Isomeric molecules have equal empirical formulas, but different structural formulas.

Identical constituents (atoms, group of atoms) are connected to each other in different ways in such compounds. The example gives the respective formulas for acetaldehyde and the respective enole.

Stereoisomery is an important, special form of isomery: In this case the constituents are connected in a similar way, but distributed differently in space. Important examples are cis-trans isomery, and chiral molecules.