Term: molecular entity
https://doi.org/10.1351/goldbook.M03986

Definition:
Any constitutionally or isotopically distinct atom, molecule, ion, ion pair, radical, radical ion, complex, conformer etc., identifiable as a separately distinguishable entity. Molecular entity is used in this Compendium as a general term for singular entities, irrespective of their nature, while chemical species stands for sets or ensembles of molecular entities. Note that the name of a compound may refer to the respective molecular entity or to the chemical species, e.g. methane, may mean a single molecule of CH4 (molecular entity) or a molar amount, specified or not (chemical species), participating in a reaction. The degree of precision necessary to describe a molecular entity depends on the context. For example 'hydrogen molecule' is an adequate definition of a certain molecular entity for some purposes, whereas for others it is necessary to distinguish the electronic state and/or vibrational state and/or nuclear spin, etc. of the hydrogen molecule.

Related Terms:
1) ion pair (http://doi.org/10.1351/goldbook.I03231).
2) radical ion (http://doi.org/10.1351/goldbook.R05073).
3) conformer (http://doi.org/10.1351/goldbook.C01262).
4) precision (http://doi.org/10.1351/goldbook.P04799).
5) electronic state (http://doi.org/10.1351/goldbook.ET07026).
6) chemical species (http://doi.org/10.1351/goldbook.CT01038).

Source: PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1142 (https://doi.org/10.1351/pac199466051077)

Citation: 'molecular entity' 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.M03986

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