View source for Argon

{{About|the chemical element}}
{{distinguish|Aragon}}
{{pp|small=yes}}
{{Use dmy dates|date=January 2020}}
{{Infobox argon}}

'''Argon''' is a [[chemical element]]; it has [[Symbol (chemistry)|symbol]] '''Ar''' and [[atomic number]] 18. It is in group 18 of the [[periodic table]] and is a [[noble gas]].<ref>In older versions of the periodic table, the noble gases were identified as Group VIIIA or as Group 0. See [[Group (periodic table)]].</ref> Argon is the third most abundant [[gas]] in [[Earth's atmosphere]], at 0.934% (9340 [[Parts-per notation|ppmv]]). It is more than twice as abundant as [[water vapor]] (which averages about 4000 ppmv, but varies greatly), 23 times as abundant as [[carbon dioxide]] (400 ppmv), and more than 500 times as abundant as [[neon]] (18 ppmv). Argon is the most abundant noble gas in [[Earth's crust]], comprising 0.00015% of the crust.

Nearly all argon in Earth's atmosphere is [[radiogenic]] [[argon-40]], derived from the [[Radioactive decay|decay]] of [[potassium-40]] in Earth's crust. In the universe, [[argon-36]] is by far the most common argon [[isotope]], as it is the most easily produced by stellar [[nucleosynthesis]] in [[supernova]]s.

The name "argon" is derived from the [[Ancient Greek]] word {{lang|grc|ἀργόν}}, neuter singular form of {{lang|grc|ἀργός}} meaning 'lazy' or 'inactive', as a reference to the fact that the element undergoes almost no chemical reactions. The complete [[octet rule|octet]] (eight electrons) in the outer atomic shell makes argon stable and resistant to bonding with other elements. Its [[triple point]] temperature of 83.8058&nbsp;[[Kelvin|K]] is a defining fixed point in the [[International Temperature Scale of 1990]].

Argon is extracted industrially by the [[fractional distillation]] of [[liquid air]]. It is mostly used as an [[Inert gas|inert]] [[shielding gas]] in welding and other high-temperature industrial processes where ordinarily unreactive substances become reactive; for example, an argon atmosphere is used in [[graphite]] electric furnaces to prevent the graphite from burning. It is also used in [[Incandescent light bulb|incandescent]] and [[fluorescent lighting]], and other gas-discharge tubes. It makes a distinctive [[Ion laser#Argon laser|blue-green gas laser]]. It is also used in fluorescent glow starters.

==Characteristics==
[[File:Argon ice 1.jpg|upright|thumb|left|A small piece of rapidly melting solid argon]]

Argon has approximately the same [[solubility]] in water as [[oxygen]] and is 2.5 times more soluble in water than [[nitrogen]]. Argon is colorless, odorless, nonflammable and nontoxic as a solid, liquid or gas.<ref>{{cite web |url= http://www.uigi.com/MSDS_gaseous_Ar.html |title=Material Safety Data Sheet Gaseous Argon |website= UIGI.com| publisher= Universal Industrial Gases, Inc. |access-date=14 October 2013 }}</ref> Argon is chemically [[Inert gas|inert]] under most conditions and forms no confirmed stable compounds at room temperature.

Although argon is a [[noble gas]], it can form some compounds under various extreme conditions. [[Argon fluorohydride]] (HArF), a compound of argon with [[fluorine]] and [[hydrogen]] that is stable below {{cvt|17|K}}, has been demonstrated.<ref>{{cite journal
 |display-authors=4
 |first1= Leonid | last1= Khriachtchev
 |first2=Mika | last2= Pettersson
 |first3=Nino | last3=Runeberg
 |first4=Jan | last4= Lundell
 |first5= Markku | last5= Räsänen
 |s2cid=4382128
 |name-list-style=amp
 |date=2000
 |title=A stable argon compound
 |journal=[[Nature (journal)|Nature]]
 |volume=406 |issue=6798
 |pages=874–876
 |doi = 10.1038/35022551
 |pmid=10972285|bibcode=2000Natur.406..874K
 }}</ref><ref name="sciencenews-harf">
{{cite news
 |last=Perkins |first=S.
 |date=26 August 2000
 |title=HArF! Argon's not so noble after all – researchers make argon fluorohydride
 |url=http://www.sciencenews.org/view/generic/id/795/description/HArF_Argons_not_so_noble_after_all |work= Science News}}</ref> Although the neutral ground-state chemical compounds of argon are presently limited to HArF, argon can form [[clathrates]] with water when atoms of argon are trapped in a lattice of water molecules.<ref>
{{cite journal
 |display-authors=4
 |author=Belosludov, V. R.
 |author2=Subbotin, O. S.
 |author3=Krupskii, D. S.
 |author4=Prokuda, O. V.
 |author5=Belosludov, R. V.
 |author6=Kawazoe, Y.
 |date=2006
 |title=Microscopic model of clathrate compounds
 |journal=[[Journal of Physics: Conference Series]]
 |volume=29 |issue=1
 |pages=1–7
 |doi = 10.1088/1742-6596/29/1/001
|bibcode = 2006JPhCS..29....1B |doi-access=free }}</ref> [[Ions]], such as {{chem|ArH|+}}, and [[exciplex|excited-state complexes]], such as ArF, have been demonstrated. Theoretical calculation predicts several more [[argon compounds]] that should be stable<ref>
{{cite journal
 |last1=Cohen |first1=A.
 |last2=Lundell |first2=J.
 |last3=Gerber |first3=R. B.
 |s2cid=95850840
 |date=2003
 |title=First compounds with argon–carbon and argon–silicon chemical bonds
|journal=[[Journal of Chemical Physics]]
|volume=119 |page = 6415
|doi=10.1063/1.1613631
|bibcode = 2003JChPh.119.6415C
|issue=13 }}</ref> but have not yet been synthesized.

