Argon
Pronunciation | / ˈ ɑːr ɡ ɒ n/ ​ ( AR-gon )
Appearance | colorless 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
Group | group 18 (noble gases)
Period | period 3
Block | p-block
Element category | Noble gas
Electron configuration | [Ne] 3s 2 3p 6
Electrons per shell | 2, 8, 8
Phase at STP | gas
Melting point | 83.81 K ​(−189.34 °C, ​−308.81 °F)
Boiling point | 87.302 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 point | 83.8058 K, ​68.89 kPa
Critical point | 150.687 K, 4.863 MPa
Heat of fusion | 1.18 kJ/mol
Heat of vaporization | 6.53 kJ/mol
Molar heat capacity | 20.85 J/(mol·K)
Oxidation states | 0
Electronegativity | Pauling scale: no data
Ionization energies | 1st: 1520.6 kJ/mol 2nd: 2665.8 kJ/mol 3rd: 3931 kJ/mol (more)
Covalent radius | 106±10 pm
Van der Waals radius | 188 pm
Natural occurrence | primordial
Speed of sound | 323 m/s (gas, at 27 °C)
Thermal conductivity | 17.72 ×10 − 3 W/(m·K)
Magnetic ordering | diamagnetic
Magnetic susceptibility | −19.6·10 −6 cm 3/mol
CAS Number | 7440-37-1
Discovery and first isolation | Lord Rayleigh and William Ramsay (1894)