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

Answer

Both `BCl_(3)` and `AlCl_(3)` are electron deficient molecules having six electrons in the valence shell of their respective central atoms.To complete their octets, the central atom in each case can accept a pair of electrons from the chlorine atom of another molecule forming dimeric structures. However, because of small size of `B`,it cannot accommodate four big sized `Cl` atoms around it.Therefore, `BCl_(3)` prefers to exist as a monomeric planar molecule in which `B` atom is `sp^(2)-` hybridized.
On the other hand, `Al` because of its bigger size can easily accommodate four `Cl` atoms around it.As a result, `AlCl_(3)` exists as a dimer, since the covalency of `Al` has increased to `4`.Therefore, `Al` is `sp^(3)`-hybridized and the four `Cl` atoms are held tetrahedrally around it.