Oxygen

nitrogen ← oxygen → fluorine – ↑ O ↓ S

Atomic properties Atomic number 8 Standard atomic weight 15.9994(3) Electron configuration [He] 2s2 2p4 Physical properties (O2) Melting point 54.8(2) K (−218.8 °C) Boiling point 90.2(2) K (−183.0 °C) Critical point 154.58 K, 50.43 bar Chemical properties Electronegativity 3.44 (Pauling) Ionization energies 1st 13.618 06 eV, 1313.943 kJ mol−1 2nd 35.1211 eV, 3388.67 kJ mol−1 3rd 54.9355 eV, 5300.47 kJ mol−1 4th 77.4135 eV 5th 113.8989 eV 6th 138.1196 eV 7th 739.3268 eV 8th 871.4097 eV Total 2043.8432 eV Electron affinity 140.9759(42) kJ mol−1 Atomic radii Covalent radius 66 pm Van der Waals radius 152 pm Thermodynamic properties (O2) Standard entropy 205.152(5) J K−1 mol−1 Enthalpy change of fusion 0.444 kJ mol−1 Enthalpy change of vaporization 6.82 kJ mol−1 Molar heat capacity (Cp) 29.378 J K−1 mol−1 Miscellaneous CAS number 7782-44-7 (O2) 17778-80-2 (atomic) EC number 231-956-9 Where appropriate, and unless otherwise stated, data are given for 100 kPa (1 bar) and 298.15 K (25 °C).

Oxygen (ˈɒksɨdʒɨn) is a colourless gas which makes up about one fifth of the Earth's atmosphere. Its name comes from the Greek ὀξύς (oxys; acid, literally "sharp", from the taste of acids) and -γενής (-genēs; producer, literally "begetter").