Oxygen

nitrogen ← oxygen → fluorine — ↑ O ↓ S

Atomic properties Atomic number 8 Standard atomic weight 15.9994(3) Electron configuration [He]2s22p4 Physical properties Melting point −218.8 °C Boiling point −183.0 °C Critical point 154.59 K, 5.043 MPa Chemical properties Electronegativity 3.44 (Pauling) Ionization energies 1st 1313.5 kJ/mol 2nd 3388.3 kJ/mol 3rd 5300.5 kJ/mol Atomic radii Covalent radius 66 Van der Waals radius 152 Thermodynamic properties Enthalpy change of fusion (O2) 0.444 Enthalpy change of vaporization (O2) 6.82 Molar heat capacity (Cp) (O2) 29.378 Miscellaneous CAS number 7782-44-7 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").