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I've been trying to understand why NaF has the highest melting point among all of the given solids. I thought it would be NaI as it will have the biggest molar mass among them which would mean it has the strongest intermolecular forces. Is it electronegativity difference or molar mass that you look at when analysing intermolecular forces? Am I missing something or misunderstanding something? Thank you very much.

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  • $\begingroup$ NaF has better packed crystal structure, which is harder to break. $\endgroup$ – ABC Sep 6 '17 at 9:56
  • $\begingroup$ Another point. There is no such thing as a NaCl "molecule" in solid NaCl. All the ionic charges interact. Lookup the "Madelung Constant." $\endgroup$ – MaxW Sep 6 '17 at 14:00
  • $\begingroup$ A simple way of comparing the M.P.s of ionic solids is through calculating their lattice energies. $\endgroup$ – Tan Yong Boon Sep 7 '17 at 3:39
  • $\begingroup$ Evidently, NaF has the highest L.E. anong the four as fluoride has the smallest ionic radius among the four anions. $\endgroup$ – Tan Yong Boon Sep 7 '17 at 3:40
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All those are ionic compounds, so it is just Coulomb forces that hold them together. Two charges are attracted with the same force regardless of their mass. The distance, however, does matter a great deal. And the distance depends on the ionic radii, and I'm sure you know how those change down the group.

Things could have turned out otherwise if the compounds had different crystal structures. Luckily for us, this is not the case.

So if you are confronted with an unknown compound, you first look at the electronegativity difference, and if it tells you the compound is ionic, you use the same line of thought as I just did. Otherwise it is probably a molecular compound held together with dispersion forces, and they tend to grow with molar mass. Oh, and there might be a covalent solid...

See, chemistry is complicated, and it is not always possible to get by with a few simple rules of thumb. But at least I think I've answered your original question in the narrow sense, so let's leave it at that.

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  • The point that is crucial here is that ionic compounds are held together in a crystal lattice structure .
  • ionic structure of table salt

  • What you have misunderstood is that there are no intermolecular forces between these ions , as number one the forces are not between molecules but ions , and number two if you break the bonds between an Na and a Cl ion , you have effectively broken an intramolecular bond resulting in a sodium ion and a chlorine ion .

  • Hence mass has no effect on the strength of the bond but factors affecting coulombs law will .
  • The reason why NaF would have the highest melting point is because , if you look at the electronegativity values fluorine has the highest hence it have the greatest pull or attraction towards the electrons in the ionic bond , resulting in a really strong sodium positive ion and a strong negative fluorine ion ,this thereby results in a high melting point . You can also look up lattice enthalpy values to be sure .
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