Argon is achemical element; it hassymbolAr andatomic number 18. It is in group 18 of theperiodic table and is anoble gas.^[10] Argon is the third most abundantgas inEarth's atmosphere, at 0.934% (9340ppmv). It is more than twice as abundant aswater vapor (which averages about 4000 ppmv, but varies greatly), 23 times as abundant ascarbon dioxide (400 ppmv), and more than 500 times as abundant asneon (18 ppmv). Argon is the most abundant noble gas inEarth's crust, comprising 0.00015% of the crust.

Argon, ₁₈Ar

Argon
Pronunciation	/ˈɑːrɡɒn/​(AR-gon)
Appearance	colorless gas exhibiting a lilac/violet glow when placed in an electric field
Standard atomic weightAr°(Ar)
	[39.792, 39.963][1] 39.95±0.16 (abridged)[2]
Argon in theperiodic table
Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson Ne ↑ Ar ↓ Kr chlorine ←argon →potassium
Atomic number(Z)	18
Group	group 18 (noble gases)
Period	period 3
Block	p-block
Electron configuration	[Ne] 3s2 3p6
Electrons per shell	2, 8, 8
Physical properties
Phaseat STP	gas
Melting point	83.81 K ​(−189.34 °C, ​−308.81 °F)
Boiling point	87.302 K ​(−185.848 °C, ​−302.526 °F)
Density(at STP)	1.784 g/L
when liquid (at b.p.)	1.3954 g/cm3
Triple point	83.8058 K, ​68.89 kPa[3]
Critical point	150.687 K, 4.863 MPa[3]
Heat of fusion	1.18 kJ/mol
Heat of vaporization	6.53 kJ/mol
Molar heat capacity	20.85[4] J/(mol·K)
Vapor pressure P (Pa) 1 10 100 1 k 10 k 100 k at T (K)   47 53 61 71 87
Atomic properties
Oxidation states	common: (none) 0[5]
Electronegativity	Pauling scale: no data
Ionization energies	1st: 1520.6 kJ/mol 2nd: 2665.8 kJ/mol 3rd: 3931 kJ/mol (more)
Covalent radius	106±10 pm
Van der Waals radius	188 pm
Spectral lines of argon
Other properties
Natural occurrence	primordial
Crystal structure	​face-centered cubic (fcc) (cF4)
Lattice constant	a = 546.91 pm (at triple point)[6]
Thermal conductivity	17.72×10−3  W/(m⋅K)
Magnetic ordering	diamagnetic[7]
Molar magnetic susceptibility	−19.6×10−6 cm3/mol[8]
Speed of sound	323m/s (gas, at 27 °C)
CAS Number	7440-37-1
History
Naming	from the Greekἀργόν, meaning 'lazy' or 'inactive', in reference to its inertness
Discovery and first isolation	Lord Rayleigh andWilliam Ramsay (1894)
Isotopes of argon v e
Main isotopes[9] Decay abun­dance half-life(t1/2) mode pro­duct 36Ar 0.334% stable 37Ar trace 35 d ε 37Cl 38Ar 0.0630% stable 39Ar trace 268 y β− 39K 40Ar 99.6% stable 41Ar trace 109.34 min β− 41K 42Ar synth 32.9 y β− 42K
Category: Argon view talk edit |references

Nearly all argon in Earth's atmosphere isradiogenicargon-40, derived from thedecay ofpotassium-40 in Earth's crust. In the universe,argon-36 is by far the most common argonisotope, as it is the most easily produced by stellarnucleosynthesis insupernovas.

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.

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.

Prior to 1957, the symbol for argon was "A". This was changed to Ar after theInternational Union of Pure and Applied Chemistry published the workNomenclature of Inorganic Chemistry in 1957.^[22]

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]

The predominance ofradiogenic⁴⁰
Ar is the reason thestandard 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 hisperiodic table in order of atomic weight, but the inertness of argon suggested a placementbefore the reactivealkali metal.Henry Moseley later solved this problem by showing that the periodic table is actually arranged in order ofatomic number (seeHistory of the periodic table).

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 is extracted industrially by thefractional distillation ofliquid air in acryogenicair separation unit; a process that separatesliquid nitrogen, which boils at 77.3 K, from argon, which boils at 87.3 K, andliquid oxygen, which boils at 90.2 K. About 700,000tonnes of argon are produced worldwide every year.^[25][35]

Argon has several desirable properties:

• Argon is a chemicallyinert gas.
• Argon is the cheapest alternative whennitrogen is not sufficiently inert.
• Argon has lowthermal conductivity.
• 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.

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]

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.

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]

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.

Argon may be used as the carrier gas ingas chromatography and inelectrospray ionization mass spectrometry; it is the gas of choice for the plasma used inICPspectroscopy. Argon is preferred for the sputter coating of specimens forscanning electron microscopy. Argon gas is also commonly used forsputter deposition of thin films as inmicroelectronics and forwafer cleaning in microfabrication.

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]

Incandescent lights are filled with argon, to preserve thefilaments at high temperature from oxidation. It is used for the specific way it ionizes and emits light, such as inplasma globes andcalorimetry in experimentalparticle physics.Gas-discharge lamps filled with pure argon provide lilac/violet light; with argon and some mercury, blue light. Argon is also used for blue and greenargon-ion lasers.

Argon is used forthermal insulation inenergy-efficient windows.^[47] Argon is also used in technicalscuba diving to inflate adry suit because it is inert and has low thermal conductivity.^[48]

Argon is used as a propellant in the development of theVariable Specific Impulse Magnetoplasma Rocket (VASIMR). Compressed argon gas is allowed to expand, to cool the seeker heads of some versions of theAIM-9 Sidewinder missile and other missiles that use cooled thermal seeker heads. The gas isstored at high pressure.^[49]

Argon-39, with a half-life of 269 years, has been used for a number of applications, primarilyice core andground water dating. Also,potassium–argon dating and relatedargon-argon dating are used to datesedimentary,metamorphic, andigneous rocks.^[25]

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]

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]

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

• 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.
• 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 978-0-8493-0486-6. On triple point pressure at 69 kPa.
• Preston-Thomas, H. (1990)."The International Temperature Scale of 1990 (ITS-90)".Metrologia.27 (1):3–10.Bibcode:1990Metro..27....3P.doi:10.1088/0026-1394/27/1/002.S2CID 250785635. On triple point pressure at 83.8058 K.

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