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?

  • $\begingroup$ See here for previous answers on the subject. $\endgroup$ Commented Jul 7, 2014 at 13:19

1 Answer 1


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.

Also this paper here (credit to orthocresol for finding that). The abstract mentions "the equilibrium structures of the monomer (NaCl) and the dimer $\ce{Na2Cl2}$"

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    $\begingroup$ 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 $\rm{1\ atm}$. $\endgroup$ Commented Jul 7, 2014 at 15:33
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    $\begingroup$ @Nicolau Saker Neto Good point, thanks for adding it. $\endgroup$
    – ron
    Commented Jul 7, 2014 at 15:50
  • $\begingroup$ @ron Does this imply that all ionic compounds in gas phase are found as monomers and dimers? $\endgroup$
    – Rishi
    Commented Jul 8, 2014 at 1:07
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    $\begingroup$ 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. $\endgroup$
    – ron
    Commented Jul 8, 2014 at 1:11
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    $\begingroup$ 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. $\endgroup$ Commented Jul 8, 2014 at 16:39

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