Wikipedia

Argon

This article is about the chemical element. For other uses, seeArgon (disambiguation).
Not to be confused withAragon.

Argonis achemical element;it hassymbolArandatomic number18. It is in group 18 of theperiodic tableand is anoble gas.[9]Argon is the third most abundantgasinEarth'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
Vial containing a violet glowing gas
Argon
Pronunciation/ˈɑːrɡɒn/​(AR-gon)
Appearancecolorless gas exhibiting a lilac/violet glow when placed in an electric field
Standard atomic weightAr°(Ar)
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
Groupgroup 18 (noble gases)
Periodperiod 3
Blockp-block
Electron configuration[Ne] 3s23p6
Electrons per shell2, 8, 8
Physical properties
PhaseatSTPgas
Melting point83.81K​(−189.34 °C, ​−308.81 °F)
Boiling point87.302 K ​(−185.848 °C, ​−302.526 °F)
Density(at STP)1.784 g/L
when liquid (atb.p.)1.3954 g/cm3
Triple point83.8058 K, ​68.89 kPa[3]
Critical point150.687 K, 4.863 MPa[3]
Heat of fusion1.18kJ/mol
Heat of vaporization6.53 kJ/mol
Molar heat capacity20.85[4]J/(mol·K)
Vapor pressure
P(Pa) 1 10 100 1 k 10 k 100 k
atT(K) 47 53 61 71 87
Atomic properties
Oxidation states0
ElectronegativityPauling scale: no data
Ionization energies
  • 1st: 1520.6 kJ/mol
  • 2nd: 2665.8 kJ/mol
  • 3rd: 3931 kJ/mol
  • (more)
Covalent radius106±10pm
Van der Waals radius188 pm
Color lines in a spectral range
Spectral linesof argon
Other properties
Natural occurrenceprimordial
Crystal structure​face-centered cubic(fcc) (cF4)
Lattice constant
Face-centered cubic crystal structure for argon
a= 546.91 pm (at triple point)[5]
Thermal conductivity17.72×10−3W/(m⋅K)
Magnetic orderingdiamagnetic[6]
Molar magnetic susceptibility−19.6×10−6cm3/mol[7]
Speed of sound323m/s(gas, at 27 °C)
CAS Number7440-37-1
History
Discoveryand first isolationLord RayleighandWilliam Ramsay(1894)
Isotopes of argon
Main isotopes[8] 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
|references

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

The name "argon" is derived from theGreekwordἀργόν,neuter singular form ofἀργόςmeaning '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 pointtemperature of 83.8058Kis a defining fixed point in theInternational Temperature Scale of 1990.

Argon is extracted industrially by thefractional distillationofliquid air.It is mostly used as aninertshielding gasin welding and other high-temperature industrial processes where ordinarily unreactive substances become reactive; for example, an argon atmosphere is used ingraphiteelectric furnaces to prevent the graphite from burning. It is also used inincandescent,fluorescent 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 samesolubilityin water asoxygenand is 2.5 times more soluble in water thannitrogen.Argon is colorless, odorless, nonflammable and nontoxic as a solid, liquid or gas.[10]Argon is chemicallyinertunder 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 withfluorineandhydrogenthat is stable below 17 K (−256.1 °C; −429.1 °F), has been demonstrated.[11][12]Although the neutral ground-state chemical compounds of argon are presently limited to HArF, argon can formclathrateswith water when atoms of argon are trapped in a lattice of water molecules.[13]Ions,such asArH+
,andexcited-state complexes,such as ArF, have been demonstrated. Theoretical calculation predicts several moreargon compoundsthat should be stable[14]but have not yet been synthesized.

History

A: test-tube, B: dilute alkali, C: U-shaped glass tube, D: platinum electrode

Argon(Greekἀργόν,neuter singular form ofἀργόςmeaning "lazy" or "inactive" ) is named in reference to its chemical inactivity. This chemical property of this firstnoble gasto be discovered impressed the namers.[15][16]An unreactive gas was suspected to be a component of air byHenry Cavendishin 1785.[17]

Argon was first isolated from air in 1894 byLord Rayleighand SirWilliam RamsayatUniversity College Londonby removingoxygen,carbon dioxide,water, andnitrogenfrom a sample of clean air.[18][19][20]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 dilutealkalisolution (B), which in Cavendish's original experiment was potassium hydroxide,[17]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 cellsand aRuhmkorff coilof 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.

