Editing Atom (section)

== 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 />