==History==
[[Image:Isolation of Argon.png|thumb|left |upright=0.6|A: test-tube, B: dilute alkali, C: U-shaped glass tube, D: platinum electrode]]

''Argon'' ([[Ancient Greek]] {{lang|grc|ἀργόν}}, neuter singular form of {{lang|grc|ἀργός}} meaning "lazy" or "inactive") is named in reference to its chemical inactivity. This chemical property of this first [[noble gas]] to be discovered impressed the namers.<ref name="lazyone1">
{{cite book
 |last = Hiebert |first = E. N.
 |date = 1963
 |chapter = In Noble-Gas Compounds
 |editor = Hyman, H. H.
 |title = Historical Remarks on the Discovery of Argon: The First Noble Gas
 |publisher = [[University of Chicago Press]]
 |pages = 3–20
}}</ref><ref name="lazyone2">
{{cite book
 |last=Travers |first = M. W.
 |date=1928
 |title=The Discovery of the Rare Gases
 |url=https://archive.org/details/discoveryofrareg0000trav |url-access=registration |pages=[https://archive.org/details/discoveryofrareg0000trav/page/1 1–7]
 |publisher=Edward Arnold & Co.
}}</ref> An unreactive gas was suspected to be a component of air by [[Henry Cavendish]] in 1785.<ref name="Cave1785">{{cite journal |author=Cavendish, Henry |title=Experiments on Air |journal=Philosophical Transactions of the Royal Society |year=1785 |volume=75 |pages=372–384 |bibcode=1785RSPT...75..372C |doi=10.1098/rstl.1785.0023|url=https://zenodo.org/record/1432276|doi-access=free }}</ref>

Argon was first isolated from air in 1894 by [[John William Strutt, 3rd Baron Rayleigh|Lord Rayleigh]] and Sir [[William Ramsay]] at [[University College London]] by removing [[oxygen]], [[carbon dioxide]], water, and [[nitrogen]] from a sample of clean air.<ref>{{Unbulleted list citebundle|
{{cite journal
 |author=Lord Rayleigh
 |author-link=Lord Rayleigh
 |author2=Ramsay, William
 |author2-link=William Ramsay
 |date=1894–1895
 |title=Argon, a New Constituent of the Atmosphere
 |journal=[[Proceedings of the Royal Society]]
 |volume=57 |issue=1 |pages=265–287
 |doi=10.1098/rspl.1894.0149
 |jstor=115394
|doi-access=free
 }}|
{{cite journal
 |author=Lord Rayleigh
 |author2=Ramsay, William
 |date = 1895
 |title = VI. Argon: A New Constituent of the Atmosphere
 |journal = Philosophical Transactions of the Royal Society A
 |volume = 186 |pages = 187–241
 |doi= 10.1098/rsta.1895.0006
 |jstor=90645
|bibcode = 1895RSPTA.186..187R |doi-access=free}}|
{{cite web
 |last=Ramsay |first=W.
 |date=1904
 |title=Nobel Lecture
 |url=http://nobelprize.org/nobel_prizes/chemistry/laureates/1904/ramsay-lecture.html
 |publisher=[[The Nobel Foundation]]
}}}}</ref> They first accomplished this by replicating an experiment of [[Henry Cavendish]]'s. They trapped a mixture of atmospheric air with additional oxygen in a test-tube (A) upside-down over a large quantity of dilute [[alkali]] solution (B), which in Cavendish's original experiment was potassium hydroxide,<ref name="Cave1785" /> and conveyed a current through wires insulated by U-shaped glass tubes (CC) which sealed around the platinum wire electrodes, leaving the ends of the wires (DD) exposed to the gas and insulated from the alkali solution. The arc was powered by a battery of five [[Grove cell]]s and a [[Ruhmkorff coil]] of medium size. The alkali absorbed the oxides of nitrogen produced by the arc and also carbon dioxide. They operated the arc until no more reduction of volume of the gas could be seen for at least an hour or two and the spectral lines of nitrogen disappeared when the gas was examined. The remaining oxygen was reacted with alkaline pyrogallate to leave behind an apparently non-reactive gas which they called argon.

[[File:Lord Rayleigh Vanity Fair 21 December 1899.jpg|thumb|160px|Captioned "Argon", caricature of [[John William Strutt, 3rd Baron Rayleigh|Lord Rayleigh]] in ''[[Vanity Fair (British magazine)|Vanity Fair]]'', 1899]]
Before isolating the gas, they had determined that nitrogen produced from chemical compounds was 0.5% lighter than nitrogen from the atmosphere. The difference was slight, but it was important enough to attract their attention for many months. They concluded that there was another gas in the air mixed in with the nitrogen.<ref>
{{cite news
 |date=3 March 1895
 |title=About Argon, the Inert; The New Element Supposedly Found in the Atmosphere
 |url=https://query.nytimes.com/gst/abstract.html?res=9B04E3D61139E033A25750C0A9659C94649ED7CF
 |work=[[The New York Times]]
 |access-date = 1 February 2009
}}</ref> Argon was also encountered in 1882 through independent research of H. F. Newall and W. N. Hartley.<ref>{{cite book |last1=Emsley |first1=John |title=Nature's Building Blocks: An A-Z Guide to the Elements |date=2003 |publisher=Oxford University Press |isbn=0-19-850340-7 |page=36 |url=https://books.google.com/books?id=j-Xu07p3cKwC&pg=PA36 |access-date=12 June 2020}}</ref> Each observed new lines in the [[emission spectrum]] of air that did not match known elements.

