molecularity
/ muh-LEK-yoo-LAR-ih-tee /
When two cars must meet at exactly the same place and time to swap a passenger, the event depends on both arriving; when a single car simply drops a passenger off by itself, only that one car matters. Molecularity asks the same question for a single elementary step: how many molecules must come together for that step to happen?
If just one molecule reacts all by itself, breaking or rearranging without help, the step is unimolecular. If two molecules must collide together in one event, it is bimolecular. (Three at once, termolecular, is rare because a precise three-way collision is unlikely.) Molecularity is a property of a single elementary step, and because such a step is one physical event, you can write its rate law directly from its molecularity: a unimolecular step is first order, a bimolecular step is second order. This is the basis of the names SN1 and E1 (the slow step is unimolecular) versus SN2 and E2 (the slow step is bimolecular).
Molecularity is closely related to, but not the same as, reaction order. Molecularity is a theoretical count of molecules in a proposed elementary step and is always a small whole number. Reaction order is an experimental fact, measured from how the rate changes with concentration, and for an overall multistep reaction it need not match any single step's molecularity (it can even be fractional or zero). The two coincide only for a true elementary step; confusing them is a classic mistake.
The slow step of an SN2 reaction is bimolecular: the nucleophile and the substrate must collide together, so the rate is rate = k[substrate][nucleophile], second order overall.
The '2' in SN2 marks the bimolecular slow step; the '1' in SN1 marks a unimolecular one.
Molecularity (a theoretical count for an elementary step) and reaction order (an experimental number for the overall reaction) match only for a true single step; for multistep reactions the order can differ and even be fractional.