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Is a single molecule of sodium chloride (say) or a cluster of molecules of NaCl unstable, although macroscopically NaCl is in fact, stable? How can I reason this based on Earnshaw's theorem?

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    $\begingroup$ Earnshaw's theorem is valid for classical point particles. However, $\text{Na}^+$ and $\text{Cl}^-$ ions are diffuse quantum objects, so Earnshaw's theorem is not necessarily applicable. However, you might be able to apply it to a gas of $\text{Na}^+$ and $\text{Cl}^-$ ions to qualitatively suggest that it's unstable, and thus should collapse (ie, condense) to form the ionic solid. In the gas phase, the classical point-particle approximation is probably more valid than in the solid phase. $\endgroup$ May 7, 2014 at 19:04

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There is a related question in Is NaCl vapour plasma?. According to my answer to that question this paper states that sodium chloride vapour contains NaCl molecules, dimers and indeed polymers, so these are all stable. However when I've just tried the link I got an error from the AIP server. Some Googling found a similar and possibly the same paper here.

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