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Argon (pronounced /ˈɑrɡɒn/) 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 gases). Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide. The complete octet in the outer atomic shell makes it 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
A small piece of rapidly melting argon ice.

Argon has approximately the same solubility in water as oxygen gas and is 2.5 times more soluble in water than nitrogen gas. Argon is colorless, odorless, and nontoxic as a solid, liquid, and gas. 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 fluorohydride (HArF), a marginally stable compound of argon with fluorine and hydrogen, was reported by researchers at the University of Helsinki in 2000.[2] Although the neutral ground-state chemical compounds of argon are presently limited to HArF, argon can form clathrates with water when atoms of it are trapped in a lattice of the water molecules.[3] Also argon-containing ions and excited state complexes, such as ArH+ and ArF, respectively, are known to exist. Theoretical calculations have predicted several argon compounds that should be stable,[4] but for which no synthesis routes are currently known.

History
Lord Rayleigh's method for the isolation of argon, based on an experiment of Henry Cavendish's. The gases are contained in a test-tube (A) standing over a large quantity of weak alkali (B), and the current is conveyed in wires insulated by U-shaped glass tubes (CC) passing through the liquid and round the mouth of the test-tube. The inner platinum ends (DD) of the wire receive a current from a battery of five Grove cells and a Ruhmkorff coil of medium size.

Argon (αργος, Greek meaning "inactive", in reference to its chemical inactivity)[5][6] was suspected to be present in air by Henry Cavendish in 1785 but was not isolated until 1894 by Lord Rayleigh and Sir William Ramsay in Scotland in an experiment in which they removed all of the oxygen, carbon dioxide, water and nitrogen from a sample of clean air.[7][8][9] They had determined that nitrogen produced from chemical compounds was one-half percent lighter than nitrogen from the atmosphere. The difference seemed insignificant, 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.[10] 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.[11]

Occurrence

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.[12]

Isotopes
Main article: Isotopes of argon

The main isotopes of argon found on Earth are 40Ar (99.6%), 36Ar (0.34%), and 38Ar (0.06%). Naturally occurring 40K with a half-life of 1.25 × 109 years, decays to stable 40Ar (11.2%) by electron capture and positron emission, and also to stable 40Ca (88.8%) via beta decay. These properties and ratios are used to determine the age of rocks.[13]

In the Earth's atmosphere, 39Ar is made by cosmic ray activity, primarily with 40Ar. In the subsurface environment, it is also produced through neutron capture by 39K or alpha emission by calcium. 37Ar is created from the neutron spallation of 40Ca as a result of subsurface nuclear explosions. It has a half-life of 35 days.[13]

Argon is notable in that its isotopic composition varies greatly between different locations in the solar system. Where the major source of argon is the decay of potassium-40 in rocks, Argon-40 will be the dominant isotope, as it is on earth. Argon produced directly by stellar nucleosynthesis, in contrast, is dominated by the alpha process nuclide, argon-36. Correspondingly, solar argon contains 84.6% argon-36 based on solar wind measurements.[14]

The predominance of radiogenic argon-40 is responsible for the fact that the standard atomic weight of terrestrial argon is greater than that of the next element, potassium. This was puzzling at the time when argon was discovered, since Mendeleev had placed the elements in his periodic table in order of atomic weight, although the inertness of argon implies that it must be placed before the reactive alkali metal potassium. 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).

The Martian atmosphere contains 1.6% of argon-40 and 5 ppm of argon-36. The Mariner space probe fly-by of the 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 Huygens probe also discovered the presence of argon-40 on Titan, the largest moon of Saturn.[15]

Compounds
See also: H2-Ar

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 fluorohydride (HArF) was formed.[2][16] It is stable up to 40 kelvin (−233 °C).
Production
Industrial

Argon is produced industrially by the fractional distillation of liquid air, a process that separates liquid nitrogen, which boils at 77.3 K, from argon, which boils at 87.3 K and oxygen, which boils at 90.2 K. About 700,000 tonnes of argon are produced worldwide every year.[17]
In radioactive decays

40Ar, the most abundant isotope of argon, is produced by the decay of 40K with a half-life of 1.25 × 109 years by electron capture or positron emission. Because of this, it is used in potassium-argon dating to determine the age of rocks.
Applications
Cylinders 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 gases 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.
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 various types of metal inert gas welding such as tungsten inert gas 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 an asphyxiant in the poultry industry, either for mass culling following disease outbreaks, or as a means of slaughter more humane than the electric bath. Argon's relatively high density causes it to remain close to the ground during gassing. 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.[18]

Argon is sometimes used for extinguishing fires where damage to equipment is to be avoided (see photo).
Preservative
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. Aerial oxidation, hydrolysis, and other chemical reactions which degrade the products are retarded or prevented entirely. Bottles of high-purity chemicals and certain pharmaceutical products are available in sealed bottles or ampoules packed in argon. In wine making, argon is used to top-off barrels to avoid the aerial oxidation of ethanol to acetic acid during the aging process.

