View source for Argon

{{Infobox argon}}

{{SI}}

==Overview==
'''Argon''' ({{pronEng|ˈɑrgɒn}}) is a [[chemical element]] designated by the symbol '''Ar'''.  Argon has [[atomic number]] 18 and is the third element in group 18 of the [[periodic table]] ([[noble gas]]es).  Argon is present in the [[Earth's atmosphere]] at slightly less than 1%, making it the most common noble gas on Earth. Its full outer shell makes argon  stable and resistant to bonding with other elements. Its [[triple point]] temperature of 83.8058 K is a defining fixed point in the [[International Temperature Scale of 1990]].

== Characteristics ==
Argon has approximately the same [[solubility]] in water as oxygen gas and is 2.5 times more soluble in water than [[nitrogen]] gas. This highly stable chemical element is colorless, odorless, tasteless and nontoxic in both its liquid and gaseous forms.  Argon is inert under most conditions and forms no confirmed stable compounds at room temperature.

Although argon is a [[noble gas]], it has been found to have the capability of forming some compounds. For example, the creation of [[argon hydrofluoride]] (HArF), a metastable compound of argon with [[fluorine]] and [[hydrogen]], has been reported by researchers at the [[University of Helsinki]] in 2000.<ref>"[http://www.lenntech.com/Periodic-chart-elements/Ar-en.htm Periodic Table of the Elements: Argon]." ''[http://www.lenntech.com/ Lenntech].'' 1998. Retrieved on [[September 3]], [[2007]].</ref> Although the neutral ground-state chemical compounds of argon are presently limited to HArF, argon can form [[clathrates]] with [[water (molecule)|water]] when atoms of it are trapped in a lattice of the water molecules.<ref>{{cite web |url=http://ej.iop.org/links/r5qIClVyX/UJoriTXQ2xG1FnZqav5vpA/jpconf6_29_001.pdf|title=Microscopic model of clathrate compounds |accessdate=2007-03-08 |last=Belosludov |first=V. R. |authorlink= |coauthors=O. S. Subbotin, D. S. Krupskii, O. V. Prokuda, and Y. Kawazoe |year=2006 |publisher=Institute of Physics (has blown up once in a while) Publishing |pages=1 |language=English |archiveurl= |archivedate= |quote= }}</ref> Also argon-containing [[ions]] e.g. ArH+ and [[exciplex|excited state complexes]] e.g. ArF are well known. Theoretical calculations on computers have shown several argon compounds that should be stable but for which no synthesis routes are currently known.

== History ==
Argon ([[Greek language|Greek]] ''αργόν'' meaning "the lazy one," in reference to its chemical inactivity)<ref name="lazyone1">Hiebert, E. N. Historical Remarks on the Discovery of Argon: The First Noble Gas. In Noble-Gas Compounds; Hyman, H. H., Ed.; University of Chicago Press: Chicago, IL, 1963; pp 3–20.</ref><ref name="lazyone2">Travers, M. W. The Discovery of the Rare Gases; Edward Arnold & Co.: London, 1928; pp 1–7.</ref><ref name="lazyone3">Rayleigh, Lord; Ramsay, W. Argon: A New Constituent of the Atmosphere. Chem. News 1895 (February 1), 71, 51–58.</ref> was suspected to be present in air by [[Henry Cavendish]] in [[1785]] but was not discovered until [[1894]] by [[John William Strutt, 3rd Baron Rayleigh|Lord Rayleigh]] and Sir [[William Ramsay]] in an experiment in which they removed all of the [[oxygen]] and [[nitrogen]] from a sample of air.<ref>{{cite journal | title = Argon, a New Constituent of the Atmosphere. | author = [[Lord Rayleigh]];[[ William Ramsay]]  | journal = Proceedings of the Royal Society of London | volume = 57 | issue = 1 | pages = 265-287 | year = 1894 - 1895 | url = http://links.jstor.org/sici?sici=0370-1662%281894%2F1895%2957%3C265%3AAANCOT%3E2.0.CO%3B2-X}}</ref> Argon was also encountered in 1882 through independent research of H.F. Newall and W.N. Hartley.  Each observed new lines in the color spectrum of air but were unable to identify the element responsible for the lines. Argon became the first member of the noble gases to be discovered. The symbol for argon is now '''Ar''', but up until 1957 it was '''A'''.<ref>{{cite web |url= http://www.nndc.bnl.gov/content/elements.html |title=  History of the Origin of the Chemical Elements and Their Discoverers |accessdate= |author=Holden, Norman E. |authorlink= |coauthors= |date=12 |year=04 |month=03 |format= |work= |publisher=National Nuclear Data Center (NNDC) |pages= |language=English |archiveurl= |archivedate= |quote= }}</ref>