Captioned "Argon", caricature ofLord RayleighinVanity Fair,1899

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.[21]Argon was also encountered in 1882 through independent research of H. F. Newall and W. N. Hartley.[22]Each observed new lines in theemission spectrumof air that did not match known elements.

Until 1957, the symbol for argon was "A", but now it is "Ar".[23]

Occurrence

Argon constitutes 0.934% by volume and 1.288% by mass ofEarth's atmosphere.[24]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,kryptonandxenon.[25]Earth's crust and seawater contain 1.2 ppm and 0.45 ppm of argon, respectively.[26]

Isotopes

Main article:Isotopes of argon

The mainisotopesof argon found on Earth are40
Ar(99.6%),36
Ar(0.34%), and38
Ar(0.06%). Naturally occurring40
K,with ahalf-lifeof 1.25×109years, decays to stable40
Ar(11.2%) byelectron captureorpositron emission,and also to stable40
Ca(88.8%) bybeta decay.These properties and ratios are used to determine the age ofrocksbyK–Ar dating.[26][27]

In Earth's atmosphere,39
Aris made bycosmic rayactivity, primarily by neutron capture of40
Arfollowed by two-neutron emission. In the subsurface environment, it is also produced throughneutron captureby39
K,followed by proton emission.37
Aris created from theneutron captureby40
Cafollowed by anAlpha particleemission as a result of subsurfacenuclear explosions.It has a half-life of 35 days.[27]

Between locations in theSolar System,the isotopic composition of argon varies greatly. Where the major source of argon is the decay of40
Kin rocks,40
Arwill be the dominant isotope, as it is on Earth. Argon produced directly bystellar nucleosynthesisis dominated by theAlpha -processnuclide36
Ar.Correspondingly, solar argon contains 84.6%36
Ar(according tosolar windmeasurements),[28]and the ratio of the three isotopes36Ar:38Ar:40Ar in the atmospheres of the outer planets is 8400: 1600: 1.[29]This contrasts with the low abundance ofprimordial36
Arin 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,MercuryandTitan(the largest moon ofSaturn) contain argon, predominantly as40
Ar.[30]

The predominance ofradiogenic40
Aris the reason thestandard atomic weightof terrestrial argon is greater than that of the next element,potassium,a fact that was puzzling when argon was discovered.Mendeleevpositioned the elements on hisperiodic tablein order of atomic weight, but the inertness of argon suggested a placementbeforethe reactivealkali metal.Henry Moseleylater solved this problem by showing that the periodic table is actually arranged in order ofatomic number(seeHistory of the periodic table).

Compounds

Main article:Argon compounds
Space-filling modelofargon fluorohydride

Argon's complete octet ofelectronsindicates full s and p subshells. This fullvalence shellmakes 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.[31]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 fluoridewithcaesium iodide.This discovery caused the recognition that argon could form weakly bound compounds, even though it was not the first.[12][32][33]It is stable up to 17 kelvins (−256 °C). ThemetastableArCF2+
2dication, which is valence-isoelectronicwithcarbonyl fluorideandphosgene,was observed in 2010.[34]Argon-36,in the form of argon hydride (argonium) ions, has been detected ininterstellar mediumassociated with theCrab Nebulasupernova;this was the firstnoble-gas moleculedetected inouter space.[35][36]

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 H2molecules in Ar(H2)2dissociate above 175 GPa.[37]

Production

Argon is extracted industrially by thefractional distillationofliquid airin acryogenicair separationunit; 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 700,000tonnesof argon are produced worldwide every year.[26][38]

Applications

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

Argon has several desirable properties:

  • Argon is a chemicallyinert gas.
  • Argon is the cheapest alternative whennitrogenis not sufficiently inert.
  • Argon has lowthermal conductivity.
  • Argon has electronic properties (ionization and/or the emission spectrum) desirable for some applications.

