According to IUPAC Gold Book[1] a molecular entity is 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 as a general term for any singular entities, irrespective of their nature, in order to concisely express any type of chemical particle that can experiment some process: for example, atoms, molecules, ions... can undergo a chemical reaction.
Chemical species is the macroscopic equivalent of molecular entities and it stands for sets or ensembles of molecular entities.
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.
Notes and references
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