Prior to 1957, the symbol for argon was "A". This was changed to Ar after the [[International Union of Pure and Applied Chemistry]] published the work ''[[Nomenclature of Inorganic Chemistry]]'' in 1957.<ref>{{Unbulleted list citebundle|
{{cite web
 |last=Holden |first=N. E.
 |date=12 March 2004
 |title=History of the Origin of the Chemical Elements and Their Discoverers
 |url=http://www.nndc.bnl.gov/content/elements.html
 |publisher=[[National Nuclear Data Center]]
}}|{{Citation |title=Commission II.2: Nomenclature of Inorganic Chemistry |date=1957 |publisher=International Union of Pure and Applied Chemistry (IUPAC) |via=Science History Institute Archives |url=https://sciencehistory.libraryhost.com/repositories/3/archival_objects/38117|access-date=September 3, 2024}}}}</ref>

==Occurrence==
Argon constitutes 0.934% by volume and 1.288% by mass of [[Earth's atmosphere]].<ref>
{{cite encyclopedia
 |title=Argon (Ar)|encyclopedia=Encyclopædia Britannica
 |access-date=14 January 2014
 |url= https://www.britannica.com/EBchecked/topic/33896/argon-Ar
}}</ref> Air is the primary industrial source of purified argon products. Argon is isolated from air by fractionation, most commonly by [[cryogenics|cryogenic]] [[fractional distillation]], a process that also produces purified [[nitrogen]], [[oxygen]], [[neon]], [[krypton]] and [[xenon]].<ref>
{{cite web
 |title=Argon, Ar 
 |url=http://elements.etacude.com/Ar.php 
 |work=Etacude.com 
 |access-date=8 March 2007 
 |url-status=unfit
 |archive-url=https://web.archive.org/web/20081007175238/http://elements.etacude.com/Ar.php 
 |archive-date=7 October 2008 
}}</ref> Earth's crust and seawater contain 1.2 ppm and 0.45 ppm of argon, respectively.<ref name="emsley">{{cite book
 |last=Emsley |first=J.
 |date=2001
 |title=Nature's Building Blocks
 |publisher=[[Oxford University Press]]
 |pages=44–45| url=https://books.google.com/books?id=2EfYXzwPo3UC&pg=PA44
 |isbn=978-0-19-960563-7
}}</ref>

==Isotopes==
{{Main|Isotopes of argon}}

The main [[isotope]]s of argon found on Earth are {{chem|40|Ar}} (99.6%), {{chem|36|Ar}} (0.34%), and {{chem|38|Ar}} (0.06%). Naturally occurring {{chem|40|K|link=potassium-40}}, with a [[half-life]] of 1.25{{e|9}} years, decays to stable {{chem|40|Ar}} (11.2%) by [[electron capture]] or [[positron emission]], and also to stable {{chem|40|Ca}} (88.8%) by [[beta decay]]. These properties and ratios are used to determine the age of [[Rock (geology)|rocks]] by [[K–Ar dating]].<ref name="emsley" /><ref name="iso">
{{cite web
 |url=http://www.geoberg.de/text/geology/07011601.php
 |title=<sup>40</sup>Ar/<sup>39</sup>Ar dating and errors
 |access-date=7 March 2007
 |archive-url = https://web.archive.org/web/20070509023017/http://www.geoberg.de/text/geology/07011601.php
 |archive-date = 9 May 2007
}}</ref>

In Earth's atmosphere, {{chem|39|Ar}} is made by [[cosmic ray]] activity, primarily by neutron capture of {{chem|40|Ar}} followed by two-neutron emission. In the subsurface environment, it is also produced through [[neutron capture]] by {{chem|39|K}}, followed by proton emission. {{chem|37|Ar}} is created from the [[neutron capture]] by {{chem|40|Ca}} followed by an [[alpha particle]] emission as a result of subsurface [[nuclear testing|nuclear explosions]]. It has a half-life of 35 days.<ref name="iso" />

Between locations in the [[Solar System]], the isotopic composition of argon varies greatly. Where the major source of argon is the decay of {{chem|40|K}} in rocks, {{chem|40|Ar}} will be the dominant isotope, as it is on Earth. Argon produced directly by [[stellar nucleosynthesis]] is dominated by the [[alpha process|alpha-process]] nuclide {{chem|36|Ar}}. Correspondingly, solar argon contains 84.6% {{chem|36|Ar}} (according to [[solar wind]] measurements),<ref>
{{cite journal
 |last=Lodders |first=K.|author-link=Katharina Lodders
 |s2cid=59150678
 |date = 2008
 |title=The solar argon abundance
 |journal=[[Astrophysical Journal]]
 |volume=674 |issue=1
 |pages=607–611
 |arxiv=0710.4523
 |doi=10.1086/524725
|bibcode=2008ApJ...674..607L
}}</ref> and the ratio of the three isotopes <sup>36</sup>Ar&nbsp;:&nbsp;<sup>38</sup>Ar&nbsp;:&nbsp;<sup>40</sup>Ar in the atmospheres of the outer planets is 8400&nbsp;:&nbsp;1600&nbsp;:&nbsp;1.<ref>{{Cite journal 
| last1 = Cameron | first1 = A. G. W. 
| s2cid = 119861943 
| title = Elemental and isotopic abundances of the volatile elements in the outer planets 
| doi = 10.1007/BF00214750 
| journal = Space Science Reviews 
| volume = 14 
| issue = 3–4 
| pages = 392–400 
| year = 1973 
|bibcode = 1973SSRv...14..392C }}</ref> This contrasts with the low abundance of [[Primordial nuclide|primordial]] {{chem|36|Ar}} in Earth's atmosphere, which is only 31.5 ppmv (= 9340 ppmv × 0.337%), comparable with that of neon (18.18 ppmv) on Earth and with interplanetary gasses, measured by [[interplanetary probe|probe]]s.

