Question

Explain briefly why boron trichloride is a gas and aluminium trichloride is a dimer solid.

Answer 1

Hint : Both have a trigonal planar structure. Boron has no vacant d orbitals but aluminium does. Boron forms $p\pi - p\pi$ intermolecular bonding whereas aluminium forms $p\pi - p\pi$ bonding with other aluminium chloride molecules.

Complete Step By Step Answer:
Boron trichloride is a colorless gas with the molecular formula $BC{l_3}$ . It is a reagent in organic synthesis. It is highly reactive toward the water. $BC{l_3}$ has a very pungent odor. The structure of $BC{l_3}$ is trigonal planar. It has a bond length of $175pm$ . In the structure the central atom boron is surrounded by three chlorine atoms, giving rise to the trigonal planar structure. Aluminium trichloride $AlC{l_3}$ also known as aluminium chloride is a white color solid. It also has a pungent odor. The structure of $AlC{l_3}$ is also trigonal planar, where the central atom Al is surrounded by three chlorine atoms.
The structure of $BC{l_3}$ is:

The structure of $AlC{l_3}$ is:

The valence shell configuration of Boron is $1{s^2}2{s^2}2{p^1}$ . It can be seen that boron does not have any vacant 2d-orbitals and so it cannot extend its coordination number beyond $3$ . The valence shell configuration of Al is $1{s^2}2{s^2}2{p^6}3{s^2}3{p^1}$ has vacant 3d orbitals. So it can extend its coordination number beyond $3$ . Also both $BC{l_3}$ and $AlC{l_3}$ are electron-deficient species. $BC{l_3}$ has intermolecular $\pi - p\pi$ bonding to compensate for this electron deficiency. And in case $AlC{l_3}$ it shows $p\pi - p\pi$ bonding with another $AlC{l_3}$ molecule to compensate for the electron deficiency. $AlC{l_3}$ dimerizes to attain the octet by forming a dative bond between Cl and Al atoms. Therefore because of the above reasons, boron trichloride is a gas, and aluminium trichloride is a dimer solid.

Note :
boron does not have any vacant shell as aluminium has. And aluminium has only $6$ electrons in its valence shell so to complete its octet state Al accepts $2$ more electrons from another $AlCl_3$ molecule and it forms a coordinate bond with Cl thus it exists as a dimer.