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. Argon is the third-most abundant gas in 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 Earth's atmosphere is radiogenic argon-40, derived from the decay of potassium-40 in 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 extracted 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.

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Atomic properties

Standard atomic weight39.95 ±0.16 [39.792 … 39.963]
Atomic mass39.9481 u

Atomic radii

Radius (calculated)71 pm
Covalent radius106 ±10 pm
Van der Waals radius188 pm

Atomic shell

Electron configurationNe 3s2 3p6
Ionization energy(1st) 15.759611 eV
(2nd) 27.62967 eV
(3rd) 40.735 eV
(4th) 59.58 eV
(5th) 74.84 eV
Shell model

Physical properties

Density0.001784 g·cm−3 (273 K)
Molar volumesolid: 2.256·10-5 m3·mol−1
Speed of sound319 m·s−1 (293.1 K)


Melting point83.81 K
Boiling point87.3 K
Liquid range3.492 K
Triple point83.8 K @ 68.89 kPa
Critical point150.6 K @ 4.863 MPa


Melting enthalpy1.18 kJ·mol-1
Enthalpy of vaporization6.5 kJ·mol-1
Binding energy0 kJ·mol-1

Heat and conductivity

Specific heat capacity319 J·kg−1·K−1 (293.1 K)
Thermal conductivity0.01772 W·m-1·K-1


Magnetic susceptibility-1.96·10-5 cm3·mol−1

Optical properties

Refractive index1

Chemical properties

Oxidation state-


Sanderson scale3.31
Allred-Rochow scale3.2
Mulliken scale3.19
Allen scale3.242
Ghosh-Gupta scale7.651 eV
Nagle scale3.18

Other properties

Natural occurrenceprimordial
Crystal structureFace-centered cubic
Goldschmidt Classificationatmophile
Superconductorwithout transition tempperature
Price/kg0.931 USD

Natural abundances

200,000 ppb ≈ 2.01·1015 M☉
70,000 ppb ≈ 1.39·1014 Mt
Earth’s crust
1,500 ppb ≈ 41,500 Mt
450 ppb ≈ 616 Gt