Sodium Chloride in gas forms an $\ce{NaCl}$ monomer and $\ce{Na2Cl2}$ dimer. link So, no lattice, and those formulae show the number of atoms in the ionic compound, so not just an empirical formula that shows a ratio. So in that sense those are like molecular formulae.

Does Sodium Chloride in gas state meet this definition of molecule in the IUPAC gold book

"An electrically neutral entity consisting of more than one atom (n>1). Rigorously, a molecule, in which n>1 must correspond to a depression on the potential energy surface that is deep enough to confine at least one vibrational state."

I understand that the "electrically neutral entity" part of the definition might be problematic, even sodium chloride aside. e.g. $\ce{H2O}$ has partial charges on hydrogen and oxygen that i'm sure wouldn't cancel out so there wouldn't be an overall neutral charge on $\ce{H2O}$. And $\ce{H2O}$ is of course a molecule.

And I understand that there is a concept of a zwitterion, which is composed of atoms that have a mix of charges but overall neutral charge and that's a molecule. So, electrically neutral entity is meant to refer to overall charge as neutral, rather than each individual atom.

I'm sure that Na+Cl- would be electrically neutral..

So then I wonder, does Na+Cl- meet the other part of the definition.

The vibrational state part of the definition.. i.e. the part that states "a molecule, in which n>1 must correspond to a depression on the potential energy surface that is deep enough to confine at least one vibrational state.""

  • 1
    $\begingroup$ Does Na+Cl-(g) ion pair meet the molecule definition better than 4mm big salt crystal? $\endgroup$
    – Poutnik
    Jan 8 at 13:49
  • $\begingroup$ On the other hand, a part of the $\ce{NaCl}$ must be decomposed into atoms in the flame, because the yellow flame of sodium is due to atomic sodium, and not to sodium ions ! $\endgroup$
    – Maurice
    Jan 8 at 14:57
  • $\begingroup$ @Maurice How about a solid , not heated, sodium chloride lattice, wouldn't that meet the definition of molecule included here as much as an ionic monomer or ionic dimer?! $\endgroup$
    – barlop
    Jan 8 at 19:42
  • $\begingroup$ @Barlop I am not talking about molecules or ions. I am just speaking about the probably small proportion of $\ce{NaCl}$ being decomposed into atoms in the flame. $\endgroup$
    – Maurice
    Jan 8 at 19:54
  • $\begingroup$ @Poutnik I think it doesn't meet the definition because although it matches the vibration state part, a crystal or 3d polymer is not considered to be a "chemical entity" $\endgroup$
    – barlop
    Jan 16 at 20:30

1 Answer 1


Certainly the electrostatic attraction between the ions can hold the $\ce{Na+Cl-}$ entity together when it vibrates, so yes, this does satisfy the vibrational stability definition of a molecule.

So we have a molecule. Given the ionic bonding I would call it an ion pair, which is a specific type of molecule.

Such ion-pair molecules are not as far-fetched as gaseous sodium chloride. We see them, in solvated form, for instance when magnesium sulfate is dissolved in water.

  • $\begingroup$ Thanks, though I notice on this question chemistry.stackexchange.com/questions/162004/… you commented "I would not really call the gas-phase entities "molecules", or at least that is not the most specific term. The monomer is more lime an ion pair, and the dimer would be more like a larger cluster of ions. – Oscar Lanzi " Whereas in this answer to this question here, you seem to take the view that not only they meet that definition of molecule, but you take the view that you consider it to be a molecule. $\endgroup$
    – barlop
    Jan 8 at 18:56
  • $\begingroup$ I wouldcsay "molecule" is not specific, so would prefer "ion pair". $\endgroup$ Jan 8 at 19:42
  • $\begingroup$ Would even a solid sodium chloride lattice, fit the definition? $\endgroup$
    – barlop
    Jan 8 at 19:43

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