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Sodium chloride in the gas forms $\ce{NaCl}$ monomer and $\ce{Na2Cl2}$ dimer. 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 the following 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.

Regarding the "electrically neutral entity" aspect of the definition, I understand that it means discounting any partial charges. Sodium chloride has no partial charges, only formal charges, and they balance, so it meets the definition. In the case of e.g. $\ce{H2O}$ , which has partial charges on hydrogen and oxygen then whether or not the partial charges cancel out, they'd say that there would be an overall neutral charge on $\ce{H2O}$ because only formal charge is counted. And $\ce{H2O}$ is of course considered by all to be a molecule and thus considered an electrically neutral entity. So Sodium Chloride meets the "electrically neutral" part of the definition.

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, so it's not an ion, it's merely composed of ions, but since it has an overall neutral charge, it's a molecule. So, electrically neutral entity is meant to refer to overall charge as neutral, rather than each individual atom.

And Na+Cl- would be electrically neutral since it's an ionic compound, compounds are electrically neutral, so it's not an ion / polyatomic ion, it's ionic in the sense of composed of ions.

So that covers the electrically neutral aspect of the definition.

So then I wonder, does $\ce{Na+Cl-}$ meet the other 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." ?

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Buck Thorn
    Oct 10, 2022 at 16:17

2 Answers 2

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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.

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  • $\begingroup$ 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, 2022 at 18:56
  • $\begingroup$ I wouldcsay "molecule" is not specific, so would prefer "ion pair". $\endgroup$ Jan 8, 2022 at 19:42
  • $\begingroup$ Would even a solid sodium chloride lattice, fit the definition? $\endgroup$
    – barlop
    Jan 8, 2022 at 19:43
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Looking at the definition

https://goldbook.iupac.org/terms/view/M04002

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.

Looking at "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."

An ionic monomer and ionic dimer or even a large ionic crystal, would fit that part of the definition.

Speaking to a chemistry guy, they mentioned to me that they think that part of the definition is just to rule out very weak forces like van der waal forces. So, the ionic bonds within an NaCl crystal or NaCl dimer or monomer, would meet that part of the defintion. (I mention NaCl crystal for the sake of understanding the principle. Of course NaCl in gas form won't form anything big).

So now that makes a smaller question.

Does NaCl in gaseous state, meet this part of the definition

An electrically neutral entity consisting of more than one atom (n>1).

There's a very significant word that I overlooked when asking the question. The word "entity".

That's where an NaCl crystal gets ruled out. And even an NaCl dimer gets ruled out. Since a crystal and I think a dimer too, has an entity NaCl. But wouldn't be considered to be an entity itself.

The word entity is a bit ambiguous 'cos it can mean "molecular entity", or it can include the term "formula unit"(which is a physical object pointed to by an empirical formula).

An ionic monomer is certainty meeting the defintion since it's an entity, even, a molecular entity.. As it's an "ion pair"

Molecular Entity https://goldbook.iupac.org/terms/view/M03986

Any constitutionally or isotopically distinct atom, molecule, ion, ion pair, radical, radical ion, complex, conformer etc., identifiable as a separately distinguishable entity. Molecular entity is used in this Compendium as a general term for singular entities, irrespective of their nature, while chemical species stands for sets or ensembles of molecular entities...

As for a question in comment, what about an ionic NaCl crystal. (And maybe one could consider, if an ionic NaCl dimer is too).

In the case of an ionic crystal. I'm not entirely sure.

And also with the term entity, do they mean just molecular entity. In their definition of "Chemical substance" https://goldbook.iupac.org/terms/view/C01039 they mention "formula unit" as an entity.

I did hear a suggestion that an ionic crystal is a species, but an expert I spoke to said it isn't. It might be possible to say that there is a species NaCl present in the crystal. Another thing I heard which I don't think is correct, is that if it's a species it's not an entity, I don't think that's correct, 'cos that which is a molecular entity can be a species and vice versa, for n=1. So it comes down to is it an entity. If it has loads of atoms then it'd have an empirical formula, and the thing referred to by an empirical formula is known as a formula unit. A formula unit meets a definition of entity though not molecular entity.

If a crystal is considered to be a molecule then a dimer would be.

If it isn't, then maybe a dimer isn't either.

I will do some more looking into things re a crystal. And, likewise, for a dimer. (The situation for a dimer should be related in principle to the situation for a crystal).

But an ionic monomer is a molecule, it meets the "entity" definition along with the rest of the definition.

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