why is BCl3 planar but AlCl3 tetrahedral?

The Lewis structure of BCl3 is given below. BCl3 has 3 B-Cl single bonds and no lone pair around B, hence 6 valence electrons around B. As per VSEPR notation, this molecule takes AX3 notation. The geometry of BCl3 is trigonal planar. It is a flat molecule with all three bond angles 120 degree C. The three B-Cl bonds are evenly arranged in the same plane. The polarities of the molecules cancel out each other with no dipole moment.

The Lewis structure of AlCl3 is given below. AlCl3 has 3 Al-Cl single bonds and no lone pair around Al, hence 6 valence electrons around Al. As per VSEPR notation, this molecule takes AX3 notation. The geometry of AlCl3 is trigonal planar. It is a flat molecule with all three bond angles 120 degree C. The three Al-Cl bonds are evenly arranged in the same plane. The polarities of the molecules cancel out each other with no dipole moment.