Fajans' rules

[4]Thus sodium chloride (with a low positive charge (+1), a fairly large cation (~1 Å) and relatively small anion (0.2 Å) is ionic; but aluminium iodide (AlI3) (with a high positive charge (+3) and a large anion) is covalent.

Polarization will be increased by: The "size" of the charge in an ionic bond depends on the number of electrons transferred.

In this case, the aluminum ion's charge will "tug" on the electron cloud of iodine, drawing it closer to itself.

In this case, iodine is replaced by fluorine, a relatively small highly electronegative atom.

Thus, we get an ionic compound (metal bonded to a nonmetal) with a slight covalent character.