The minimum number of reacting particles (molecules, atoms or ions) that come together or collide in a rate determining step to form product is called the molecularity of a reaction. Molecularity greater than three is generally rare because higher molecularity decreases the chance of their coming together and effective collision to product formation.
Decomposition of H2O2 takes place in the following two steps-
H2O2 → H2O + 1/2O2
Step 1: it is a slow step
H2O2 → H2O + O
Step 2: it is a fast step
O + O → O2
We know that the slowest step is rate-determining. So,from step-1 it is a unimolecular.