The atmospheres of [[Atmosphere of Mars|Mars]], [[Mercury (planet)|Mercury]] and [[Titan (moon)|Titan]] (the largest moon of [[Saturn]]) contain argon, predominantly as {{chem|40|Ar}}.<ref>{{cite journal|doi=10.1126/science.1237966 |pmid=23869014 |title=Abundance and Isotopic Composition of Gases in the Martian Atmosphere from the Curiosity Rover |journal=Science |volume=341 |issue=6143 |pages=263–6 |year=2013 |last1=Mahaffy |first1=P. R. |last2=Webster |first2=C. R. |last3=Atreya |first3=S. K. |last4=Franz |first4=H. |last5=Wong |first5=M. |last6=Conrad |first6=P. G. |last7=Harpold |first7=D. |last8=Jones |first8=J. J. |last9=Leshin |first9=L. A. |last10=Manning |first10=H. |last11=Owen |first11=T. |last12=Pepin |first12=R. O. |last13=Squyres |first13=S. |last14=Trainer |first14=M. |last15=Kemppinen |first15=O. |last16=Bridges |first16=N. |last17=Johnson |first17=J. R. |last18=Minitti |first18=M. |last19=Cremers |first19=D. |last20=Bell |first20=J. F. |last21=Edgar |first21=L. |last22=Farmer |first22=J. |last23=Godber |first23=A. |last24=Wadhwa |first24=M. |author24-link=Meenakshi Wadhwa |last25=Wellington |first25=D. |last26=McEwan |first26=I. |last27=Newman |first27=C. |last28=Richardson |first28=M. |last29=Charpentier |first29=A. |last30=Peret |first30=L. |s2cid=206548973 |display-authors=29 |bibcode=2013Sci...341..263M }}</ref>

The predominance of [[radiogenic]] {{chem|40|Ar}} is the reason the [[standard atomic weight]] of terrestrial argon is greater than that of the next element, [[potassium]], a fact that was puzzling when argon was discovered. [[Dmitri Mendeleev|Mendeleev]] positioned the elements on his [[periodic table]] in order of atomic weight, but the inertness of argon suggested a placement ''before'' the reactive [[alkali metal]]. [[Henry Moseley]] later solved this problem by showing that the periodic table is actually arranged in order of [[atomic number]] (see [[History of the periodic table]]).

==Compounds==
{{Main|Argon compounds}}
[[File:Argon-fluorohydride-3D-vdW.png|thumb|upright|[[Space-filling model]] of [[argon fluorohydride]]]]
Argon's complete octet of [[electron]]s indicates full s and p subshells. This full [[valence shell]] makes argon very stable and extremely resistant to bonding with other elements. Before 1962, argon and the other noble gases were considered to be chemically inert and unable to form compounds; however, compounds of the heavier noble gases have since been synthesized. The first argon compound with tungsten pentacarbonyl, W(CO)<sub>5</sub>Ar, was isolated in 1975. However, it was not widely recognised at that time.<ref>{{cite journal|last1=Young|first1=Nigel A.|title=Main group coordination chemistry at low temperatures: A review of matrix isolated Group 12 to Group 18 complexes|journal=Coordination Chemistry Reviews|date=March 2013|volume=257|issue=5–6|pages=956–1010|doi=10.1016/j.ccr.2012.10.013|url=https://hull-repository.worktribe.com/output/429135 }}</ref> In August 2000, another argon compound, [[argon fluorohydride]] (HArF), was formed by researchers at the [[University of Helsinki]], by shining ultraviolet light onto frozen argon containing a small amount of [[hydrogen fluoride]] with [[caesium iodide]]. This discovery caused the recognition that argon could form weakly bound compounds, even though it was not the first.<ref name="sciencenews-harf" /><ref>{{Unbulleted list citebundle|{{cite book|author = Kean, Sam|chapter = Chemistry Way, Way Below Zero|title = The Disappearing Spoon|date = 2011|publisher=Black Bay Books}}|
{{cite journal
 |url=http://pubs.acs.org/cen/80th/noblegases.html
 |title=The Noble Gases
 |author=Bartlett, Neil
 |author-link=Neil Bartlett (chemist)
 |journal=Chemical & Engineering News |date=8 September 2003|volume=81|issue=36
|pages=32–34
 |doi=10.1021/cen-v081n036.p032
 |url-access=subscription
 }}}}</ref> It is stable up to 17&nbsp;kelvins (−256&nbsp;°C). The [[metastable]] {{chem|ArCF|2|2+}} dication, which is valence-[[isoelectronicity|isoelectronic]] with [[carbonyl fluoride]] and [[phosgene]], was observed in 2010.<ref>
{{cite journal
 |display-authors=4
 |author=Lockyear, JF
 |author2=Douglas, K
 |author3=Price, SD
 |author4=Karwowska, M
 |author5=Fijalkowski, KJ
 |author6=Grochala, W
 |author7=Remeš, M
 |author8=Roithová, J
 |author9=Schroder, D
 |name-list-style=amp
 |date=2010
 |title=Generation of the ArCF<sub>2</sub><sup>2+</sup> Dication
 |journal=[[Journal of Physical Chemistry Letters]]
 |volume=1 |issue=1
 |page=358
 |doi=10.1021/jz900274p
|bibcode=2010JPCL....1..358L
 }}</ref> [[Argon-36]], in the form of argon hydride ([[argonium]]) ions, has been detected in [[interstellar medium]] associated with the [[Crab Nebula]] [[supernova]]; this was the first [[Noble gas|noble-gas molecule]] detected in [[outer space]].<ref>{{Unbulleted list citebundle|
{{cite journal
 |last1=Barlow |first1 = M. J.
 |last2=Swinyard
 |display-authors=1
 |date=2013
 |title=Detection of a Noble Gas Molecular Ion, <sup>36</sup>ArH<sup>+</sup>, in the Crab Nebula
 |journal=[[Science (journal)|Science]]
 |volume=342 |issue=6164 |pages=1343–1345
 |doi=10.1126/science.1243582
|arxiv = 1312.4843 |bibcode = 2013Sci...342.1343B |pmid=24337290|s2cid = 37578581
 }}|{{cite news |last=Quenqua |first=Douglas |title=Noble Molecules Found in Space |url=https://www.nytimes.com/2013/12/17/science/space/noble-molecules-found-in-space.html |archive-url=https://ghostarchive.org/archive/20220101/https://www.nytimes.com/2013/12/17/science/space/noble-molecules-found-in-space.html |archive-date=2022-01-01 |url-access=limited |date=13 December 2013 |work=[[The New York Times]] |access-date=13 December 2013 }}{{cbignore}}}}</ref>

