= Molecular entity
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.<IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book) by International Union of Pure and Applied Chemistry>[M03986, molecular entity]{ref}
One <methane> <molecule>, $\mathrm{CH_4}$, 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.<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]{ref}
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