1.htm |archive-date=29 October 2007 }}</ref>

<ref name=chemguide>{{cite web |url=http://www.chemguide.co.uk/atoms/bonding/covalent.html |title=Covalent bonding – Single bonds |publisher=chemguide |year=2000|archive-url=https://web.archive.org/web/20081101203444/http://www.chemguide.co.uk/atoms/bonding/covalent.html |archive-date=1 November 2008 |url-status=live }}</ref>

<ref name=husted20031211>{{cite web |last=Husted | first=Robert |date=11 December 2003 |url=http://periodic.lanl.gov/default.htm |title=Periodic Table of the Elements |publisher=Los Alamos National Laboratory| archive-url= https://web.archive.org/web/20080110103232/http://periodic.lanl.gov/default.htm| archive-date= 10 January 2008 | url-status= live|display-authors=etal}}</ref>

<ref name=baum2003>{{cite magazine |first=Rudy |last=Baum |year=2003 |url=http://pubs.acs.org/cen/80th/elements.html |title=It's Elemental: The Periodic Table |magazine=Chemical & Engineering News|archive-url=https://web.archive.org/web/20110406121140/http://pubs.acs.org/cen/80th/elements.html |archive-date=6 April 2011 |url-status=live }}</ref>

<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=719–724 |doi=10.1070/PU2006v049n07ABEH006013 |bibcode = 2006PhyU...49..719B |s2cid=93168446 }}</ref>

<ref name=nist_bec>{{cite news |author=Staff|date=9 October 2001 |title=Bose–Einstein Condensate: A New Form of Matter |publisher=National Institute of Standards and Technology |url=https://www.nist.gov/public_affairs/releases/bec_background.cfm| archive-url= https://web.archive.org/web/20080103192918/https://www.nist.gov/public_affairs/releases/BEC_background.htm| archive-date= 3 January 2008 | url-status= live}}</ref>

<ref name=colton_fyffe1999>{{cite web |last=Colton|first=Imogen|author2=Fyffe, Jeanette
|date=3 February 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| archive-url = https://web.archive.org/web/20070829200820/http://www.ph.unimelb.edu.au/~ywong/poster/articles/bec.html| archive-date = 29 August 2007}}</ref>

<ref name=jacox1997>{{cite web |last=Jacox|first=Marilyn|author2=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| archive-url= https://web.archive.org/web/20080107133132/http://physics.nist.gov/GenInt/STM/stm.html| archive-date= 7 January 2008 | url-status= live}}</ref>

<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 |access-date=11 January 2008 |archive-url=https://web.archive.org/web/20080917103215/http://nobelprize.org/nobel_prizes/physics/laureates/1986/index.html |archive-date=17 September 2008 |url-status=live }} In particular, see the Nobel lecture by G. Binnig and H. Rohrer.</ref>

<ref name=sab53_13_1739>{{cite journal |first1=N.|last1=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 = 1739–1763|bibcode = 1998AcSpB..53.1739J |last2=Moens |first2=Luc |last3=Vanhaecke |first3=Frank }}</ref>

