Question

Explain briefly why boron trichloride is a gas and aluminium trichloride is a dimeric solid at room temperature.

Answer 1

Borontrichloride is highly reactive as compared to aluminium trichloride
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Answer 2

The electron deficiency of B$Cl_{3}$ is compensated by forming bond between the lone pair of chlorine and the empty unhybridised p orbital of Boron atom and form p$\pi$- p$\pi$ bond. Thus, the electron deficiency of B$Cl_{3}$ is compensated by forming of p$\pi$ - p$\pi$ bonds within the molecule. So, they does not dimerise.

The electron deficiency of Al atom in $AlCl_{3}$ is compensated by forming coordinate bond between lone pair of Cl of another $AlCl_{3}$ molecule and thus the empty unhybridised p orbital of Al atom and form dimer.