A molecular entity is an individually identifiable chemical unit, such as an atom, molecule, ion or molecular complex, distinguished by its composition and structure. The term is broader than molecule: a single sodium ion is a molecular entity even though it is not a neutral assembly of several atoms.[ref]
One methane molecule, , is a molecular entity; a collection of chemically identical methane molecules is a chemical species. This distinction helps separate the individual objects in a reaction mechanism from the macroscopic amount of substance used in laboratory bookkeeping.[ref]
IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book) by International Union of Pure and Applied Chemistry. M03986, note distinguishing a single methane molecule from an amount of a chemical species
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