Argon is also available in aerosol-type cans, which may be used to preserve compounds such as varnish, polyurethane, paint, etc. for storage after opening.[19]

Since 2001, the American National Archives stores important national documents such as the Declaration of Independence and the Constitution within argon-filled cases to retard their degradation. Using argon reduces gas leakage, compared with the helium used in the preceding five decades.[20]
Laboratory equipment
Gloveboxes are typically filled with argon, which recirculates over scrubbers to maintain an oxygen- and moisture-free atmosphere
See also: Air-free technique

Argon may be used as the inert gas within Schlenk lines and gloveboxes. The use of argon over comparatively less expensive dinitrogen is preferred where nitrogen may react.

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 ICP spectroscopy. Argon is preferred for the sputter coating of specimens for scanning electron microscopy. Argon ions are also used for sputtering in microelectronics.

Medical use

Cryosurgery procedures such as cryoablation use liquefied 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.[21] Blue argon lasers are used in surgery to weld arteries, destroy tumors, and to correct eye defects.[22] It has also used experimentally to replace nitrogen in the breathing or decompression mix, to speed the elimination of dissolved nitrogen from the blood.[23] See Argox (breathing gas).

Lighting
Argon discharge tube
Argon gas-discharge lamp forming the symbol for Argon "Ar". Small amounts of mercury are sometimes added to argon and they add blue color.

Incandescent lights are filled with argon, to preserve the filaments at high temperature from oxidation. It is used for the specific way it ionizes and emits light, such as in plasma globes and calorimetry in experimental particle physics. Gas-discharge lamps filled with argon provide blue light. Argon is also used for the creation of blue laser light.
Miscellaneous uses

It is used for thermal insulation in energy efficient windows.[24] Argon is also used in technical scuba diving to inflate a dry suit, because it is inert and has low thermal conductivity.[25]

Compressed argon is allowed to expand, to cool the seeker heads of the AIM-9 Sidewinder missile, and other missiles that use cooled thermal seeker heads. The gas is stored at high pressure.[26]

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 is used in dating igneous rocks.
Safety

Although argon is non-toxic, it does not satisfy the body's need for oxygen and is thus an asphyxiant. Argon is 25% more dense than air and is considered highly dangerous in closed areas. It is also difficult to detect because it is colorless, odorless, and tasteless. In confined spaces, it is known to result in death due to asphyxiation. A 1994 incident in Alaska that resulted in one fatality highlights the dangers of argon tank leakage in confined spaces, and emphasizes the need for proper use, storage and handling.[27]