Solid argon [[hydride]] (Ar(H<sub>2</sub>)<sub>2</sub>) has the same crystal structure as the MgZn<sub>2</sub> [[Laves phase]]. It forms at pressures between 4.3 and 220 GPa, though Raman measurements suggest that the H<sub>2</sub> molecules in Ar(H<sub>2</sub>)<sub>2</sub> dissociate above 175 GPa.<ref>{{cite journal |doi= 10.1038/srep04989 |title= New high-pressure van der Waals compound Kr(H2)4 discovered in the krypton-hydrogen binary system |journal= Scientific Reports |volume=4 |article-number=4989 |year=2014 |last1=Kleppe |first1=Annette K. |last2= Amboage |first2=Mónica |last3= Jephcoat |first3=Andrew P. |bibcode= 2014NatSR...4.4989K |doi-access=free }}</ref>

==Production==
Argon is extracted industrially by the [[fractional distillation]] of [[liquid air]] in a [[cryogenic]] [[air separation]] unit; a process that separates [[liquid nitrogen]], which boils at 77.3&nbsp;K, from argon, which boils at 87.3&nbsp;K, and [[liquid oxygen]], which boils at 90.2&nbsp;K. About 700,000 [[tonne]]s of argon are produced worldwide every year.<ref name="emsley" /><ref>
{{cite web
 |url=http://environmentalchemistry.com/yogi/periodic/Ar.html
 |title=Periodic Table of Elements: Argon – Ar
 |publisher=Environmentalchemistry.com
 |access-date=12 September 2008
}}</ref>

==Applications==
[[Image:Argon.jpg|thumb|right|upright=0.8|Cylinders containing argon gas for use in extinguishing fire without damaging server equipment]]

Argon has several desirable properties:
* Argon is a chemically [[inert gas]].
* Argon is the cheapest alternative when [[nitrogen]] is not sufficiently inert.
* Argon has low [[thermal conductivity]].
* Argon has electronic properties (ionization and/or the emission spectrum) desirable for some applications.

Other [[noble gas]]es would be equally suitable for most of these applications, but argon is by far the cheapest. It is inexpensive, since it occurs naturally in air and is readily obtained as a byproduct of [[cryogenic]] [[air separation]] in the production of [[liquid oxygen]] and [[liquid nitrogen]]: the primary constituents of air are used on a large industrial scale. The other noble gases (except [[helium]]) are produced this way as well, but argon is the most plentiful by far. The bulk of its applications arise simply because it is inert and relatively cheap.

===Industrial processes===
Argon is used in some high-temperature industrial processes where ordinarily non-reactive substances become reactive. For example, an argon atmosphere is used in graphite electric furnaces to prevent the graphite from burning.

For some of these processes, the presence of nitrogen or oxygen gases might cause defects within the material. Argon is used in some types of [[arc welding]] such as [[gas metal arc welding]] and [[gas tungsten arc welding]], as well as in the processing of [[titanium]] and other reactive elements. An argon atmosphere is also used for growing crystals of [[silicon]] and [[germanium]].

{{See also|shielding gas}}
Argon is used in the poultry industry to [[asphyxiant gas|asphyxiate]] birds, either for mass culling following disease outbreaks, or as a means of slaughter more humane than [[electronarcosis|electric stunning]]. Argon is denser than air and displaces oxygen close to the ground during [[inert gas asphyxiation]].<ref>{{Unbulleted list citebundle|
{{cite news
 |last        = Fletcher
 |first       = D. L.
 |title       = Slaughter Technology
 |url         = http://ps.fass.org/cgi/reprint/78/2/277.pdf
 |work        = Symposium: Recent Advances in Poultry Slaughter Technology
 |access-date  = 1 January 2010
 |archive-url  = https://web.archive.org/web/20110724195609/http://ps.fass.org/cgi/reprint/78/2/277.pdf
 |archive-date = 24 July 2011
}}|{{cite journal|last1=Shields|first1=Sara J.|last2=Raj|first2=A. B. M.|s2cid=11301328|title=A Critical Review of Electrical Water-Bath Stun Systems for Poultry Slaughter and Recent Developments in Alternative Technologies|journal=Journal of Applied Animal Welfare Science|volume=13|issue=4|year=2010|pages=281–299|issn=1088-8705|doi=10.1080/10888705.2010.507119|pmid=20865613|citeseerx=10.1.1.680.5115}}}}</ref> Its non-reactive nature makes it suitable in a food product, and since it replaces oxygen within the dead bird, argon also enhances shelf life.<ref name="FraquezaBarreto2009">{{cite journal|last1=Fraqueza|first1=M. J.|last2=Barreto|first2=A. S.|title=The effect on turkey meat shelf life of modified-atmosphere packaging with an argon mixture|journal=Poultry Science|volume=88|issue=9|year=2009|pages=1991–1998|issn=0032-5791|doi=10.3382/ps.2008-00239|pmid=19687286|doi-access=free}}</ref>

