Molecularity of a reaction

Molecularity of a reaction is no of atoms, molecules or ions involved in collision to form product for simple reaction, molecularity is equal stoichiometric coefficient, but for complex reaction molecularity is no of atoms, ions or molecules involved in rate determining step i.e. slowest step. The molecularity is a theoretical value obtained from proposed reaction mechanism and is a small whole no value like 1, 2, 3 ……..

 Cyclobutane               →             2CH2 = CH2

 The molecularity of reaction is 1, so it is unimolecular reaction.

 CH3COOCH3 + NaOH         →       CH3COONa + CH3OH

 The molecularity of reaction is 2, so it is bimolecular reaction.

 2NO + Cl2      →      2NOCl

 The molecularity of reaction is 3, so it is termolecular reaction.

 In all above reaction (examples), the molecularity is equal to stoichiometric coefficient. So, they are simple reaction and occur in single step.

 But in case of decomposition of N2O5.

 2 N2O5    →   4 NO2    + O5

The molecularity of above reaction is 1 but its stoichiometric coefficient is 2. So, it is a complex reaction occurring in multiple step in which the rate determining step includes only one molecule. The proposed mechanism for this reaction is, 

Step I:

N2O5    →   NO2 + NO3     (Slow step)

Step II:

 NO3 + N 2O5    →   3NO2    + O2     (Fast step)