Question

Why alcl3 is largely covalent while aluminium fluoride is largely ionic?

Answer 1

It's all to do with the differences in electronegativity.

Aluminium = 1.61 Chlorine = 3.16 Fluorine = 3.98 (most electronegative element there is)

So the difference tells you which atom has most of the electron density. Fluorine (being very electronegative) will "steal" a lot of electron density from aluminium making the bonds more ionic in nature than covalent.

Al- F difference = 2.37 Al-Cl difference = 1.55

Compare this to a well know ionic compound (NaCl = difference of 2.23) Based purely on electronegativity AlF3 is more ionic in character than NaCl.

But AlCl3 is covalent due to the low difference in electronegativity.

Hope this helps :)