Question

What kind of bond is in `"AlF"_3`?

Answer 1

Ultimately, I would say it is ionic (and about as ionic as in `"CsCl"`).

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If you consider the electronegativity difference:

`EN_F - EN_(Al) = 4.0 - 1.61 = 2.4`,

which suggests the bond is ionic, since the usual cutoff is `DeltaEN = 2.0`. But this isn't the only way to look at it.

Another way to look at it is to look at the percent ionic character, proposed empirically by Linus Pauling:

`bb(%"Ionic Character" = (1 - e^(-(DeltaEN//2)^2)) xx 100%)`,

where `DeltaEN` is the difference in the two electronegativities.

Physically, it means the extent to which electrons are hogged by one atom (100% ionic character) rather than shared between the atoms (0% ionic character).

So:

`%"Ionic Character" ("Al"-"F")`

`= (1 - e^(-(2.4//2)^2)) xx 100% = bbul(76.3%)`

Now, we could compare that to HCl (which we consider polar covalent) and CsCl (which we consider ionic):

`%"Ionic Character" ("H"-"Cl")`

`= (1 - e^(-((3.16 - 2.2)//2)^2)) xx 100% = ul(20.6%)`

(literature value: `17.7%`)

`%"Ionic Character" ("Cs"-"Cl")`

`= (1 - e^(-((3.16 - 0.79)//2)^2)) xx 100% = ul(75.4%)`

(literature value: `74.6%`)

So, yeah, the `"Al"-"F"` bond can be considered ionic, as its ionic character is in the vicinity of `"CsCl"`, while it is much more ionic than in `"HCl"`.