Molecularity of an Elementary Reaction

The molecularity of an elementary reaction is defined as : the number of reactant molecules involved in a reaction.

Thus the molecularity of an elementary reaction is 1, 2, 3, etc., according as one, two or three reactant molecules are participating in the reaction. The elementary reactions having molecularity 1, 2 and 3 are called unimolecular, bimolecular and termolecular respectively. Thus we have :

            (a) Unimolecular reactions : (molecularity = 1)