Oxygen (O2) (O₂) — Properties, Uses & Safety

Overview

Molecular oxygen (O2) is a diatomic gas with a molar mass of 32.00 g/mol that constitutes approximately 21% of Earth's atmosphere. It is a colorless, odorless gas at standard temperature and pressure, essential for aerobic respiration in most living organisms and required for combustion reactions. Oxygen has a unique electronic structure: molecular orbital theory predicts two unpaired electrons in its antibonding pi orbitals, making O2 a paramagnetic diradical — a property that cannot be explained by simple Lewis structures alone, making it a powerful teaching example for the superiority of molecular orbital theory. The double bond in O2 has a bond dissociation energy of 498 kJ/mol, strong enough to make the molecule stable but reactive enough to participate in oxidation reactions. Oxygen supports combustion but does not burn itself — it is the oxidizer, not the fuel. This distinction is critical in fire science and rocketry, where liquid oxygen (LOX) serves as an oxidizer for rocket propellants. Oxygen is produced industrially by fractional distillation of liquid air and in laboratories by decomposition of hydrogen peroxide (2 H2O2 → 2 H2O + O2) or electrolysis of water. Photosynthesis by plants and cyanobacteria is the primary natural source, having transformed Earth's atmosphere over billions of years in the Great Oxidation Event. Medical oxygen is used in hospitals for respiratory therapy, and supplemental oxygen supports patients with lung diseases. The GHS hazard code H270 (may cause or intensify fire; oxidizer) applies, with the flame-over-circle pictogram. Ozone (O3), an allotrope of oxygen, forms in the stratosphere and protects life from harmful UV radiation. For chemistry students, oxygen connects combustion reactions, oxidation-reduction chemistry, molecular orbital theory, paramagnetism, photosynthesis, respiration, allotropy, and atmospheric chemistry into a cohesive narrative about one of the most important elements on the periodic table.

IUPAC Name

molecular oxygen

CAS Number

7782-44-7

PubChem CID

977

Molecular Formula

O₂

Molar Mass

32 g/mol

State at STP

gas

Appearance

Colorless, odorless gas