• # question_answer 68)   Draw the structures of $BC{{l}_{3}},N{{H}_{3}}$ and$AlC{{l}_{3}}$ (dimer).

Structure of$\mathbf{BC}{{\mathbf{l}}_{\mathbf{3}}}$: In $BC{{l}_{3}}$ molecule, the boron atom is in$s{{p}^{2}}$ -hybrid state. Thus, it has planar triangular structure. Structure of$\mathbf{N}{{\mathbf{H}}_{\mathbf{3}}}$: In $N{{H}_{3}}$ molecule, the nitrogen atom is in $s{{p}^{3}}$-hybrid state. Thus, it has tetrahedral structure having one hybrid orbital with a lone pair of electrons. The actual shape of the molecule is pyramidal. The bond angle is$107.5{}^\circ$. This is less than the expected angle of${{109}^{{}^\circ }}28$? due to repulsion between lone pair present on nitrogen atom and bonded      pairs of electrons.                       Structure of $AlC{{l}_{3}}$ (dimer):                 Al atom undergoes $s{{p}^{2}}$ -hybridisation for planar triangular structure. However, to achieve more stability the vacant    p-orbital of Al-atom forms a coordinate bond with one of the chlorine atom of the other molecule forming a dimer.