Argon is sometimes used for [[Gaseous fire suppression|extinguishing fires]] where valuable equipment may be damaged by water or foam.<ref name="SuKim2001">{{cite journal|last1=Su|first1=Joseph Z.|last2=Kim|first2=Andrew K.|last3=Crampton|first3=George P.|last4=Liu|first4=Zhigang|title=Fire Suppression with Inert Gas Agents|journal=Journal of Fire Protection Engineering|volume=11|issue=2|year=2001|pages=72–87|issn=1042-3915|doi=10.1106/X21V-YQKU-PMKP-XGTP}}</ref>

===Scientific research===
Liquid argon is used as the target for neutrino experiments and direct [[dark matter]] searches. The interaction between the hypothetical [[Weakly interacting massive particles|WIMP]]s and an argon nucleus produces [[scintillation (physics)|scintillation]] light that is detected by [[photomultiplier tubes]]. Two-phase detectors containing argon gas are used to detect the ionized electrons produced during the WIMP–nucleus scattering. As with most other liquefied noble gases, argon has a high scintillation light yield (about 51 photons/keV<ref>
{{cite journal
 |display-authors= 4
 |author= Gastler, Dan
 |author2= Kearns, Ed
 |author3= Hime, Andrew
 |author4= Stonehill, Laura C.
 |author5= Seibert, Stan
 |author6= Klein, Josh
 |author7= Lippincott, W. Hugh
 |author8= McKinsey, Daniel N.
 |author9= Nikkel, James A
 |s2cid= 6876533
 |date= 2012
 |title=Measurement of scintillation efficiency for nuclear recoils in liquid argon
 |doi= 10.1103/PhysRevC.85.065811
 |journal= Physical Review C
 |volume= 85
 |issue= 6
 |article-number= 065811
 |arxiv=1004.0373
|bibcode = 2012PhRvC..85f5811G }}</ref>), is transparent to its own scintillation light, and is relatively easy to purify. Compared to [[xenon]], argon is cheaper and has a distinct scintillation time profile, which allows the separation of electronic recoils from nuclear recoils. On the other hand, its intrinsic beta-ray background is larger due to {{chem|39|Ar}} contamination, unless one uses argon from underground sources, which has much less {{chem|39|Ar}} contamination. Most of the argon in Earth's atmosphere was produced by electron capture of long-lived {{chem|40|K}} ({{chem|40|K}} + e<sup>−</sup> → {{chem|40|Ar}} + ν) present in natural potassium within Earth. The {{chem|39|Ar}} activity in the atmosphere is maintained by cosmogenic production through the knockout reaction {{chem|40|Ar}}(n,2n){{chem|39|Ar}} and similar reactions. The half-life of {{chem|39|Ar}} is only 269&nbsp;years. As a result, the underground Ar, shielded by rock and water, has much less {{chem|39|Ar}} contamination.<ref>
{{Cite journal|author= Xu, J.
 |author2= Calaprice, F.
 |author3= Galbiati, C.
 |author4= Goretti, A.
 |author5= Guray, G.
 |s2cid= 117711599
 |name-list-style= amp
 |date= 26 April 2012
 |title=A Study of the Residual {{Chem|39|Ar}} Content in Argon from Underground Sources
 |journal= Astroparticle Physics
 |volume= 66
 |issue= 2015
 |pages= 53–60
 |arxiv=1204.6011 |display-authors=etal|doi= 10.1016/j.astropartphys.2015.01.002
 |bibcode= 2015APh....66...53X
 }}</ref> Dark-matter detectors currently operating with liquid argon include [[DarkSide (dark matter experiment)|DarkSide]], [[WIMP Argon Programme|WArP]], [[ArDM]], [[Cryogenic Low-Energy Astrophysics with Neon|microCLEAN]] and [[DEAP]]. Neutrino experiments include [[ICARUS (experiment)|ICARUS]] and [[MicroBooNE]], both of which use high-purity liquid argon in a [[time projection chamber]] for fine grained three-dimensional imaging of neutrino interactions.

At Linköping University, Sweden, the inert gas is being utilized in a vacuum chamber in which plasma is introduced to ionize metallic films.<ref>{{Cite web|title=Plasma electrons can be used to produce metallic films|url=https://phys.org/news/2020-05-plasma-electrons-metallic.html|date=May 7, 2020|website=Phys.org|access-date=May 8, 2020}}</ref> This process results in a film usable for manufacturing computer processors. The new process would eliminate the need for chemical baths and use of expensive, dangerous and rare materials.

===Preservative===
[[File:CsCrystals.JPG|thumb|A sample of [[caesium]] is packed under argon to avoid reactions with air]]
Argon is used to displace oxygen- and moisture-containing air in packaging material to extend the shelf-lives of the contents (argon has the [[E numbers|European food additive code]] E938). Aerial oxidation, hydrolysis, and other chemical reactions that degrade the products are retarded or prevented entirely. High-purity chemicals and pharmaceuticals are sometimes packed and sealed in argon.<ref name="pmid17099243">{{cite journal |vauthors=Ilouga PE, Winkler D, Kirchhoff C, Schierholz B, Wölcke J|date=November 2007 |title=Investigation of 3 industry-wide applied storage conditions for compound libraries|journal=Journal of Biomolecular Screening|volume=12 |issue=1 |pages=21–32 |doi=10.1177/1087057106295507|pmid=17099243 |doi-access=free }}</ref>

In [[winemaking]], argon is used in a variety of activities to provide a barrier against oxygen at the liquid surface, which can spoil wine by fueling both microbial metabolism (as with [[acetic acid bacteria]]) and standard [[redox]] chemistry.

