The name "argon" is derived from theGreek wordἀργόν, neuter singular form ofἀργός meaning 'lazy' or 'inactive', as a reference to the fact that the element undergoes almost no chemical reactions. The completeoctet (eight electrons) in the outer atomic shell makes argon stable and resistant to bonding with other elements. Itstriple point temperature of 83.8058 K is a defining fixed point in theInternational Temperature Scale of 1990.
Argon is extracted industrially by thefractional distillation ofliquid air. It is mostly used as aninertshielding gas in welding and other high-temperature industrial processes where ordinarily unreactive substances become reactive; for example, an argon atmosphere is used ingraphite electric furnaces to prevent the graphite from burning. It is also used inincandescent andfluorescent lighting, and other gas-discharge tubes. It makes a distinctiveblue-green gas laser. It is also used in fluorescent glow starters.
Characteristics
A small piece of rapidly melting solid argon
Argon has approximately the samesolubility in water asoxygen and is 2.5 times more soluble in water thannitrogen. Argon is colorless, odorless, nonflammable and nontoxic as a solid, liquid or gas.[11] Argon is chemicallyinert under most conditions and forms no confirmed stable compounds at room temperature.
Although argon is anoble gas, it can form some compounds under various extreme conditions.Argon fluorohydride (HArF), a compound of argon withfluorine andhydrogen that is stable below 17 K (−256.1 °C; −429.1 °F), has been demonstrated.[12][13] Although the neutral ground-state chemical compounds of argon are presently limited to HArF, argon can formclathrates with water when atoms of argon are trapped in a lattice of water molecules.[14]Ions, such asArH+ , andexcited-state complexes, such as ArF, have been demonstrated. Theoretical calculation predicts several moreargon compounds that should be stable[15] but have not yet been synthesized.
Argon (Greekἀργόν, neuter singular form ofἀργός meaning "lazy" or "inactive") is named in reference to its chemical inactivity. This chemical property of this firstnoble gas to be discovered impressed the namers.[16][17] An unreactive gas was suspected to be a component of air byHenry Cavendish in 1785.[18]
Argon was first isolated from air in 1894 byLord Rayleigh and SirWilliam Ramsay atUniversity College London by removingoxygen,carbon dioxide, water, andnitrogen from a sample of clean air.[19] They first accomplished this by replicating an experiment ofHenry Cavendish's. They trapped a mixture of atmospheric air with additional oxygen in a test-tube (A) upside-down over a large quantity of dilutealkali solution (B), which in Cavendish's original experiment was potassium hydroxide,[18] 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 fiveGrove cells and aRuhmkorff 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.
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.[20] Argon was also encountered in 1882 through independent research of H. F. Newall and W. N. Hartley.[21] Each observed new lines in theemission spectrum of air that did not match known elements.
Argon constitutes 0.934% by volume and 1.288% by mass ofEarth's atmosphere.[23] Air is the primary industrial source of purified argon products. Argon is isolated from air by fractionation, most commonly bycryogenicfractional distillation, a process that also produces purifiednitrogen,oxygen,neon,krypton andxenon.[24] Earth's crust and seawater contain 1.2 ppm and 0.45 ppm of argon, respectively.[25]
The mainisotopes of argon found on Earth are40 Ar (99.6%),36 Ar (0.34%), and38 Ar (0.06%). Naturally occurring40 K, with ahalf-life of 1.25×109 years, decays to stable40 Ar (11.2%) byelectron capture orpositron emission, and also to stable40 Ca (88.8%) bybeta decay. These properties and ratios are used to determine the age ofrocks byK–Ar dating.[25][26]
In Earth's atmosphere,39 Ar is made bycosmic ray activity, primarily by neutron capture of40 Ar followed by two-neutron emission. In the subsurface environment, it is also produced throughneutron capture by39 K, followed by proton emission.37 Ar is created from theneutron capture by40 Ca followed by analpha particle emission as a result of subsurfacenuclear explosions. It has a half-life of 35 days.[26]
Between locations in theSolar System, the isotopic composition of argon varies greatly. Where the major source of argon is the decay of40 K in rocks,40 Ar will be the dominant isotope, as it is on Earth. Argon produced directly bystellar nucleosynthesis is dominated by thealpha-process nuclide36 Ar. Correspondingly, solar argon contains 84.6%36 Ar (according tosolar wind measurements),[27] and the ratio of the three isotopes36Ar : 38Ar : 40Ar in the atmospheres of the outer planets is 8400 : 1600 : 1.[28] This contrasts with the low abundance ofprimordial36 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 byprobes.
