In case of chemical reactions i.e. reactions involving a number of atoms, ions, molecules of the reactants in the balanced equation, the chances for all the atoms, ions or molecules of the reactants to come together and collide are very rare. Hence in such cases, the reactions are supposed to take place in a number of steps.
The slowest step is the rate determining step.
The number of atoms, ions or molecules taking place in the slowest step i.e. the rate determining step is called the molecularity of the reaction of complex reactions.
Hence, the molecularity of the reaction must always be a whole number whereas the order of a reaction can be fractional.
