Argon
Pronunciation/ ˈɑːrɡɒn/ ( AR-gon )
Appearancecolorless gas exhibiting a lilac/violet glow when placed in an electric field
Standard atomic weight Ar, std(Ar) [ 39.792 39.963 ] conventional:  39.95
Atomic number ( Z) 18
Groupgroup 18 (noble gases)
Periodperiod 3
Blockp-block
Element category Noble gas
Electron configuration[Ne] 3s 2 3p 6
Electrons per shell
2, 8, 8
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/cm 3
Triple point83.8058 K, ​68.89 kPa
Critical point150.687 K, 4.863 MPa
Heat of fusion1.18 kJ/mol
Heat of vaporization6.53 kJ/mol
Molar heat capacity20.85 J/(mol·K)
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
Natural occurrenceprimordial
Speed of sound323 m/s (gas, at 27 °C)
Thermal conductivity17.72 ×10 3  W/(m·K)
Magnetic orderingdiamagnetic
Magnetic susceptibility−19.6·10 −6 cm 3/mol
CAS Number7440-37-1
Discovery and first isolationLord Rayleigh and William Ramsay (1894)