The formation of AlCl3 can be explained as :
Electronic configuration of 13 Al = 1s2 2s2 2p6 3s2 3p1
Electronic configuration of 17 Cl = 1s2 2s2 2p6 3s2 3p5
In excited state, one electron in Al gets promoted to p-orbital. It can be shown as:
Thus, it has three unpaired electrons. Chlorine needs one electron to complete its octet. Three Cl atoms hybridise with Al to undergo sp2 hybridisation to give a trigonal planar arrangement in AlCl3. But in the formation of AlCl-4 the empty 3pz orbital will also be involved in the hybridisation and thus the hybridization changes from sp2 to sp3. As a result, the shape gets changed to tetrahedral.