Is KF the most ionic compound?

I saw somewhere (can't recall where) that KF is the most ionic compound. I expected CsF. Does the greater polarizability of Cs allow it to more easily form covalent bonds compared to K? Does this overcome the fact that K being in n = 4 should bond better with F in n = 2?




Yes, this is a subtle thing.


Using the [Pauling electronegativities](http://en.wikipedia.org/wiki/Electronegativity), one would expect CsF to have the larger electronegativity difference (3.2). So in principal, it should be "more ionic."


Unfortunately, an ionic bond requires separating charge, so $\ce{Cs+F-}$. The problem is that $\ce{Cs+}$ is much larger than $\ce{K+}$ and so the dipole moment for a fully ionic $\ce{Cs+F-}$ would be much larger.


We find that the charge isn't fully separated, likely because of the need to stabilize the large dipole moment.


So some people plot a curve of "% ionic character" as determined by the ***actual*** dipole moment vs. the expected dipole moment (i.e, full charge separation). We find that this curve reaches a limit ~75-85% ionic character.


IIRC, I think on that basis LiF is more Ionic than KF.


![enter image description here](https://i.stack.imgur.com/xAl9g.png)


For more, I really like Bruce Robinson's lecture notes:
[http://courses.washington.edu/bhrchem/c152/Lec23.pdf](https://web.archive.org/web/20160406074454/http://courses.washington.edu/bhrchem/c152/Lec23.pdf) via [the Internet Archive](https://web.archive.org/web/20160406074454/http://courses.washington.edu/bhrchem/c152/Lec23.pdf)


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