= Ion pair
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>.<The Feynman Lectures on Physics, Volume II by Richard Feynman and others>[§8–3, distinction between vaporized NaCl pairs and separated gaseous ions]{ref}<Conductivity of Solutions by Harvard Natural Sciences Lecture Demonstrations>{ref}
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