Correct Answer - In `BCl_(3)`, the central `B` atom has six electrons in the valence shell.It is therefore, an electron deficient molecule and needs two more electrons to complete its octet.It other words, `BCl_(3)` acts as a Lewis acid `NH_(3)`, on the other hand, has a lone pair of electrons which it can donate easily.Therefore, `NH_(3)` acts as a Lewis base.The Lewis acid `(BCl_(3))` and the Lewis base `(NH_(3))` combine together to form an adduct as shown below:
In `AlCl_(3),Al` has six electrons in the valence shell.Therefore, it is an electron deficient molecule and needs two more electrons to complete its octet.
Chlorine, on other hand, has three lone pair of electrones.Therefore to complete its octet, the central `Al` atom of one molecule accepts a lone pair of electrons from `Cl` atom of the other molecule forming a dimeric structure as shown above.
