Molecularity

From rate laws to mechanism. Kinetics does not stop at measuring an overall rate law -- it also aims to propose a believable step-by-step REACTION MECHANISM that could produce that rate law. Each individual step of a proposed mechanism is called an elementary reaction, and every elementary step carries its own molecularity: simply the total number of reactant species (atoms, ions or molecules) that must come together in that one step for it to occur. Unlike order, molecularity is a purely theoretical, structural count for one step -- it is always a whole positive number (1, 2, or rarely 3), never zero and never fractional, because you cannot have 'half a molecule' colliding.

Unimolecular example. The hydrolysis of t-butyl bromide (from Class XI) has a rate-determining step involving only the t-butyl bromide molecule itself -- one reactant species is enough to define the step, so it is called a Unimolecular Nucleophilic Substitution ($S_N1$) reaction.

Bimolecular example, and why molecularity can reveal a hidden mechanism. The iodide-catalysed decomposition of hydrogen peroxide, $2H_2O_2(aq)\xrightarrow{I^-}2H_2O(l)+O_2(g)$, is found experimentally to be FIRST order in both $H_2O_2$ AND $I^-$ -- a strong clue that $I^-$, despite being written as a mere catalyst over the reaction arrow, genuinely takes part in the mechanism rather than sitting passively on the sidelines. The proposed two-step mechanism is:

Step 1 (slow, rate-determining): $H_2O_2(aq)+I^-(aq)\rightarrow H_2O(l)+OI^-(aq)$

Step 2 (fast): $H_2O_2(aq)+OI^-(aq)\rightarrow H_2O(l)+I^-(aq)+O_2(g)$

Adding the two steps together, the intermediate $OI^-$ and the catalyst $I^-$ both cancel appropriately, regenerating exactly the overall reaction. Step 1, involving two distinct reactant species ($H_2O_2$ and $I^-$) colliding, is BIMOLECULAR -- and because Step 1 is also the slow, RATE-DETERMINING step, the observed overall order (first in each of $H_2O_2$ and $I^-$) matches this one step's molecularity exactly, which is how kinetic data is used as evidence for a proposed mechanism. …