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

Jul 6, 2017

Ultimately, I would say it is ionic (and about as ionic as in $\text{CsCl}$).

If you consider the electronegativity difference:

$E {N}_{F} - E {N}_{A l} = 4.0 - 1.61 = 2.4$,

which suggests the bond is ionic, since the usual cutoff is $\Delta E N = 2.0$. 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 $\Delta E N$ 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%)

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

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

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