MolecularityOrder It represents the number of the reactant molecules taking part in the elementary reaction. It represents the sum of the exponents to which the concentration term in the rate law are raised. It is theoretical property and indicates the number of reactant molecules involved in each act leading to the reaction.It is an experimental property and indicates the dependence of observed reaction rate on the concentration of the reactants. It is always an integer and can never be fraction or zero.It may be an integer, fraction or zero. It is the property of only elementary reaction and has no meaning for complex reactions.It is property of both elementary and complex reactions.