Othernoble gaseswould 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 separationin the production ofliquid oxygenandliquid 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 weldingsuch asgas metal arc weldingandgas tungsten arc welding,as well as in the processing oftitaniumand other reactive elements. An argon atmosphere is also used for growing crystals ofsiliconandgermanium.

See also:shielding gas

Argon is used in the poultry industry toasphyxiatebirds, 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.[39][40]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.[41]

Argon is sometimes used forextinguishing fireswhere valuable equipment may be damaged by water or foam.[42]

Scientific research

Liquid argon is used as the target for neutrino experiments and directdark mattersearches. The interaction between the hypotheticalWIMPsand an argon nucleus producesscintillationlight 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[43]), 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
Arcontamination, unless one uses argon from underground sources, which has much less39
Arcontamination. 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
Aractivity in the atmosphere is maintained by cosmogenic production through the knockout reaction40
Ar(n,2n)39
Arand similar reactions. The half-life of39
Aris only 269 years. As a result, the underground Ar, shielded by rock and water, has much less39
Arcontamination.[44]Dark-matter detectors currently operating with liquid argon includeDarkSide,WArP,ArDM,microCLEANandDEAP.Neutrino experiments includeICARUSandMicroBooNE,both of which use high-purity liquid argon in atime projection chamberfor 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.[45]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 ofcaesiumis 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 codeE938). 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.[46]

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 standardredoxchemistry.

Argon is sometimes used as the propellant inaerosolcans.

Argon is also used as a preservative for such products asvarnish,polyurethane,and paint, by displacing air to prepare a container for storage.[47]

Since 2002, the AmericanNational Archivesstores important national documents such as theDeclaration of Independenceand theConstitutionwithin 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.[48]

Laboratory equipment

See also:Air-free technique
Gloveboxesare often filled with argon, which recirculates over scrubbers to maintain anoxygen-,nitrogen-, and moisture-free atmosphere

Argon may be used as theinert gaswithinSchlenk linesandgloveboxes.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 chromatographyand 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 depositionof thin films as inmicroelectronicsand forwafer cleaning in microfabrication.

Medical use

Cryosurgeryprocedures such ascryoablationuse liquid argon to destroy tissue such ascancercells. It is used in a procedure called "argon-enhanced coagulation", a form of argonplasma beamelectrosurgery.The procedure carries a risk of producinggas embolismand has resulted in the death of at least one patient.[49]

Blueargon lasersare used in surgery to weld arteries, destroy tumors, and correct eye defects.[26]

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.[50]

Lighting

Argongas-discharge lampforming the symbol for argon "Ar"

Incandescent lightsare filled with argon, to preserve thefilamentsat high temperature from oxidation.[51]It is used for the specific way it ionizes and emits light, such as inplasma globesandcalorimetryin experimentalparticle physics.Gas-discharge lampsfilled with pure argon provide lilac/violet light; with argon and some mercury, blue light. Argon is also used for blue and greenargon-ion lasers.

Miscellaneous uses

Argon is used forthermal insulationinenergy-efficient windows.[52]Argon is also used in technicalscuba divingto inflate adry suitbecause it is inert and has low thermal conductivity.[53]

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 Sidewindermissile and other missiles that use cooled thermal seeker heads. The gas isstored at high pressure.[54]

Argon-39, with a half-life of 269 years, has been used for a number of applications, primarilyice coreandground waterdating. Also,potassium–argon datingand relatedargon-argon datingare used to datesedimentary,metamorphic,andigneous rocks.[26]

Argon has been used by athletes as a doping agent to simulatehypoxicconditions. In 2014, theWorld Anti-Doping Agency(WADA) added argon andxenonto the list of prohibited substances and methods, although at this time there is no reliable test for abuse.[55]

Safety

Although argon is non-toxic, it is 38% moredensethan air and therefore considered a dangerousasphyxiantin closed areas. It is difficult to detect because it is colorless, odorless, and tasteless. A 1994 incident, in which a man wasasphyxiatedafter 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.[56]

See also

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Further reading

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