The following points are given in my textbook about order and molecularity:-
(i) Order of a reaction is an experimental quantity. It can be zero and even a fraction but molecularity cannot be zero or a non-integer. Its values are limited to 1,2 or 3.
(ii) Order is applicable to elementary as well as complex reactions whereas molecularity is applicable only for elementary reactions.For a complex reaction, molecularity has no meaning.
(iii) Order is given by the slowest step(also known as RDS) and molecularity of the slowest step is same as the order of the overall reaction.
(iv) Molecularity is equal to the sum of stoichiometric coefficients of reactants in the elementary reaction.
However, I have observed an inconsistency in the above points and would like a clarification.
Let us consider 2 cases:-
Case 1: Complex Reaction
By (iii)-order of overall reaction=molecularity of RDS. Since molecularity is 1,2 or 3, order must also be 1, 2 or 3.
Case 2:Elementary Reaction
By (iii)-order of overall reaction=molecularity of the reaction(since there is only 1 step so that must be the RDS). Since molecularity is 1,2 or 3, order must also be 1,2 or 3.
Now, combining these two cases, order =1,2 or 3. However, (i) mentions that the order can be fractional or zero. How is this possible?