== Applications ==
[[Image:Argon.jpg|thumb|left|180px|Canisters containing argon gas for use in extinguishing [[fire]] without damaging server equipment]]
There are several different reasons why argon is used in particular applications:
*An [[inert]] gas is needed.  In particular, argon is the cheapest alternative when diatomic [[nitrogen]] is not sufficiently inert.
*Low [[thermal conductivity]] is required.
*The electronic properties (ionization and/or the emission spectrum) are necessary.

Other [[noble gas]]es would probably work as well in most of these applications, but argon is by far the cheapest.  Argon is inexpensive since it is a byproduct of the production of [[liquid oxygen]] and [[liquid nitrogen]], both of which 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 since it has the highest concentration in the atmosphere. 
The bulk of argon applications arise simply because it is inert and relatively cheap.  Argon is used:
*As a fill gas in [[incandescent lighting]], because argon will not react with the [[electrical filament|filament]] of [[light bulb]]s even at high temperatures.
*As an inert gas shield in many forms of [[welding]], including [[metal inert gas welding]] and [[tungsten inert gas welding]].
*For extinguishing fires where damage to equipment is to be avoided (see photo).
*As the gas of choice for the plasma used in [[Inductively coupled plasma|ICP]] [[spectroscopy]]
*As a non-reactive blanket in the processing of [[titanium]] and other reactive elements, 
*As a protective atmosphere for growing [[silicon]] and [[germanium]] [[crystal]]s, and in  partial pressure heat treat furnaces.
*By museum conservators to protect old materials or documents, which are prone to gradual oxidation in the presence of air. <ref>[http://www.archives.gov/national-archives-experience/charters/treasure/declaration_facts.html USA National Archives description of how the Declaration of Independence is stored and displayed].  More detail can be found in this [http://www.archives.gov/national-archives-experience/charters/charters_preservation_01.html  more technical explanation], specially [http://www.archives.gov/national-archives-experience/charters/charters_preservation_04.html Page 4], which talks about the argon keeping the oxygen out.</ref>
*To keep open bottles of wine from oxidizing, and in a number of dispensing units and keeper cap systems.
*In winemaking to top off barrels,  displacing oxygen and thus preventing the wine from turning to vinegar during the aging process.
*In the pharmaceutical industry to top off bottles of [[intravenous]] drug preparations (for example intravenous [[paracetamol]]), again displacing oxygen and therefore prolonging the drug's [[shelf-life]].
*Used to cool the seeker head of the US Air Force version of the [[AIM-9 Sidewinder]] missile.  The [[AIM-9 Sidewinder#Design Architecture|gas is stored at high pressure]], and the expansion of the gas cools the seeker<ref>[http://home.wanadoo.nl/tcc/rnlaf/aim9.html Description of Aim-9 Operation]</ref>.

The next most common reason for using argon is its low thermal conductivity.  It is used for [[thermal insulation]] in energy efficient [[window]]s.<ref>{{cite web |url=http://www.bchydro.com/powersmart/elibrary/elibrary644.html |title=Energy-Efficient Windows |accessdate=2007-03-08 |publisher=Bc Hydro}}</ref>  Argon is also used in technical [[scuba diving]] to inflate a [[dry suit]], because it is inert and has low thermal conductivity. 

