Isomer

Summary: Molecules with the same molecular formula but different structural formula (different arrangement of atoms). Tags: igcse chemistry definition organic-chem Created: 2026-07-14 Last Updated: 2026-07-14

An isomer is one of two or more compounds that share the same molecular formula but have different structural arrangements of atoms, resulting in distinct physical properties and sometimes different chemical behaviour. At IGCSE level, the focus is on structural isomerism in alkanes — for instance, butane (straight-chain) and methylpropane (branched) both have the formula C₄H₁₀ yet boil at -0.5 °C and -12 °C respectively, illustrating how branching lowers boiling point by reducing surface contact and therefore intermolecular forces. As molecular size increases, the number of possible isomers grows rapidly: C₅H₁₂ has three isomers, while C₆H₁₄ has five. IGCSE candidates must be able to draw and name both straight-chain and branched isomers of alkanes up to about four or five carbon atoms. A common exam pitfall is failing to recognise that isomers must have different structural formulae — simply rotating the molecule or flipping it in space does not create an isomer, because the connectivity of atoms is unchanged.

Key term — IGCSE Chemistry (0620/0971) glossary.