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 13.61806 eV 1313.5 kJ/mol 2nd 3388.3 kJ/mol 3rd 5300.5 kJ/mol Electron affinity 1.461112(44) eV 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 Enthalpy change of vaporization 6.82 kJ/mol 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").