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Argon
Argon
is a chemical element with symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas.[6] Argon
Argon
is the third-most abundant gas in the Earth's atmosphere, at 0.934% (9340 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
Argon
is the most abundant noble gas in Earth's crust, comprising 0.00015% of the crust. Nearly all of the argon in the Earth's atmosphere
Earth's atmosphere
is radiogenic argon-40, derived from the decay of potassium-40 in the 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 supernovas. The name "argon" is derived from the Greek word ἀργόν, neuter singular form of ἀργός meaning "lazy" or "inactive", as a reference to the fact that the element undergoes almost no chemical reactions. The complete 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 K is a defining fixed point in the International Temperature Scale of 1990. Argon
Argon
is produced industrially by the fractional distillation of liquid air. Argon
Argon
is mostly used as an 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. Argon
Argon
is also used in incandescent, fluorescent lighting, and other gas-discharge tubes. Argon
Argon
makes a distinctive blue-green gas laser. Argon
Argon
is also used in fluorescent glow starters.

Contents

1 Characteristics 2 History 3 Occurrence 4 Isotopes 5 Compounds 6 Production

6.1 Industrial 6.2 In radioactive decays

7 Applications

7.1 Industrial processes 7.2 Scientific research 7.3 Preservative 7.4 Laboratory equipment 7.5 Medical use 7.6 Lighting 7.7 Miscellaneous uses

8 Safety 9 See also 10 References 11 Further reading 12 External links

Characteristics[edit]

A small piece of rapidly melting solid argon

Argon
Argon
has approximately the same solubility in water as oxygen and is 2.5 times more soluble in water than nitrogen. Argon
Argon
is colorless, odorless, nonflammable and nontoxic as a solid, liquid or gas.[7] Argon
Argon
is chemically 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 extreme conditions. Argon fluorohydride
Argon fluorohydride
(HArF), a compound of argon with fluorine and hydrogen that is stable below 17 K (−256.1 °C; −429.1 °F), has been demonstrated.[8][9] 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.[10] Ions, such as ArH+, and excited-state complexes, such as ArF, have been demonstrated. Theoretical calculation predicts several more argon compounds that should be stable[11] but have not yet been synthesized. History[edit]

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
Ruhmkorff coil
of medium size.

Argon
Argon
(Greek ἀργόν, neuter singular form of 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.[12][13] An unreactive gas was suspected to be a component of air by Henry Cavendish
Henry Cavendish
in 1785. Argon was first isolated from air in 1894 by Lord Rayleigh
Lord Rayleigh
and Sir William Ramsay at University College London
University College London
by removing oxygen, carbon dioxide, water, and nitrogen from a sample of clean air.[14][15][16] 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.[17] Argon
Argon
was also encountered in 1882 through independent research of H. F. Newall and W. N. Hartley.[citation needed] Each observed new lines in the emission spectrum of air that did not match known elements. Until 1957, the symbol for argon was "A", but now is "Ar".[18] Occurrence[edit] Argon
Argon
constitutes 0.934% by volume and 1.288% by mass of the Earth's atmosphere,[19] and air is the primary industrial source of purified argon products. Argon
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.[20] The Earth's crust and seawater contain 1.2 ppm and 0.45 ppm of argon, respectively.[21] Isotopes[edit] 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 or positron emission, and also to stable 40Ca (88.8%) by beta decay. These properties and ratios are used to determine the age of rocks by K–Ar dating.[21][22] In the Earth's atmosphere, 39Ar is made by cosmic ray activity, primarily by neutron capture of 40Ar followed by two-neutron emission. In the subsurface environment, it is also produced through neutron capture by 39K, followed by proton emission. 37Ar is created from the neutron capture by 40Ca followed by an alpha particle emission as a result of subsurface nuclear explosions. It has a half-life of 35 days.[22] Between locations in the Solar System, the isotopic composition of argon varies greatly. Where the major source of argon is the decay of 40K in rocks, 40Ar will be the dominant isotope, as it is on Earth. Argon
Argon
produced directly by stellar nucleosynthesis, is dominated by the alpha-process nuclide 36Ar. Correspondingly, solar argon contains 84.6% 36Ar (according to solar wind measurements),[23] and the ratio of the three isotopes 36Ar : 38Ar : 40Ar in the atmospheres of the outer planets is 8400 : 1600 : 1.[24] This contrasts with the low abundance of primordial 36Ar 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 probes. The atmospheres of Mars, Mercury and Titan (the largest moon of Saturn) contain argon, predominantly as 40Ar, and its content may be as high as 1.93% (Mars).[25] The predominance of radiogenic 40Ar 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. 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
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[edit] Main article: Argon
Argon
compounds