References

1. ^ Magnetic susceptibility of the elements and inorganic compounds, in Handbook of Chemistry and Physics 81st edition, CRC press.
2. ^ a b "HArF! Argon's not so noble after all – researchers make argon fluorohydride". http://findarticles.com/p/articles/mi_m1200/is_9_158/ai_65368548.
3. ^ Belosludov, V. R.; Subbotin, O S; Krupskii, D S; Prokuda, O V; Belosludov, R V; Kawazoe, Y (2006). "Microscopic model of clathrate compounds". J. Phys.: Conf. Ser. 29: 1. doi:10.1088/1742-6596/29/1/001.
4. ^ Cohen, Arik; Lundell, Jan; Gerber, R. Benny (2003). "First compounds with argon–carbon and argon–silicon chemical bonds". The Journal of Chemical Physics 119: 6415. doi:10.1063/1.1613631.
5. ^ Hiebert, E. N. (1963). "In Noble-Gas Compounds". in Hyman, H. H.. Historical Remarks on the Discovery of Argon: The First Noble Gas. Chicago, IL: University of Chicago Press. pp. 3–20.
6. ^ Travers, M. W. (1928). The Discovery of the Rare Gases. London: Edward Arnold & Co.. pp. 1–7.
7. ^ Lord Rayleigh; Ramsay, William (1894–1895). "Argon, a New Constituent of the Atmosphere". Proceedings of the Royal Society of London 57 (1): 265–287. doi:10.1098/rspl.1894.0149. http://www.jstor.org/pss/115394.
8. ^ Lord Rayleigh; Ramsay, William (1895). "VI. Argon: A New Constituent of the Atmosphere". Philosophical Transactions of the Royal Society of London. A 186: 187. http://www.jstor.org/pss/90645.
9. ^ William Ramsay. "Nobel Lecture in Chemistry, 1904". http://nobelprize.org/nobel_prizes/chemistry/laureates/1904/ramsay-lecture.html.
10. ^ "About Argon, the Inert; The New Element Supposedly Found in the Atmosphere". The New York Times. 1895-03-03. http://query.nytimes.com/gst/abstract.html?res=9B04E3D61139E033A25750C0A9659C94649ED7CF. Retrieved 2009-02-01.
11. ^ Holden, Norman E. (12). "History of the Origin of the Chemical Elements and Their Discoverers". National Nuclear Data Center (NNDC). http://www.nndc.bnl.gov/content/elements.html.
12. ^ "Argon, Ar". http://elements.etacude.com/Ar.php. Retrieved 2007-03-08.
13. ^ a b "40Ar/39Ar dating and errors". http://www.geoberg.de/text/geology/07011601.php. Retrieved 2007-03-07.
14. ^ Lodders, Katharina (2008). "the solar argon abundance". The Astrophysical Journal 674: 607. doi:10.1086/524725. http://arxiv.org/ftp/arxiv/papers/0710/0710.4523.pdf.
15. ^ "Seeing, touching and smelling the extraordinarily Earth-like world of Titan". European Space Agency. 21. http://www.esa.int/esaCP/SEMHB881Y3E_index_0.html.
16. ^ Bartlett, Neil. "The Noble Gases". Chemical & Engineering News. http://pubs.acs.org/cen/80th/noblegases.html.
17. ^ "Periodic Table of Elements: Argon – Ar". Environmentalchemistry.com. http://environmentalchemistry.com/yogi/periodic/Ar.html. Retrieved 2008-09-12.
18. ^ Fletcher, D. L.. "Symposium: Recent Advances in Poultry Slaughter Technology Slaughter Technology". http://ps.fass.org/cgi/reprint/78/2/277.pdf. Retrieved 2010-01-01.
19. ^ Zawalick, Steven Scott "Method for preserving an oxygen sensitive liquid product" U.S. Patent 6,629,402 Issue date: Octover 7, 2003
20. ^ "Schedule for Renovation of the National Archives Building". http://www.archives.gov/press/press-kits/charters.html#pressrelaese1. Retrieved 2009-07-07.
21. ^ "Fatal Gas Embolism Caused by Overpressurization during Laparoscopic Use of Argon Enhanced Coagulation". MDSR. 24. http://www.mdsr.ecri.org/summary/detail.aspx?doc_id=8248.
22. ^ Fujimoto, James; Rox Anderson, R. (2006). "Tissue Optics, Laser-Tissue Interaction, and Tissue Engineering" (pdf). Biomedical Optics. pp. 77–88. http://www.spie.org/Conferences/Programs/06/pw/BiOSAbstracts.pdf. Retrieved 2007-03-08.
23. ^ Pilmanis Andrew A, Balldin UI, Webb James T, Krause KM (December 2003). "Staged decompression to 3.5 psi using argon-oxygen and 100% oxygen breathing mixtures". Aviation, Space, Environmental Medicine 74 (12): 1243–50. PMID 14692466.
24. ^ "Energy-Efficient Windows". FineHomebuilding.com. http://www.finehomebuilding.com/how-to/articles/understanding-energy-efficient-windows.aspx. Retrieved 2009-08-01.
25. ^ Nuckols ML, Giblo J, Wood-Putnam JL. (September 15–18, 2008). "Thermal Characteristics of Diving Garments When Using Argon as a Suit Inflation Gas.". Proceedings of the Oceans 08 MTS/IEEE Quebec, Canada Meeting (MTS/IEEE). http://archive.rubicon-foundation.org/7962. Retrieved 2009-03-02.
26. ^ "Description of Aim-9 Operation". planken.org. http://home.wanadoo.nl/tcc/rnlaf/aim9.html. Retrieved 2009-02-01.
27. ^ Middaugh, John (1994-06-23). "Welder's Helper Asphyxiated in Argon-Inerted Pipe (FACE AK-94-012)". State of Alaska Department of Public Health. http://www.hss.state.ak.us/dph/ipems/occupation_injury/reports/docs/94ak012.htm. Retrieved 2009-02-01.