Difference between Molecularity and Order of Reaction :
The molecularity in defined as the number of molecules, atoms or ions participating in the rate determining step.
The order of reaction is defined as the sum of the exponents of the concentration terms in the rate Iaw equation.
Molecularity of a reaction is a theoretical quantity to satisfy the experimental findings whereas the order of reaction is entirely an experimental quantity.
Molecularity of a reaction must be a whole number and never be zero or a fraction, but the order of a reaction may be whole number, zero or a fraction to.