`BCl_(3)` is trigonal planr while `AlCl_(3)` is tetrahedral in dimeric state. Explain.

Explanation

Boron atom in `BCl_(3)` is `sp^(2)` hybridised and its hybrid and its hybrid orbitals are involved in the bond formation with the electrons of the chloride atoms. Due to its small size, boron cannot accommodate another atom of chlorine althoug `BCl_(2)` is a Lewis acid. However, the size of aluminium atom is comparatively large and it can accept and electron pair from the becomes tetrahedral in the dimeric form `(Al_(2)Cl_(6))`.