Question

Describe the change in hybridisation (if any) of the Al atom in the following reaction.
\(AlCl_3+Cl^- \rightarrow AlCl_4^-\)

Answer 1

Given reaction:

AlCl₃ + Cl⁻ → AlCl₄⁻

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Hybridisation of Al before reaction:

In AlCl₃, the aluminium atom forms three σ bonds with chlorine atoms.

• Aluminium uses three hybrid orbitals and has an incomplete octet.
• The geometry around Al is trigonal planar.
• Hybridisation of Al = sp²

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Hybridisation of Al after reaction:

In AlCl₄⁻, the aluminium atom accepts a lone pair of electrons from the chloride ion, forming a coordinate (dative) bond.

• Aluminium now forms four σ bonds with four chloride ions.
• The geometry becomes tetrahedral.
• Hybridisation of Al = sp³

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Conclusion:

Thus, during the reaction the hybridisation of aluminium changes from sp² in AlCl₃ to sp³ in AlCl₄⁻.