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As you can see from the graph below, as we go down the blue fluoride-alkali metal series (alkali metal ion is varied from Lithium to Rubidium, which is represented by an increase in ionic mass on the graph), the boiling point increases then decreases. This is contrary to my expectation that the boiling point should continuously decrease due to an increase in ionic radius. The charge is obviously not a factor because it is consistent throughout. What could be the factor that causes this deviation?

Thanks!

The graph

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  • $\begingroup$ You also have to account for the geometry, which is highly influenced by charge (which is not a factor here) and radius (which is a big factor here). $\endgroup$ – DHMO Oct 16 '16 at 8:42
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Polarizability of the fluoride ion by the alkali metal ion decreases going down the group, this decreases the covalent character in each ionic bond in question.

See the related questions and answers here:

Does covalent character increase or decrease bond strength?

Melting points of alkali metal halides

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  • $\begingroup$ Does this mean that since covalent character decreases down the group, sodium fluoride would have a higher b.p. than lithium fluoride, and potassium flouride has even less covalent character regains its ionic properties, thus resuming the 'normal' trend? $\endgroup$ – Brian Lin Oct 18 '16 at 8:39

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