Space-filling model
Space-filling model
of argon fluorohydride

Argon's complete octet of electrons 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)5Ar, was isolated in 1975. However it was not widely recognised at that time.[26] 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.[9][27][28] It is stable up to 17 kelvins (−256 °C). The metastable ArCF2+ 2 dication, which is valence-isoelectronic with carbonyl fluoride and phosgene, was observed in 2010.[29] Argon-36, in the form of argon hydride (argonium) ions, has been detected in interstellar medium associated with the Crab Nebula
Crab Nebula
supernova; this was the first noble-gas molecule detected in outer space.[30][31] Solid argon hydride (Ar(H2)2) has the same crystal structure as the MgZn2 Laves 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.[32] Production[edit] Industrial[edit] Argon
Argon
is produced 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 K, from argon, which boils at 87.3 K, and liquid oxygen, which boils at 90.2 K. About 700,000 tonnes of argon are produced worldwide every year.[21][33] In radioactive decays[edit] 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[edit]

Cylinders containing argon gas for use in extinguishing fire without damaging server equipment

Argon
Argon
has several desirable properties:

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

Other noble gases would be equally suitable for most of these applications, but argon is by far the cheapest. Argon
Argon
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 argon applications arise simply because it is inert and relatively cheap. Industrial processes[edit] Argon
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
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
Argon
is used in the poultry industry to asphyxiate birds, either for mass culling following disease outbreaks, or as a means of slaughter more humane than the electric bath. Argon
Argon
is denser than air and displaces oxygen close to the ground during gassing.[34][35] 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.[36] Argon
Argon
is sometimes used for extinguishing fires where valuable equipment may be damaged by water or foam.[37] Scientific research[edit] Liquid argon is used as the target for neutrino experiments and direct dark matter searches. The interaction between the hypothetical WIMPs and an argon nucleus produces 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[38]), 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 39Ar contamination, unless one uses argon from underground sources, which has much less 39Ar contamination. Most of the argon in the Earth’s atmosphere was produced by electron capture of long-lived 40K (40K + e− → 40Ar + ν) present in natural potassium within the Earth. The 39Ar activity in the atmosphere is maintained by cosmogenic production through the knockout reaction 40Ar(n,2n)39Ar and similar reactions. The half-life of 39Ar is only 269 years. As a result, the underground Ar, shielded by rock and water, has much less 39Ar contamination.[39] Dark-matter detectors currently operating with liquid argon include DarkSide, WArP, ArDM, microCLEAN and DEAP. Neutrino experiments include 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. Preservative[edit]

A sample of caesium is packed under argon to avoid reactions with air

Argon
Argon
is used to displace oxygen- and moisture-containing air in packaging material to extend the shelf-lives of the contents (argon has the 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. 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
Argon
is sometimes used as the propellant in aerosol cans for such products as varnish, polyurethane, and paint, and to displace air when preparing a container for storage after opening.[40] Since 2002, the American National Archives
National Archives
stores important national documents such as the Declaration of Independence and the Constitution within argon-filled cases to inhibit their degradation. Argon
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.[41] Laboratory equipment[edit]

Gloveboxes are often filled with argon, which recirculates over scrubbers to maintain an oxygen-, nitrogen-, and moisture-free atmosphere

See also: Air-free technique Argon
Argon
may be used as the inert gas within Schlenk lines and gloveboxes. Argon
Argon
is preferred to less expensive nitrogen in cases where nitrogen may react with the reagents or apparatus. Argon
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
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 wafer cleaning in microfabrication. Medical use[edit] Cryosurgery
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 beam electrosurgery. The procedure carries a risk of producing gas embolism and has resulted in the death of at least one patient.[42] Blue argon lasers are used in surgery to weld arteries, destroy tumors, and correct eye defects.[21] Argon
Argon
has also been used experimentally to replace nitrogen in the breathing or decompression mix known as Argox, to speed the elimination of dissolved nitrogen from the blood.[43] Lighting[edit]

