Oxygen(O)
Oxygen is a highly reactive nonmetal essential for respiration and combustion. It is the most abundant element by mass in the Earth's crust.
KEY PROPERTIES
Electron configuration & shells
[He] 2s² 2p⁴
Shell model is schematic — see lesson on atomic structure for orbitals. Oxidation states: -2. Common ions: O²⁻.
Measured properties
Values at stated conditions; missing data shown as “—”. Radii distinguished by type; densities at stated phase/temperature.
- Electronegativity (Pauling)
- 3.44
- First ionization energy
- 1314 kJ·mol⁻¹ (first ionization, gas phase)
- Atomic radius
- 48 pm (calculated)
- Covalent radius
- 66 pm (covalent)
- Van der Waals radius
- 152 pm (van der Waals)
- Density
- 0.001429 g·cm⁻³ (gas at STP)
- Melting point
- 54.36 K (1 atm)
- Boiling point
- 90.2 K (1 atm)
Standard conditions: 1 atm unless noted. Values with null are not established or not comparable across allotropes. Radii are distinguished by type. Densities at stated phase/temperature.
| Mass number | Abundance | Stability | Half-life |
|---|---|---|---|
| 16 | 99.757% | stable | — |
| 17 | 0.038% | stable | — |
| 18 | 0.205% | stable | — |
Isotopic abundances are natural terrestrial values where applicable; synthetic isotopes shown as trace.
Occurrence & uses
Most abundant by mass in Earth's crust (~47%) and ~21% of air by volume as O₂.
- Respiration
- Steelmaking
- Medical oxygen
BIOLOGICAL ROLE
Biologically relevant — see uses/occurrence for context.
Discovery
Carl Wilhelm Scheele / Joseph Priestley — 1772
Safety
Compounds featuring O
Reactions involving O
RELATED CONTENT
SOURCES
- [nist-atomic-weights]nist-atomic-weights (2022)/#nist-atomic-weights
- [nist-2022]nist-2022 (2022)/#nist-2022
- [iupac-gold]iupac-gold (2022)/#iupac-gold
Last reviewed: 2026-08-22 • Reviewer: dr-example