Isomers are chemical compounds having identical chemical composition and molecular formula, but differing in the arrangement of atoms in their molecules, and having different properties. Isomerism arises because the comparatively large number of atoms which make up many organic compounds can be arranged in more than one way.

The two chief types of isomers are stereoisomers, which have the same structural formula, and structural isomers, which have different structural formulas. The latter may be subdivided into positional isomers, which have the same functional groups occupying the different positions on the carbon skeleton; and functional isomers, which have different functional groups.