- Molecularity is defined as the number of reactant species (atoms, ions, or molecules) that collide simultaneously in an elementary reaction step. - By definition, molecularity must be a whole number (1, 2, or rarely 3). It cannot be zero, negative, or fractional because it represents a count of colliding entities in a single step. - Reaction order, on the other hand, is determined experimentally from the rate law and can be a whole number, zero, or a fraction. - For elementary reactions, the order of reaction is equal to its molecularity. For complex reactions (multi-step reactions), the overall order is determined by the slowest step (rate-determining step), and the concept of molecularity applies only to each individual elementary step, not the overall reaction.