The molecularity is the number of the reacting species (atoms, molecules) in a reaction occurring in a single step. These are the species that collide in order to form a product.

Hence, the molecularity of a complete reaction can never be zero or non-integer.

For the complex reaction molecularity of the slowest step of the reaction is always equal to the order of the overall reaction.

As the number of reacting species in the slowest step is equal to the experimental value of the order of the overall reaction.