molecularity

Molecularity in chemistry refers to the number of molecules or atoms involved in a single step of a chemical reaction. This concept is crucial for understanding reaction mechanisms.12

Reactions can be classified by their molecularity:
* Unimolecular: Only one molecule is involved in the rate-determining step.23
* Bimolecular: Two molecules are involved in the rate-determining step.24
* Termolecular (or Trimolecular): Three molecules are involved in the rate-determining step.5

The molecularity of a reaction step is distinct from its order, which is determined experimentally by how reaction rates depend on reactant concentrations. While often related, they are not always the same, especially in multi-step reactions.62