Definition of Elementary reaction

An elementary reaction is a chemical reaction in which one or more chemical species react directly to form products in a single reaction step and with a single transition state.

In a unimolecular elementary reaction a molecule, A, dissociates or isomerises to form the products(s).

The rate of such a reaction, at constant temperature, is proportional to the concentration of the species A

In a bimolecular elementary reaction two atoms, molecules, ions or radicals, A and B, react together to form the product(s)

The rate of such a reaction, at constant temperature, is proportional to the product of the concentrations of the species A and B.

This rate expression can be derived from first principles by using collision theory. The rate expression for an elementary bimolecular reaction is sometimes referred to as the Law of Mass Action as it was first proposed by Guldberg and Waage in 1864. An example of this type of reaction is a cycloaddition reaction.