Inchemistry andphysics, amolecular entity, orchemical entity, is "any constitutionally orisotopically distinctatom,molecule,ion, ion pair,radical, radical ion,complex,conformer, etc., identifiable as a separately distinguishable entity".[1] A molecular entity is any singular entity, irrespective of its nature, used to concisely express any type of chemical particle that can exemplify some process: for example, atoms, molecules, ions, etc. can all undergo achemical reaction.
Chemical species is the macroscopic equivalent of molecular entity and refers to sets or ensembles of molecular entities.
According toIUPAC, "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 theelectronic state and/orvibrational state and/ornuclear spin, etc. of the hydrogen molecule."