Argon is also used for the specific way it ionizes and emits light.  It is used in [[plasma globe]]s and [[Calorimeter (particle physics)|calorimetry]] in experimental [[particle physics]]. Blue argon lasers are used in surgery to weld arteries, destroy tumors, and to correct eye defects.<ref>{{cite web |url=http://www.spie.org/Conferences/Programs/06/pw/BiOSAbstracts.pdf |title=Tissue Optics, Laser-Tissue Interaction, and Tissue Engineering |accessdate=2007-03-08 |last=Fujimoto |first=James |authorlink= |coauthors=Rox Anderson, R. |date= |year=2006 |month= |format= |work= |publisher=Biomedical Optics |pages=77-88 |language=English |archiveurl= |archivedate= |quote= }}
</ref>  In microelectronics, argon ions are used for [[sputtering]].

Finally, there are a number of miscellaneous uses.  Argon-39, with a half life of 269 years, has been used for a number of applications, primarily [[ice core]] and [[ground water]] dating. The argon-40/[[potassium]]-40 ratio is used in dating [[igneous rock]]s.

[[Cryosurgery]] procedures such as [[cryoablation]] use liquified argon to destroy [[cancer]] cells.  In surgery it is used in a procedure called "argon enhanced coagulation" which is a form of argon plasma beam [[electrosurgery]]. The procedure carries a risk of producing [[gas embolism]] in the patient and has resulted in the death of one person via this type of accident. <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 |author= |authorlink= |coauthors= |date=24 |year=1994 |month=June |format= |work= |publisher=MDSR |pages= |language=English |archiveurl= |archivedate= |quote= }}</ref>

== Occurrence ==
[[Image:ArTube.jpg||thumb|left|An argon & mercury vapour discharge tube.]]
Argon constitutes 0.934% by volume and 1.29% by mass of the [[Earth's atmosphere]], and air is the primary raw material used by industry to produce purified argon products.   Argon is isolated from air by [[fractionation]], most commonly by [[cryogenic]] [[fractional distillation]], a process that also produces purified [[nitrogen]], [[oxygen]], [[neon]], [[krypton]] and [[xenon]].<ref>{{cite web |url=http://elements.etacude.com/Ar.php |title=Argon, Ar |accessdate=2007-03-08}}</ref>  

The [[Mars (planet)|Martian]] atmosphere in contrast contains 1.6% of argon-40 and 5 [[part per million|ppm]] of argon-36. The [[Mariner]] spaceprobe fly-by of the [[planet]] [[Mercury (planet)|Mercury]] in 1973 found that Mercury has a very thin atmosphere with 70% argon, believed to result from releases of the gas as a decay product from radioactive materials on the planet.  In 2005, the ''[[Cassini-Huygens|Huygens]]'' probe also discovered the presence of argon-40 on [[Titan (moon)|Titan]], the largest moon of [[Saturn (planet)|Saturn]].<ref>{{cite web |url=http://www.esa.int/esaCP/SEMHB881Y3E_index_0.html |title= Seeing, touching and smelling the extraordinarily Earth-like world of Titan |accessdate=|author=|authorlink= |coauthors= |date=21 |year=05 |month=01 |format= |work= |publisher=European Space Agency |pages= |language=English |archiveurl= |archivedate= |quote= }}</ref>

== Compounds ==
[[Image:Argon ice 1.jpg|250px|thumb|A small piece of rapidly melting argon ice.]]
Argon’s complete octet of [[electrons]] indicates full s and p subshells.  This full outer energy level 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. In August 2000, the first argon compounds were formed by researchers at the [[University of Helsinki]]. By shining ultraviolet light onto frozen argon containing a small amount of hydrogen fluoride, [[argon hydrofluoride]] (HArF) was formed.<ref>{{cite web |url=http://pubs.acs.org/cen/80th/noblegases.html |title=The Noble Gases |accessdate= |author=Bartlett, Neil |authorlink= |coauthors= |date= |year= |month= |format= |work= |publisher=Chemical & Engineering News |pages= |language=English |archiveurl= |archivedate= |quote= }}</ref> It is stable up to 40 [[kelvin]]s (−233&nbsp;°[[Celsius|C]]).

