Molecularity is

• A. The energy barrier of an elementary step.
• B. The number of steps in the mechanism.
• C. The number of molecules in the rate-determining step.
• D. The number of reactant particles involved in a proposed reaction mechanism.

Answer: (D)

Molecularity is the number of reactant particles involved in a single elementary step of a reaction mechanism. It’s defined for elementary steps, not for the overall reaction, and it links directly to the form of the rate law for that step. A unimolecular step involves one molecule transforming or rearranging, giving a first-order dependence on that species; a bimolecular step involves two particles colliding, giving a rate proportional to the product of their concentrations (or to the square if the two reactants are the same); a termolecular step involves three particles colliding, though such steps are rare due to the low probability of three-body encounters. The correct statement reflects this idea: the number of reactant particles involved in a proposed reaction mechanism.

The other ideas describe different concepts: the energy barrier is the activation energy of a step, not its particle count; the number of steps in the mechanism refers to mechanism complexity, not molecularity; and the number of molecules in the rate-determining step relates to that specific step’s importance, but molecularity is specifically about how many reactant particles participate in an elementary step.