The minimum number of reacting particles that take part in a rate determining step to form product is called molecularity of reaction.

Rate determining step:

The occurrence of a chemical reaction is not a single step procedure but it depends on the mechanism by which reaction complete. The rate of reaction is determined by the slowest step in the sequence and thus it is called rate determining step.

Example of elementary reactions:

This is unimoleculer reaction because it involve one molecule few reactions and molecules in rate determining step.

Example of the complex non elementary reactions:
For complex reactions the molecularity is determine by studying their mechanism.

(a)       Example:

(b)       Example:

(c)       Example:

Molecularity and order:

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