Argon is sometimes used as the propellant in [[aerosol]] cans.

Argon is also used as a preservative for such products as [[varnish]], [[polyurethane]], and paint, by displacing air to prepare a container for storage.<ref>Zawalick, Steven Scott "Method for preserving an oxygen sensitive liquid product" {{US patent|6629402}} Issue date: 7 October 2003.</ref>

Since 2002, the American [[National Archives]] stores important national documents such as the [[United States Declaration of Independence|Declaration of Independence]] and the [[United States Constitution|Constitution]] within argon-filled cases to inhibit their degradation. Argon is preferable to the helium that had been used in the preceding five decades, because helium gas escapes through the intermolecular pores in most containers and must be regularly replaced.<ref>
{{cite web
 |url=https://www.archives.gov/press/press-kits/charters.html#pressrelaese1
 |title=Schedule for Renovation of the National Archives Building
 |access-date=7 July 2009
}}</ref>

===Laboratory equipment===
{{See also|Air-free technique}}
[[Image:Glovebox.jpg|thumb|[[Glovebox]]es are often filled with argon, which recirculates over scrubbers to maintain an [[oxygen]]-, [[nitrogen]]-, and moisture-free atmosphere]]

Argon may be used as the [[inert gas]] within [[Schlenk line]]s and [[glovebox]]es. Argon is preferred to less expensive nitrogen in cases where nitrogen may react with the reagents or apparatus.

Argon may be used as the carrier gas in [[gas chromatography]] and in [[electrospray ionization mass spectrometry]]; it is the gas of choice for the plasma used in [[Inductively coupled plasma|ICP]] [[spectroscopy]]. Argon is preferred for the sputter coating of specimens for [[scanning electron microscopy]]. Argon gas is also commonly used for [[sputter deposition]] of thin films as in [[microelectronics]] and for [[microfabrication|wafer cleaning in microfabrication]].

===Medical use===
[[Cryosurgery]] procedures such as [[cryoablation]] use liquid argon to destroy tissue such as [[cancer]] cells. It is used in a procedure called "argon-enhanced coagulation", a form of argon [[plasma torch|plasma beam]] [[electrosurgery]]. The procedure carries a risk of producing [[gas embolism]] and has resulted in the death of at least one patient.<ref>{{cite web
 |url=http://www.mdsr.ecri.org/summary/detail.aspx?doc_id=8248
 |title=Fatal Gas Embolism Caused by Overpressurization during Laparoscopic Use of Argon Enhanced Coagulation
 |date=24 June 1994
 |publisher=MDSR
 |access-date=10 January 2007
 |archive-date=12 July 2021
 |archive-url=https://web.archive.org/web/20210712040208/http://www.mdsr.ecri.org/summary/detail.aspx?doc_id=8248
 }}</ref>

Blue [[argon laser]]s are used in surgery to weld arteries, destroy tumors, and correct eye defects.<ref name="emsley" />

Argon has also been used experimentally to replace nitrogen in the breathing or decompression mix known as [[Argox (breathing gas)|Argox]], to speed the elimination of dissolved nitrogen from the blood.<ref>
{{cite journal
 |author=Pilmanis Andrew A.
 |author2=Balldin U. I.
 |author3=Webb James T.
 |author4=Krause K. M.
 |title=Staged decompression to 3.5 psi using argon–oxygen and 100% oxygen breathing mixtures
 |journal=Aviation, Space, and Environmental Medicine 
 |volume=74 |issue=12 |pages=1243–1250
 |date=2003
 |pmid=14692466|url=https://www.researchgate.net/publication/8945687
}}</ref>

===Lighting===
[[File:ArTube.jpg|thumb|upright=0.8|Argon [[gas-discharge lamp]] forming "Ar", the symbol for argon]]

[[Incandescent light]]s are filled with argon, to preserve the [[electrical filament|filaments]] at high temperature from oxidation. It is used for the specific way it ionizes and emits light, such as in [[plasma globe]]s and [[Calorimeter (particle physics)|calorimetry]] in experimental [[particle physics]]. [[Gas-discharge lamp]]s filled with pure argon provide lilac/violet light; with argon and some mercury, blue light. Argon is also used for blue and green [[argon laser|argon-ion lasers]].

===Miscellaneous uses===
Argon is used for [[thermal insulation]] in [[Insulated glazing|energy-efficient windows]].<ref>
{{cite web
 |url=http://www.finehomebuilding.com/how-to/articles/understanding-energy-efficient-windows.aspx
 |title=Energy-Efficient Windows
 |access-date=1 August 2009
 |publisher=FineHomebuilding.com
|date=February 1998
 }}</ref> Argon is also used in technical [[scuba diving]] to inflate a [[dry suit]] because it is inert and has low thermal conductivity.<ref name="IEEE2008">{{cite journal
 |author=Nuckols M. L.
 |author2=Giblo J.
 |author3=Wood-Putnam J. L.
 |title=Thermal Characteristics of Diving Garments When Using Argon as a Suit Inflation Gas
 |journal=Proceedings of the Oceans 08 MTS/IEEE Quebec, Canada Meeting
 |date=15–18 September 2008
 |url=http://archive.rubicon-foundation.org/7962
 |access-date=2 March 2009
 |archive-url=https://web.archive.org/web/20090721035810/http://archive.rubicon-foundation.org/7962
 |archive-date=21 July 2009
 |url-status=usurped
 }}</ref>

