An elementary reaction is defined as

• A. A series of steps that sum to the overall reaction.
• B. An intermediate's transformation.
• C. The overall rate law.
• D. A single molecular event in a proposed mechanism.

Answer: (D)

An elementary reaction is a single molecular event in a proposed mechanism. This means it represents one collision or one molecular transformation that occurs in one step, with a rate law tied directly to its molecularity (unimolecular, bimolecular, etc.). The whole sequence of steps that make up a reaction is a mechanism, not an elementary step, and the overall rate law of the reaction is not necessarily the same as the rate law of any single step unless that step is the rate-determining one. So describing a single, indivisible event within a mechanism best captures what an elementary reaction is.