The atmospheres ofMars,Mercury andTitan (the largest moon ofSaturn) contain argon, predominantly as40 Ar.[29]
Argon's complete octet ofelectrons indicates full s and p subshells. This fullvalence 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)5Ar, was isolated in 1975. However, it was not widely recognised at that time.[30] In August 2000, another argon compound,argon fluorohydride (HArF), was formed by researchers at theUniversity of Helsinki, by shining ultraviolet light onto frozen argon containing a small amount ofhydrogen fluoride withcaesium iodide. This discovery caused the recognition that argon could form weakly bound compounds, even though it was not the first.[13][31] It is stable up to 17 kelvins (−256 °C). ThemetastableArCF2+ 2 dication, which is valence-isoelectronic withcarbonyl fluoride andphosgene, was observed in 2010.[32]Argon-36, in the form of argon hydride (argonium) ions, has been detected ininterstellar medium associated with theCrab Nebulasupernova; this was the firstnoble-gas molecule detected inouter space.[33]
Solid argonhydride (Ar(H2)2) has the same crystal structure as the MgZn2Laves phase. It forms at pressures between 4.3 and 220 GPa, though Raman measurements suggest that the H2 molecules in Ar(H2)2 dissociate above 175 GPa.[34]
Argon has electronic properties (ionization and/or the emission spectrum) desirable for some applications.
Othernoble gases 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 ofcryogenicair separation in the production ofliquid oxygen andliquid nitrogen: the primary constituents of air are used on a large industrial scale. The other noble gases (excepthelium) 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 ofarc welding such asgas metal arc welding andgas tungsten arc welding, as well as in the processing oftitanium and other reactive elements. An argon atmosphere is also used for growing crystals ofsilicon andgermanium.
Argon is used in the poultry industry toasphyxiate birds, either for mass culling following disease outbreaks, or as a means of slaughter more humane thanelectric stunning. Argon is denser than air and displaces oxygen close to the ground duringinert gas asphyxiation.[36] 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.[37]
Argon is sometimes used forextinguishing fires where valuable equipment may be damaged by water or foam.[38]
Scientific research
Liquid argon is used as the target for neutrino experiments and directdark matter searches. The interaction between the hypotheticalWIMPs and an argon nucleus producesscintillation light that is detected byphotomultiplier 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[39]), is transparent to its own scintillation light, and is relatively easy to purify. Compared toxenon, 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 to39 Ar contamination, unless one uses argon from underground sources, which has much less39 Ar contamination. Most of the argon in Earth's atmosphere was produced by electron capture of long-lived40 K (40 K + e− →40 Ar + ν) present in natural potassium within Earth. The39 Ar activity in the atmosphere is maintained by cosmogenic production through the knockout reaction40 Ar(n,2n)39 Ar and similar reactions. The half-life of39 Ar is only 269 years. As a result, the underground Ar, shielded by rock and water, has much less39 Ar contamination.[40] Dark-matter detectors currently operating with liquid argon includeDarkSide,WArP,ArDM,microCLEAN andDEAP. Neutrino experiments includeICARUS andMicroBooNE, both of which use high-purity liquid argon in atime 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.[41] 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
A sample ofcaesium 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 theEuropean 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.[42]
Inwinemaking, 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 withacetic acid bacteria) and standardredox chemistry.
Argon is sometimes used as the propellant inaerosol cans.
Argon is also used as a preservative for such products asvarnish,polyurethane, and paint, by displacing air to prepare a container for storage.[43]
Since 2002, the AmericanNational Archives stores important national documents such as theDeclaration of Independence and theConstitution 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.[44]
Gloveboxes are often filled with argon, which recirculates over scrubbers to maintain anoxygen-,nitrogen-, and moisture-free atmosphere
Argon may be used as theinert gas withinSchlenk lines andgloveboxes. Argon is preferred to less expensive nitrogen in cases where nitrogen may react with the reagents or apparatus.
Cryosurgery procedures such ascryoablation use liquid argon to destroy tissue such ascancer cells. It is used in a procedure called "argon-enhanced coagulation", a form of argonplasma beamelectrosurgery. The procedure carries a risk of producinggas embolism and has resulted in the death of at least one patient.[45]
Blueargon lasers are used in surgery to weld arteries, destroy tumors, and correct eye defects.[25]
Argon has also been used experimentally to replace nitrogen in the breathing or decompression mix known asArgox, to speed the elimination of dissolved nitrogen from the blood.[46]
Argon has been used by athletes as a doping agent to simulatehypoxic conditions. In 2014, theWorld Anti-Doping Agency (WADA) added argon andxenon to the list of prohibited substances and methods, although at this time there is no reliable test for abuse.[50]
Safety
Although argon is non-toxic, it is 38% moredense than air and therefore considered a dangerousasphyxiant in closed areas. It is difficult to detect because it is colorless, odorless, and tasteless. A 1994 incident, in which a man wasasphyxiated after entering an argon-filled section of oil pipe under construction inAlaska, highlights the dangers of argon tank leakage in confined spaces and emphasizes the need for proper use, storage and handling.[51]
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