The discovery of [[argon difluoride]] (ArF<sub>2</sub>) was announced in 2003. But this is unconfirmed and most probably incorrect.

== Isotopes ==
The main [[isotope]]s of argon found on Earth are <sup>40</sup>Ar (99.6%), <sup>36</sup>Ar (0.34%), and <sup>38</sup>Ar (0.06%). Naturally occurring <sup>40</sup>[[potassium|K]] with a [[half-life]] of 1.25{{e|9}} years, decays to stable <sup>40</sup>Ar (11.2%) by [[electron capture]] and [[positron emission]], and also to stable <sup>40</sup>Ca (88.8%) via [[beta decay]]. These properties and ratios are used to determine the age of [[Rock (geology)|rocks]].<ref name=iso>{{cite web |url=http://www.geoberg.de/text/geology/07011601.php |title=40Ar/39Ar dating and errors |accessdate=2007-03-07}}
</ref>

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

== Potential hazards ==

Although argon is non-toxic, it does not satisfy the body's need for oxygen and is a simple asphyxiant.  People have suffocated by breathing argon by mistake.<ref>Middaugh, John; Bledsoe, Gary. "[http://www.hss.state.ak.us/dph/ipems/occupation_injury/reports/docs/94ak012.htm Welder's Helper Asphyxiated in Argon-Inerted Pipe (FACE AK-94-012)]." ''[http://www.hss.state.ak.us/dph/ State of Alaska Department of Public Health].'' [[June 23]], [[1994]]. Retrieved on [[September 3]], [[2007]].</ref>

== References ==
<references/>

==Further reading==
*[http://periodic.lanl.gov/elements/18.html Los Alamos National Laboratory &ndash; Argon]
*[http://wwwrcamnl.wr.usgs.gov/isoig/period/ar_iig.html USGS Periodic Table - Argon]
*Emsley, J., Nature’s Building Blocks; Oxford University Press: Oxford, NY, 2001; pp. 35-39.
*Brown, T. L.; Bursten, B. E.; LeMay, H. E., In ''Chemistry: The Central Science'', 10th ed.; Challice, J.; Draper, P.; Folchetti, N. et al.; Eds.; Pearson Education, Inc.: Upper Saddle River, NJ, 2006; pp. 276 and 289.
* Triple point temperature: 83.8058 K - {{cite journal | first=H. | last=Preston-Thomas | title=The International Temperature Scale of 1990 (ITS-90) | journal=Metrologia | volume=27 | pages=3-10 | date=1990 | url=http://www.bipm.org/en/publications/its-90.html}}
* Triple point pressure: 69 kPa - {{cite book | title=CRC Handbook of Chemistry and Physics | edition=85th edition | publisher=CRC Press | date=2005 | location=Boca Raton, Florida | chapter=Section 4, Properties of the Elements and Inorganic Compounds; Melting, boiling, triple, and critical temperatures of the elements}}

== External links ==

*[http://www.webelements.com/webelements/elements/text/Ar/index.html WebElements.com &ndash; Argon]
*Diving applications: [http://www.decompression.org/maiken/Why_Argon.htm Why Argon?]
*[http://www.uigi.com/argon.html Argon Ar Properties, Uses, Applications]
*[http://www.compchemwiki.org/index.php?title=Argon Computational Chemistry Wiki]

{{E number infobox 930-949}}
[[Category:Chemical elements]]
[[Category:Noble gases]]
[[Category:Argon|*]]

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