1. The number of molecules that participate as reactants in an elementary chemical reaction; the sum of the exponents in the rate law for a single elementary step.
Usage: chemistry; plural form of molecularity; used in kinetics and reaction mechanism studies

Examples

• The molecularities of different elementary steps in a reaction mechanism can vary.
• A unimolecular reaction has a molecularity of one, while a bimolecular reaction has a molecularity of two.
• Chemists determine molecularities by analyzing experimental rate laws and proposing reaction mechanisms.
• The sum of the molecularities of all elementary steps equals the overall order of the reaction.
• Understanding molecularities helps predict how fast chemical reactions will proceed.