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

1 Answer
Jul 6, 2017

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


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"#.