Question

Describe the change in hybridisation (if any) of the Al atom in the following reaction.

\[\ce{AlCl_3 + Cl- -> AlCl_4-}\]

Answer 1

Electronic configuration of ₁₃Al = 1s² 2s² 2p⁶ 3s¹ 3p_x¹3p_y¹
(excited state)

Hence, hybridisation will be SP²
In AlCl^–₄, the empty 3p_z orbital is also involved. So, the hybridisation is sp³ and the shape is tetrahedral.

Answer 2

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 3p_z orbital also gets involved and the hybridization changes from sp² to sp³. As a result, the shape gets changed to tetrahedral.