Molecularity is determined theoretically whereas order is a practical quantity. Molecularity is applicable only for elementary reactions as they are the single step reactions and the rate depends on the concentration of each molecule, whereas in case of complex reactions there are multiple reactions involved and thus molecularity holds no meaning. Molecularity is the collisions of reactant molecules and to increase the rate it requires a proper orientation at the time of collisions which is only possible in less molecules.In complex reactions order is determined by the slow step and it can also be easily determined in the elementary reactions as well.