Living Reference Work Entry

Encyclopedia of Geochemistry

Part of the series Encyclopedia of Earth Sciences Series pp 1-4

Date: Latest Version


  • Mark A. KendrickAffiliated withResearch School of Earth Sciences, The Australian National University Email author 

Element Data

Atomic Symbol: Ar

Atomic Number: 18

Atomic Weight: 39.984

Isotopes and Abundances:40Ar (99.6%), 38Ar (0.06%), and 36Ar (0.34%)

1 Atm Melting Point: 83.95 K

1 Atm Boiling Point: 87.29 K

Common Valences: 0

First Ionization Potential: 1520.6 kJ mol−1

Chondritic (CI) Abundance: ~10−6 mol g−1 (~93 ppm by mass)

Silicate Earth Abundance: ~10−12 mol 36Ar g−1 (parts per trillion range)

Crustal Abundance: <10−11 mol 36Ar g−1

Seawater Abundance: ~1–2 × 10−8 mol 36Ar g−1


Argon is the most abundant noble gas and accounts for 0.934 volume % of dry air. Argon has a calculated atomic radius of 0.71 Å and a van der Waals radius of 1.88 Å. Argon is a colorless, odorless, tasteless, nontoxic gas at room temperature, and like the other noble gases in group 18 of the periodic table, Ar usually has a full valence shell of electrons and does not form chemical bonds. Chemical inertness and volatility are defining features of group 18 gases which exist in monatomic states and are common ...

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