`(p pi-p pi)` back bonding occurs in the halides of boron but not in those of aluminium. Explain.

Explanation

The tendency of the central atom (A) in `AX_(3)` molecules `(A = B` or `Al, X = ` halogens) to form `(p pi-p pi)` back bonding depends on the size of central atom (A). Smaller is the size of the central atom, greater is the tendency to form `(p pi-p pi)` back bonding. Since boron (B) atom is smaller in size than aluminium `(Al)` atom, `BX_(3)` molecules have `(p pi-p pi)` back bonding while `AlX_(#)` molecules do not have this type of bonding.