Oxygen (O2)

Summary: Oxygen (O₂) is a diatomic molecule in which the two oxygen atoms share two pairs of electrons in a double covalent bond, leaving each atom with two lone pairs; it makes up about 21% of clean air and relights a glowing splint. Tags: igcse chemistry Created: 2026-07-18

Oxygen (O₂) is a diatomic Molecule of the Element oxygen and the syllabus example of a double covalent bond between two identical atoms. Each oxygen atom has six valence electrons and needs two more to complete its octet; by sharing two pairs of electrons with each other, both atoms gain access to a full outer Electron Shell of 8 without transferring electrons as in Ions and Ionic Bonds (see Covalent Bonding). Each atom keeps two lone pairs that take no part in bonding. Oxygen makes up approximately 21% of clean, dry air (see Composition of Air), where it is essential for combustion and respiration — both reactions combine fuels or glucose with oxygen, releasing energy and producing carbon dioxide and water. It is a colourless, odourless gas at room temperature and is identified in the laboratory because it relights a glowing splint, one of the standard Chemical Tests.

Bonding: Dot-and-Cross Description

In a Dot-and-Cross Diagram of O₂, one atom’s electrons are drawn as dots and the other’s as crosses:

• Each O atom contributes 2 electrons to the bond, so the region between the atoms contains 2 shared pairs (two dots and two crosses) — a double covalent bond.
• Each O atom also has 2 lone pairs (four non-bonding electrons) drawn on its outer side.
• After sharing, each atom counts 8 outer electrons: 4 in the double bond + 4 in its own lone pairs — a full octet, matching a Noble Gas Electronic Configuration.

Displayed formula: O=O.

Key Data

Physical Properties

• A simple molecular gas with very low melting and boiling points — boiling overcomes only the weak Intermolecular Force between molecules, not the strong O=O bond.
• Colourless, odourless, and only slightly soluble in water (enough to support aquatic life).
• Slightly denser than air.
• Does not conduct electricity — no mobile ions or delocalised electrons.

Test for Oxygen

Insert a glowing splint into the gas. Oxygen relights the glowing splint, because it supports combustion far more strongly than air does. (Contrast: hydrogen gives a squeaky pop with a lighted splint — see Hydrogen (H2) and Chemical Tests.)

Role in Combustion and Respiration

• Combustion — fuels burn in oxygen, releasing energy. Complete combustion of a hydrocarbon gives carbon dioxide and water (see Fuels and Petroleum); a limited oxygen supply causes incomplete combustion, producing carbon monoxide (see Air Pollution).
• Respiration — living cells combine glucose with oxygen to release energy, producing CO₂ and water.
• Industrially, oxygen is separated from liquid air by fractional distillation (see Separation Techniques and Composition of Air) and is used in steel-making, welding torches and medical breathing supplies.

Sources

• Cambridge IGCSE Chemistry 0620 Syllabus (2023-2025) — Section 2.5, Cambridge Assessment International Education

Common Misconceptions