Term: molecularity
https://doi.org/10.1351/goldbook.M03989

Definition:
The number of reactant molecular entities that are involved in the 'microscopic chemical event' constituting an elementary reaction. (For reactions in solution this number is always taken to exclude molecular entities that form part of the medium and which are involved solely by virtue of their solvation of solutes.) A reaction with a molecularity of one is called 'unimolecular', one with a molecularity of two 'bimolecular' and of three 'termolecular'.

Related Terms:
1) chemical reaction (http://doi.org/10.1351/goldbook.C01033).
2) order of reaction (http://doi.org/10.1351/goldbook.O04322).
3) molecular entities (http://doi.org/10.1351/goldbook.M03986).
4) elementary reaction (http://doi.org/10.1351/goldbook.E02035).
5) medium (http://doi.org/10.1351/goldbook.M03814).
6) microscopic chemical event (http://doi.org/10.1351/goldbook.M03910).
7) solvation (http://doi.org/10.1351/goldbook.S05747).

Source: PAC, 1993, 65, 2291. 'Nomenclature of kinetic methods of analysis (IUPAC Recommendations 1993)' on page 2296 (https://doi.org/10.1351/pac199365102291)

Citation: 'molecularity' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.M03989

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