Molecularity counts the reacting entities in one elementary reaction: one for a unimolecular step, two for a bimolecular step. An entity can be a molecule, ion, or chemical atom. Solvent entities involved only through solvation are excluded. Molecularity describes a microscopic event; order of reaction describes the concentration dependence of a rate law. Assigning molecularity to a net multi-step chemical equation is therefore inappropriate.[ref]
Glossary of Terms Used in Physical Organic Chemistry (IUPAC Recommendations 1994): M entries by International Union of Pure and Applied Chemistry. Glossary of Terms Used in Physical Organic Chemistry, IUPAC Recommendations 1994, entry “molecularity”: elementary event and exclusion of solvation medium
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Molecularity refers to the number of reactant molecules that participate in an elementary reaction step. It is an important concept in reaction kinetics, as it helps to characterize the mechanism of chemical reactions. There are three main types of molecularity based on the number of molecules involved: 1. **Unimolecular**: Involves a single molecule undergoing a reaction. For example, the decomposition of a compound into simpler products is a unimolecular reaction.