The molecularity of an elementary reaction is the number of reactant species (atoms, molecules, or ions). For example, a unimolecular reaction involves the reaction of a single reactant species to produce one or more molecules of product:

A ⟶ product

The rate law for a unimolecular reaction is first order:

Rate = k[A]

A unimolecular reaction may be one of several elementary reactions in a complex mechanism. For example, the reaction:

O₃ ⟶ O₂ + O

Illustrates a unimolecular elementary reaction that occurs as one part of a two-step reaction mechanism.