Oxygen
AllotropesO2, O3 (Ozone)
Appearancegas: colorless
liquid and solid: pale blue
Standard atomic weight Ar, std(O)[15.9990315.99977] conventional: 15.999
Atomic number (Z)8
Groupgroup 16 (chalcogens)
Periodperiod 2
Blockp-block
Element category  Reactive nonmetal
Electron configuration[He] 2s2 2p4
Electrons per shell
2, 6
Phase at STPgas
Melting point(O2) 54.36 K ​(−218.79 °C, ​−361.82 °F)
Boiling point(O2) 90.188 K ​(−182.962 °C, ​−297.332 °F)
Density (at STP)1.429 g/L
when liquid (at b.p.)1.141 g/cm3
Triple point54.361 K, ​0.1463 kPa
Critical point154.581 K, 5.043 MPa
Heat of fusion(O2) 0.444 kJ/mol
Heat of vaporization(O2) 6.82 kJ/mol
Molar heat capacity(O2) 29.378 J/(mol·K)
Oxidation states−2, −1, 0, +1, +2
ElectronegativityPauling scale: 3.44
Ionization energies
  • 1st: 1313.9 kJ/mol
  • 2nd: 3388.3 kJ/mol
  • 3rd: 5300.5 kJ/mol
  • (more)
Covalent radius66±2 pm
Van der Waals radius152 pm
speed of sound330 m/s (gas, at 27 °C)
Thermal conductivity26.58×10−3  W/(m·K)
Magnetic orderingparamagnetic
Magnetic susceptibility+3449.0·10−6 cm3/mol (293 K)
CAS Number7782-44-7
Named byAntoine Lavoisier (1777)