Argon
Argon
gas-discharge lamp forming the symbol for argon "Ar"

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 pure argon provide lilac/violet light; with argon and some mercury, blue light. Argon
Argon
is also used for blue and green argon-ion lasers. Miscellaneous uses[edit] Argon
Argon
is used for thermal insulation in energy-efficient windows.[44] Argon
Argon
is also used in technical scuba diving to inflate a dry suit because it is inert and has low thermal conductivity.[45] Argon
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
AIM-9 Sidewinder
missile and other missiles that use cooled thermal seeker heads. The gas is stored at high pressure.[46] 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 is used to date sedimentary, metamorphic, and igneous rocks.[21] Argon
Argon
has been used by athletes as a doping agent to simulate hypoxic conditions. In 2014, the World Anti-Doping Agency
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.[47] Safety[edit] Although argon is non-toxic, it is 38% denser than air and therefore considered a dangerous asphyxiant in closed areas. It is difficult to detect because it is colorless, odorless, and tasteless. A 1994 incident, in which a man was asphyxiated 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.[48] See also[edit]

Industrial gas Oxygen–argon ratio, a ratio of two physically similar gases, which has importance in various sectors.

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References[edit]

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Further reading[edit]

Brown, T. L.; Bursten, B. E.; LeMay, H. E. (2006). J. Challice; N. Folchetti, eds. Chemistry: The Central Science (10th ed.). Pearson Education. pp. 276 & 289. ISBN 978-0-13-109686-8.  Triple point
Triple point
temperature: 83.8058 K – Preston-Thomas, H. (1990). "The International Temperature Scale of 1990 (ITS-90)". Metrologia. 27: 3–10. Bibcode:1990Metro..27....3P. doi:10.1088/0026-1394/27/1/002.  Triple point
Triple point
pressure: 69 kPa – Lide, D. R. (2005). "Properties of the Elements and Inorganic Compounds; Melting, boiling, triple, and critical temperatures of the elements". CRC Handbook of Chemistry and Physics (86th ed.). CRC Press. §4. ISBN 0-8493-0486-5. 

External links[edit]

Argon
Argon
at The Periodic Table of Videos
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(University of Nottingham) USGS Periodic Table – Argon Diving applications: Why Argon?

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monoxide Carborundum Cyanogen
Cyanogen
radical Diatomic carbon Fluoromethylidynium Hydrogen
Hydrogen
chloride Hydrogen
Hydrogen
fluoride Hydrogen
Hydrogen
(molecular) Hydroxyl radical Iron(II) oxide Magnesium
Magnesium
monohydride cation Methylidyne radical Nitric oxide Nitrogen
Nitrogen
(molecular) Nitrogen
Nitrogen
monohydride Nitrogen
Nitrogen
sulfide Oxygen
Oxygen
(molecular) Phosphorus
Phosphorus
monoxide Phosphorus
Phosphorus
mononitride Potassium
Potassium
chloride Silicon
Silicon
carbide Silicon
Silicon
mononitride Silicon
Silicon
monoxide Silicon
Silicon
monosulfide Sodium
Sodium
chloride Sodium
Sodium
iodide Sulfur
Sulfur
monohydride Sulfur
Sulfur
monoxide Titanium
Titanium
oxide

Triatomic

Aluminium
Aluminium
hydroxide Aluminium
Aluminium
isocyanide Amino radical Carbon
Carbon
dioxide Carbonyl sulfide CCP radical Chloronium Diazenylium Dicarbon monoxide Disilicon carbide Ethynyl radical Formyl radical Hydrogen
Hydrogen
cyanide (HCN) Hydrogen
Hydrogen
isocyanide (HNC) Hydrogen
Hydrogen
sulfide Hydroperoxyl Iron
Iron
cyanide Isoformyl Magnesium
Magnesium
cyanide Magnesium
Magnesium
isocyanide Methylene radical N2H+ Nitrous oxide Nitroxyl Ozone Phosphaethyne Potassium
Potassium
cyanide Protonated molecular hydrogen Sodium
Sodium
cyanide Sodium
Sodium
hydroxide Silicon
Silicon
carbonitride c- Silicon
Silicon
dicarbide Silicon
Silicon
naphthalocyanine Sulfur
Sulfur
dioxide Thioformyl Thioxoethenylidene Titanium
Titanium
dioxide Tricarbon Water

