Oxygen
Allotropes | O2, O3 (Ozone)
Appearance | gas: colorless liquid and solid: pale blue
Standard atomic weight Ar, std(O) | [15.99903, 15.99977] conventional: 15.999
Atomic number (Z) | 8
Group | group 16 (chalcogens)
Period | period 2
Block | p-block
Element category | Reactive nonmetal
Electron configuration | [He] 2s2 2p4
Electrons per shell | 2, 6
Phase at STP | gas
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 point | 54.361 K, ​0.1463 kPa
Critical point | 154.581 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
Electronegativity | Pauling scale: 3.44
Ionization energies | 1st: 1313.9 kJ/mol 2nd: 3388.3 kJ/mol 3rd: 5300.5 kJ/mol (more)
Covalent radius | 66±2 pm
Van der Waals radius | 152 pm
| speed of sound | 330 m/s (gas, at 27 °C)

Thermal conductivity 26.58×10−3 W/(m·K)
Magnetic ordering paramagnetic
Magnetic susceptibility+3449.0·10−6 cm3/mol (293 K)
CAS Number 7782-44-7
Named by Antoine Lavoisier (1777)