An ion pair is an associated cation and anion. An isolated ion pair lacks the extended neighbourhood of an ionic crystal. In a chemical solution, ion pairs can coexist with more widely separated ions; their abundance depends on conditions. A neutral gas-phase sodium chloride molecule can be treated as a discrete entity, with its own quantum mechanics and dissociation products. That use of molecule does not describe the constituents of ordinary solid sodium chloride.[ref]
The Feynman Lectures on Physics, Volume II by Richard Feynman and others. §8–3, distinction between vaporized NaCl pairs and separated gaseous ions
[ref]Oppositely charged ions remaining in direct contact without an intervening complete solvent shell. Their relative positions can bias substitution stereochemistry even when bond cleavage precedes nucleophile attack.
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