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Since sodium chloride is sodium and chlorine atoms bonded as a lattice and there are no discrete molecules, doesn't that mean in gas state, sodium chloride is simply sodium and chlorine atoms separate from each other, resulting in a mixture of sodium gas and chlorine gas?

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See here for previous answers on the subject. –  Nicolau Saker Neto Jul 7 at 13:19
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No, in the gas phase sodium chloride exists as a monomer (the sodium chloride molecule) along with its dimer $\ce{Na2Cl2}$. The dimer has a roughly rectangular shape and is quite floppy with chlorines located diagonally across from each other. The dimer makes up about 27% of the mix. All of the bond lengths, etc. can be found in this thesis. Go to the end and you'll see the full paper that appeared in JACS with all of the data.

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Very interesting source. To be clear, the equilibrium proportion of ionic monomers, dimers and larger aggregates varies with certain parameters, most notably temperature and pressure; high temperatures and low pressures favour more extensive dissociation into smaller units. From a quick look at the thesis, it would seem that the 1:0.27 molar proportion of the monomer and dimer was experimentally determined at $\rm{943\ K}$ for sodium chloride (which also suggests the measurements were made at a very low pressure). The proportion might be quite different for boiling salt at $\rm{1\ atm}$. –  Nicolau Saker Neto Jul 7 at 15:33
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@Nicolau Saker Neto Good point, thanks for adding it. –  ron Jul 7 at 15:50
    
@ron Does this imply that all ionic compounds in gas phase are found as monomers and dimers? –  Oleoleoleole Jul 8 at 1:07
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I dunno, my guess would be many do, many don't. I think some you'd need to heat so hot to get them vaporized they might exist as a plasma, others might decompose - but I'm just guessing. –  ron Jul 8 at 1:11
    
Roughly, somewhere around half of the lattice enthalpy of a salt comes solely from the binding of the smallest electrically neutral agglomerate; that is, it takes about as much energy to break a macroscopic solid $\ce{NaCl}$ crystal into a gas of ion pairs as it does to break all the ion pairs and create a true plasma. Thus, it is rather unlikely that, at reasonable temperatures, an ionic gas will break down any further than the smallest possible electrically neutral aggregates. –  Nicolau Saker Neto Jul 8 at 16:39
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