Molecularity
The number of reacting species that collide simultaneously in a single elementary step.
Unit
Dimensionless
In depth
Unlike order, molecularity is a theoretical property of a proposed elementary step and must be a small positive integer — one, two or rarely three. It applies only to elementary reactions; an overall reaction has an order but no molecularity, since it is a sum of steps.
Examples in the real world
The unimolecular decomposition of N₂O₅ and the bimolecular collision of two NO₂ molecules.