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}}

== Bibliography ==
* {{cite book
 |author=Oliver Manuel |year=2001
 |title=Origin of Elements in the Solar System: Implications of Post-1957 Observations
 |publisher=Springer|isbn=978-0-306-46562-8
 |oclc=228374906 |ref=refManuel2001
}}
* {{cite book |author=Andrew G. van Melsen |translator=Henry J. Koren |year=2004 |orig-year=1952 |title=From Atomos to Atom: The History of the Concept Atom |publisher=Dover Publications |isbn=0-486-49584-1 |ref=refMelsen1952}}
* {{cite book |author=J.P. Millington |year=1906 |title=John Dalton |publisher=J. M. Dent & Co. (London); E. P. Dutton & Co. (New York) |url=https://archive.org/details/in.ernet.dli.2015.30924/ |ref=refMillington1906}}
* {{cite book|author1=Charles H. Holbrow |author2=James N. Lloyd |author3=Joseph C. Amato |author4=Enrique Galvez |author5=M. Elizabeth Parks |year=2010 |title=Modern Introductory Physics |publisher=Springer Science & Business Media |isbn=978-0-387-79079-4 |ref=refHolbrowEtAl2010}}
* {{cite book
 |author=John Dalton
 |year=1808
 |title=A New System of Chemical Philosophy vol. 1
 |url=https://library.si.edu/digital-library/book/newsystemofchemi12dalt
 |ref=refDalton1808
}}
* {{cite book
 |author=John Dalton
 |year=1817
 |title=A New System of Chemical Philosophy vol. 2
 |url=https://library.si.edu/digital-library/book/newsystemofchemi21dalt
 |ref=refDalton1817
}}
* {{cite book
 |author=John L. Heilbron
 |year=2003
 |title=Ernest Rutherford and the Explosion of Atoms
 |publisher=[[Oxford University Press]]
 |isbn=0-19-512378-6
 |ref=refHeilbron2003
}}
* {{cite book
 |author=Jaume Navarro
 |year=2012
 |title=A History of the Electron: J. J. and G. P. Thomson
 |publisher=Cambridge University Press
 |isbn=978-1-107-00522-8
 |ref=refNavarro2012
}}
* {{cite book|author=Bernard Pullman |translator=Axel Reisinger |year=1998 |title=The Atom in the History of Human Thought |publisher=Oxford University Press |isbn=0-19-511447-7 |ref=refPullman1998}}
* {{cite book
 |author=Jean Perrin
 |year=1910
 |orig-year=1909
 |title=Brownian Movement and Molecular Reality
 |translator=F. Soddy
 |publisher=Taylor and Francis
 |url=https://archive.org/details/brownianmovement00perr
 |ref=refPerrin1909
}}
* {{cite book|author-link=Eric Scerri |author=Eric R. Scerri |year=2020 |title=The Periodic Table, Its Story and Its Significance |edition=2nd |publisher=Oxford University Press |location=New York |isbn=978-0-190-91436-3}}

== Further reading ==
{{refbegin}}
* {{cite book
 |last=Gangopadhyaya|first=Mrinalkanti
 |title=Indian Atomism: History and Sources
 |publisher=Humanities Press|year=1981
 |location=Atlantic Highlands, New Jersey
 |isbn=978-0-391-02177-8
 |oclc=10916778
}}
* {{cite book
 |last=Iannone|first=A. Pablo|year=2001
 |title=Dictionary of World Philosophy
 |publisher=Routledge|isbn=978-0-415-17995-9
 |oclc=44541769
}}
* {{cite book
 |last=King|first=Richard|year=1999
 |title=Indian philosophy: an introduction to Hindu and Buddhist thought
 |publisher=Edinburgh University Press
 |isbn=978-0-7486-0954-3
}}
* {{cite book
 |last=McEvilley|first=Thomas
 |title=The shape of ancient thought: comparative studies in Greek and Indian philosophies
 |publisher=Allworth Press|year=2002
 |isbn=978-1-58115-203-6
}}
* {{cite book
 |last=Siegfried|first=Robert|year=2002
 |title=From Elements to Atoms: A History of Chemical Composition
 |publisher=Diane|isbn=978-0-87169-924-4
 |oclc=186607849
}}
* {{cite book|last=Teresi|first=Dick|publisher=Simon & Schuster|title=Lost Discoveries: The Ancient Roots of Modern Science|year=2003|isbn=978-0-7432-4379-7|url=https://books.google.com/books?id=pheL_ubbXD0C|pages=213–214|access-date=25 October 2020|archive-date=4 August 2020|archive-url=https://web.archive.org/web/20200804145606/https://books.google.com/books?id=pheL_ubbXD0C|url-status=live}}
* {{cite book
 |last=Wurtz|first=Charles Adolphe|year=1881
 |title=The Atomic Theory
 |publisher=D. Appleton and company
 |location=New York
 |isbn=978-0-559-43636-9
}}
{{refend}}

== External links ==
{{Sister project links|voy=no|wikt=atom|v=The Atom|n=no|q=Atom|s=The New Student's Reference Work}}
* [https://www.feynmanlectures.caltech.edu/I_01.html Atoms in Motion – The Feynman Lectures on Physics]
* {{cite web|first=Tim|last=Sharp|title=What is an Atom?|url=https://www.livescience.com/37206-atom-definition.html|publisher=Live Science|date=8 August 2017}}

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