Argon

Argon, 18Ar
Vial containing a violet glowing gas
Argon
Pronunciationn/ (AR-gon)
Appearancecolorless gas exhibiting a lilac/violet glow when placed in an electric field
Standard atomic weight Ar, std(Ar)[39.79239.963] conventional: 39.95[1]
Argon in the periodic table
HydrogenHelium
LithiumBerylliumBoronCarbonNitrogenOxygenFluorineNeon
SodiumMagnesiumAluminiumSiliconPhosphorusSulfurChlorineArgon
PotassiumCalciumScandiumTitaniumVanadiumChromiumManganeseIronCobaltNickelCopperZincGalliumGermaniumArsenicSeleniumBromineKrypton
RubidiumStrontiumYttriumZirconiumNiobiumMolybdenumTechnetiumRutheniumRhodiumPalladiumSilverCadmiumIndiumTinAntimonyTelluriumIodineXenon
CaesiumBariumLanthanumCeriumPraseodymiumNeodymiumPromethiumSamariumEuropiumGadoliniumTerbiumDysprosiumHolmiumErbiumThuliumYtterbiumLutetiumHafniumTantalumTungstenRheniumOsmiumIridiumPlatinumGoldMercury (element)ThalliumLeadBismuthPoloniumAstatineRadon
FranciumRadiumActiniumThoriumProtactiniumUraniumNeptuniumPlutoniumAmericiumCuriumBerkeliumCaliforniumEinsteiniumFermiumMendeleviumNobeliumLawrenciumRutherfordiumDubniumSeaborgiumBohriumHassiumMeitneriumDarmstadtiumRoentgeniumCoperniciumNihoniumFleroviumMoscoviumLivermoriumTennessineOganesson
Ne

Ar

Kr
chlorineargonpotassium
Atomic number (Z)18
Groupgroup 18 (noble gases)
Periodperiod 3
Blockp-block
Element category  Noble gas
Electron configuration[Ne] 3s2 3p6
Electrons per shell2, 8, 8
Physical properties
Phase at STPgas
Melting point83.81 K ​(−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 (at b.p.)1.3954 g/cm3
Triple point83.8058 K, ​68.89 kPa[2]
Critical point150.687 K, 4.863 MPa[2]
Heat of fusion1.18 kJ/mol
Heat of vaporization6.53 kJ/mol
Molar heat capacity20.85[3] J/(mol·K)
Vapor pressure
P (Pa)1101001 k10 k100 k
at T (K) 4753617187
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±10 pm
Van der Waals radius188 pm
Color lines in a spectral range
Spectral lines of argon
Other properties
Natural occurrenceprimordial
Crystal structureface-centered cubic (fcc)
Face-centered cubic crystal structure for argon
Speed of sound323 m/s (gas, at 27 °C)
Thermal conductivity17.72×103  W/(m·K)
Magnetic orderingdiamagnetic[4]
Magnetic susceptibility−19.6·10−6 cm3/mol[5]
CAS Number7440-37-1
History
Discovery and first isolationLord Rayleigh and William Ramsay (1894)
Main isotopes of argon
Iso­topeAbun­danceHalf-life (t1/2)Decay modePro­duct
36Ar0.334%stable
37Arsyn35 dε37Cl
38Ar0.063%stable
39Artrace269 yβ39K
40Ar99.604%stable
41Arsyn109.34 minβ41K
42Arsyn32.9 yβ42K
36
Ar
and 38
Ar
content may be as high as 2.07% and 4.3% respectively in natural samples. 40
Ar
is the remainder in such cases, whose content may be as low as 93.6%.
| references

Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas.[6] 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 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 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 is produced industrially by the fractional distillation of liquid air. 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 is also used in incandescent, fluorescent lighting, and other gas-discharge tubes. Argon makes a distinctive blue-green gas laser. Argon is also used in fluorescent glow starters.

Characteristics

A small piece of rapidly melting solid argon

Argon has approximately the same solubility in water as oxygen and is 2.5 times more soluble in water than nitrogen. Argon is colorless, odorless, nonflammable and nontoxic as a solid, liquid or gas.[7] 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 various extreme conditions. 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.