Molecularity

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Molecularity in chemistry is the number of colliding molecular entities that are involved in a single reaction step. While the order of a reaction is derived experimentally, the molecularity is a theoretical concept and can only be applied to elementary reactions. In elementary reactions, the reaction order, the molecularity and the stoichiometric coefficient are the same, although only numerically, because they are different concepts.

A reaction involving one molecular entity is called unimolecular.
A reaction involving two molecular entities is called bimolecular.
A reaction involving three molecular entities is called termolecular. It is still discused if termolecular processes in fact exist, and they are in any case, very rare.