Why does AlCl3 forms covalent bond but AlF3 forms ionic bond?
Dear student!
Ionic bonding occurs results when there is a complete transfer of electrons from one atom to another and this is only possible if the difference in electronegativity between the atoms is large.
The electronegativity of Al is 1.61 and for Cl is 3.16 so the difference in electronegativity is 1.55 while the electronegativity of F is 3.98 and difference is 2.37.
So, as being most electronegative, the F- ion polarizes the electron clouds and forms a -ve charge center which is attracted by aluminium metal cation and thus an ionic bond is formed in AlF3 but, in case of Cl-atom the electronegativity difference is less and so there arises a covalent bond by sharing of electron clouds in AlCl3.
Actually, the Al3+ cation, being highly charged in nature (has a high charge density), is able to polarize the electron clouds of Cl to such an extent that electrons become shared between Al and Cl. The result is a covalent compound but the bond is a polar covalent bond.
I hope it is clear.