Molecularity

Molecularity is a concept in chemical kinetics that refers to the number of reactant molecules involved in a single elementary step of a reaction. It is used to categorize elementary steps as unimolecular (one molecule), bimolecular (two molecules), or termolecular (three molecules). The molecularity of an elementary step equals the stoichiometric factors appearing in its rate law: for an elementary step A -> products, rate = k[A]; A + B -> products, rate = k[A][B]; A + B + C -> products, rate = k[A][B][C].

For complex reactions consisting of multiple elementary steps, the observed overall rate law is determined by the rate-determining step, and need not reflect the molecularity of the overall reaction. The molecularity concept applies strictly to elementary steps; it is not generally equal to the overall reaction order, which is derived from the rate law of the rate-determining step or the mechanism.

Termolecular steps are relatively rare in practice because three-body collisions are statistically unlikely; many proposed termolecular steps are actually two successive bimolecular steps with a reactive intermediate or a fast pre-equilibrium. Similarly, unimolecular steps may be rate-limited by a preceding fast step (pseudo-unimolecular).

Molecularity is a useful guide in mechanism proposals and in interpreting rate laws but has limitations: it cannot always be determined directly from the overall rate expression, and real reactions often do not occur through a single elementary step.

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