Molecular entity

In chemistry and physics, a molecular entity, or chemical entity, is "any constitutionally or isotopically distinct atom, 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 a chemical reaction.^{[citation needed]}

Chemical species is the macroscopic equivalent of molecular entity and refers to sets or ensembles of molecular entities.^{[citation needed]}

According to IUPAC, "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."^{[citation needed]}

• Amount of substance - a quantity proportional to the number of entities in a substance
• Mole (unit) - an aggregate of entities given by the Avogadro number
• New chemical entity
• Chemical Entities of Biological Interest
• Particle

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