The valence orbital picture of aluminium in the ground state can be represented as:
The orbital picture of aluminium in the excited state can be represented as:
Hence, it undergoes sp2 hybridization to give a trigonal planar arrangement (in AlCl3). To form AlCl4 – , the empty 3pz orbital also gets involved and the hybridization changes from sp2 to sp3 . As a result, the shape gets changed to tetrahedral.