Question

Describe the change in hybridisation (if any) of the Al atom in the following reaction.
AlCl₃ + Cl^– ➝ AlCl^-₄

Answer 1

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 3p_z 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.