Argon is used as a propellant in the development of the [[Variable Specific Impulse Magnetoplasma Rocket]] (VASIMR). Compressed argon gas is allowed to expand, to cool the seeker heads of some versions of the [[AIM-9 Sidewinder]] missile and other missiles that use cooled thermal seeker heads. The gas is [[AIM-9 Sidewinder#Design|stored at high pressure]].<ref>{{cite web
 |url=http://home.wanadoo.nl/tcc/rnlaf/aim9.html 
 |title=Description of Aim-9 Operation 
 |access-date=1 February 2009 
 |publisher=planken.org 
 |archive-url=https://web.archive.org/web/20081222025556/http://home.wanadoo.nl/tcc/rnlaf/aim9.html 
 |archive-date=22 December 2008 
 }}</ref>

Argon-39, with a half-life of 269 years, has been used for a number of applications, primarily [[ice core]] and [[ground water]] dating. Also, [[potassium–argon dating]] and related [[Argon argon dating|argon-argon dating]] are used to date [[Sedimentary rock|sedimentary]], [[Metamorphic rock|metamorphic]], and [[igneous rock]]s.<ref name="emsley" />

Argon has been used by athletes as a doping agent to simulate [[Hypoxia (environmental)|hypoxic]] conditions. In 2014, the [[World Anti-Doping Agency]] (WADA) added argon and [[xenon]] to the list of prohibited substances and methods, although at this time there is no reliable test for abuse.<ref>{{cite news |title=WADA amends Section S.2.1 of 2014 Prohibited List |url=https://www.wada-ama.org/en/media/2014-05/wada-amends-section-s21-of-2014-prohibited-list#.VARJ3WNqOIl |date=31 August 2014 |access-date=1 September 2014 |archive-date=27 April 2021 |archive-url=https://web.archive.org/web/20210427160909/https://www.wada-ama.org/en/media/2014-05/wada-amends-section-s21-of-2014-prohibited-list#.VARJ3WNqOIl }}</ref>

==Safety==
Although argon is non-toxic, it is 38% more [[density|dense]] than air and therefore considered a dangerous [[asphyxiant gas|asphyxiant]] in closed areas. It is difficult to detect because it is colorless, odorless, and tasteless. A 1994 incident, in which a man was [[asphyxia]]ted after entering an argon-filled section of oil pipe under construction in [[Alaska]], highlights the dangers of argon tank leakage in confined spaces and emphasizes the need for proper use, storage and handling.<ref>
{{cite web
 |author = Alaska FACE Investigation 94AK012
 |url = https://www.cdc.gov/niosh/face/stateface/ak/94ak012.html
 |title = Welder's Helper Asphyxiated in Argon-Inerted Pipe – Alaska (FACE AK-94-012)
 |publisher = State of Alaska Department of Public Health|date = 23 June 1994
 |access-date = 29 January 2011
}}</ref>

==See also==
{{portal|Chemistry}}
* [[Industrial gas]]
* [[Oxygen–argon ratio]], a ratio of two physically similar gases, which has importance in various sectors.

==References==
{{Reflist|30em}}

==Further reading==
* {{cite book
 |last1=Brown
 |first1=T. L.
 |last2=Bursten
 |first2=B. E.
 |last3=LeMay
 |first3=H. E.
 |date=2006
 |title=Chemistry: The Central Science
 |edition=10th
 |editor=J. Challice
 |editor2=N. Folchetti
 |publisher=[[Pearson Education]]
 |pages=[https://archive.org/details/chemistry00theo_0/page/276 276]& 289
 |isbn=978-0-13-109686-8
 |url-access=registration
 |url=https://archive.org/details/chemistry00theo_0
 }}
* {{cite book
 |last=Lide |first=D. R.
 |date=2005
 |chapter=Properties of the Elements and Inorganic Compounds; Melting, boiling, triple, and critical temperatures of the elements
 |title=CRC Handbook of Chemistry and Physics
 |edition=86th
 |publisher=[[CRC Press]]
 |at=§4
 |isbn=978-0-8493-0486-6
|title-link=CRC Handbook of Chemistry and Physics
 }}<!-- Possibly 85th edition (2004) --> On triple point pressure at 69 kPa.
* {{cite journal
 |last=Preston-Thomas |first=H.
 |date=1990
 |title=The International Temperature Scale of 1990 (ITS-90)
 |url=http://www.bipm.org/en/publications/its-90.html
 |journal=[[Metrologia]]
 |volume=27 |issue=1
 |pages=3–10
 |bibcode=1990Metro..27....3P
 |doi=10.1088/0026-1394/27/1/002
|s2cid=250785635
 |doi-access=free
 }} On triple point pressure at 83.8058 K.

==External links==
{{Sister project links|auto=1|wikt=argon|s=argon |v=The periodic table/Argon |b=Wikijunior:The Elements/Argon}}
* [http://www.periodicvideos.com/videos/018.htm Argon] at ''[[The Periodic Table of Videos]]'' (University of Nottingham)
* [https://wwwrcamnl.wr.usgs.gov/isoig/period/ar_iig.html USGS Periodic Table – Argon] {{Webarchive|url=https://web.archive.org/web/20060923194910/http://wwwrcamnl.wr.usgs.gov/isoig/period/ar_iig.html |date=23 September 2006 }}
* Diving applications: [http://www.decompression.org/maiken/Why_Argon.htm Why Argon?]

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[[Category:Argon| ]]
[[Category:Chemical elements]]
[[Category:E-number additives]]
[[Category:Noble gases]]
[[Category:Industrial gases]]