A chemical reaction that takes place in one and only one step is called elementary reaction while a chemical reaction occurring in the sequence of two or more steps is called complicated reaction. The sequence of steps through which a complicated reaction takes place is called reaction-mechanism. Each step in a mechanism is an elementary step reaction.

The molecularity of an elementary reaction is defined as the minimum number of molecules, atoms or ions of the reactants(s) required for the reaction to occur and is equal to the sum of the stoichiometric coefficients of the reactants in the chemical equation of the reaction. Thus, the molecularity of some elementary reactions are as mentioned below.

`text(Elementary reactions Molecularity)`

`PCl_5 ⇋ PCl_3+Cl_2` `1`

`H_2+I_2⇋ 2HI` `1+1=2`

In general for any elementary reaction

`aA+bB-> ` Product Molecularity `=a+b`

Reactions with molecularity equal to one, two, three etc., are called unimolecular, bimolecular, trimolecular, etc., respectively.

The mathematical expression showing the dependence of rate on the concentration(s) of reactant(s) is known as rate-law or rate expression of the reaction and sum of the indices (powers) of the concentration terms appearing in the rate law as observed experimentally is called order of reaction. To understand what is order of reaction, consider the reaction:

`2NO(g)+2H_2(g)-> N_2(g)+2H_2O(g)`

Kinetic experiment carried out at `1100 K` upon this reaction has shown following rate data(See table).

From the Expt. No.1 and 2, it is evident that rate increases `4` fold when concentration of `NO` is doubled keeping the concentration of `H_2` constant i.e.

Rate = `[NO]^2` when `[H _2]` is constant

Again from Expt. No.2 and 3, it is evident that when concentration of `H_2` is doubled keeping the concentration of `NO` constant, the rate is just doubled i.e.

Rate = `[H_2]` when `[NO] ` is constant

From Expt. (1) and Ex pt. (3), the rate increases `8`-fold when concentrations of both `NO` and `H_2` are doubled simultaneously i.e.

Rate = `[NO]^2[H_2]`

This is the rate-law of reaction as observed experimentally. In the rate law, the power of nitric oxide concentration is `2` while that of hydrogen concentration is `1`. So, order of reaction w.r.t. `NO` is `2` and that w.r.t. `H_2` is `1` and overall order is `2 + 1` i.e. `3`. Note that the experimental rate law is not consistent with the stoichiometric coefficient of `H_2` in the chemical equation for the reaction. This fact immediately suggests that the reaction is complicated and it does not occur in single step as written.

i) Order is an experimental property while molecularity is the theoretical property.

ii) Order may be any number, fractional, integral zero or even negative whereas molecularity is always an integer excepting zero.

iii)Order may change with change in experimental condition while molecularity can't.