Four atoms

Acetylene Ammonia Cyanic acid Cyanoethynyl Cyclopropynylidyne Formaldehyde Fulminic acid HCCN Hydrogen
Hydrogen
peroxide Hydromagnesium isocyanide Isocyanic acid Isothiocyanic acid Ketenyl Methylene amidogen Methyl radical Propynylidyne Protonated carbon dioxide Protonated hydrogen cyanide Silicon
Silicon
tricarbide Thioformaldehyde Tricarbon
Tricarbon
monoxide Tricarbon
Tricarbon
sulfide Thiocyanic acid

Five atoms

Ammonium
Ammonium
ion Butadiynyl Carbodiimide Cyanamide Cyanoacetylene Cyanoformaldehyde Cyanomethyl Cyclopropenylidene Formic acid Isocyanoacetylene Ketene Methane Methoxy
Methoxy
radical Methylenimine Propadienylidene Protonated formaldehyde Protonated formaldehyde Silane Silicon-carbide cluster

Six atoms

Acetonitrile Cyanobutadiynyl radical E-Cyanomethanimine Cyclopropenone Diacetylene Ethylene Formamide HC4N Ketenimine Methanethiol Methanol Methyl isocyanide Pentynylidyne Propynal Protonated cyanoacetylene

Seven atoms

Acetaldehyde Acrylonitrile

Vinyl cyanide

Cyanodiacetylene Ethylene
Ethylene
oxide Hexatriynyl radical Methylacetylene Methylamine Methyl isocyanate Vinyl alcohol

Eight atoms

Acetic acid Aminoacetonitrile Cyanoallene Ethanimine Glycolaldehyde Heptatrienyl radical Hexapentaenylidene Methylcyanoacetylene Methyl formate Propenal

Nine atoms

Acetamide Cyanohexatriyne Cyanotriacetylene Dimethyl ether Ethanol Methyldiacetylene Octatetraynyl radical Propene Propionitrile

Ten atoms or more

Acetone Benzene Benzonitrile Buckminsterfullerene
Buckminsterfullerene
(C60 fullerene, buckyball) C70 fullerene Cyanodecapentayne Cyanopentaacetylene Cyanotetra-acetylene Ethylene
Ethylene
glycol Ethyl formate Methyl acetate Methyl-cyano-diacetylene Methyltriacetylene Propanal n-Propyl cyanide Pyrimidine

Deuterated molecules

Ammonia Ammonium
Ammonium
ion Formaldehyde Formyl radical Heavy water Hydrogen
Hydrogen
cyanide Hydrogen
Hydrogen
deuteride Hydrogen
Hydrogen
isocyanide Methylacetylene N2D+ Trihydrogen cation

Unconfirmed

Anthracene Dihydroxyacetone Ethyl methyl ether Glycine Graphene H2NCO+ Linear C5 Naphthalene
Naphthalene
cation Phosphine Pyrene Silylidine

Related

Abiogenesis Astrobiology Astrochemistry Atomic and molecular astrophysics Chemical formula Circumstellar envelope Cosmic dust Cosmic ray Cosmochemistry Diffuse interstellar band Earliest known life forms Extraterrestrial life Extraterrestrial liquid water Forbidden mechanism Helium
Helium
hydride ion Homochirality Intergalactic dust Interplanetary medium Interstellar medium Photodissociation region Iron–sulfur world theory Kerogen Molecules in stars Nexus for Exoplanet System Science Organic compound Outer space PAH world hypothesis Panspermia Polycyclic aromatic hydrocarbon
Polycyclic aromatic hydrocarbon
(PAH) RNA world hypothesis Spectroscopy Tholin

Book:Chemistry Category:Astrochemistry Category:Molecules Portal:Astrobiology Portal:Astronomy Portal:Chemistry

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