For a complex reaction, order of overall reaction molecularity of slowest step

As rate of overall reaction depends upon total number of molecules involved in slowest step of the reaction. Hence, molecularity of the slowest step is equal to order of overall
Since, the completion of any chemical reaction is not reaction reactants. possible in the absence of reactants hence, slowest step of any chemical reaction must contain at least one reactant. Thus, molecularity of the slowest step is never zero or non-integer.