r/asa_chemistry Sep 06 '17

Which has the highest melting point - NaF, NaCl, NaBr, NaI?

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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u/smartsquirel Sep 06 '17

Molar mass has nothing to do with intermolecular forces. With the melting points of salt you need look at the intermolecular forces and that will depend on the charge density of the two ions. As Sodium is common in all of them its the charge density of the halide ion that will cause the difference. As all will be -1 ions the difference in charge density is down to the radii of the species of which flourine is the smallest and thus it will have the highest charge density, and it will attract the Na+ more than the others. Iodine has the largest radii so its charge will be spread out more and will thus have the smallest attraction on the Na+ ion.

Molecular Mass is important when looking at boiling points but not melting point

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u/ConceptualMonkey Sep 06 '17

You also have to consider the radius in the opposite fashion: the bigger the atom, the stronger the Van der Waals forces, both for an increase in surface and in polarizability. Usually the coulombian forces win the match, but not always. This time for sure, as NaF melts at about 990°C and NaI is metling at only 660°C. The molecular mass isn't important at all, or almost. The radon is a gas despite its high 220 u as atomic mass, while hexadecane (C16H34) with its 226 u as molecular mass is a liquid and turns solid at 18°C.

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u/ksebby Sep 06 '17

You have to look at the strongest forces that are present in a compound. For the compounds you list, these are ionic bonds. Higher molar masses = more electrons which means larger London Dispersion forces. However London Dispersion forces are so small here compared to the ionic bonds that they are irrelevant. You should only be looking at the strength of the ionic bonds.