Question

A group 13 element 'X' reacts with chlorine gas to produce a compound $XCl _{3} . XCl _{3}$ is electron deficient and easily reacts with $NH _{3}$ to form $Cl _{3} X \leftarrow NH _{3}$ adduct; however, $XCl _{3}$ does not dimerise. $X$ is

• A. B
• B. Al
• C. Ga
• D. In

Answer 1

Answer: (A)

B

Answer 2

The shape of the $BCl _{3}$ molecule is a planar triangle with bond angles of $120^{\circ} .$ This is predicted by VSEPR theory as the most stable shape for three outer electron pairs round $B$. The valence bond theory also predicts a planar triangle with hybridisation of one $s$ and two $p$ orbitals used for bonding. However, the $B$ atom only has six electrons in its outer shell and this is termed electron deficient. The empty $2 p_{z}$ orbital on the boron atom in $BCl _{3}$ can also be filled by a lone pair of electrons from donor molecule such as $NH _{3} . BCl _{3}$ does not dimerise.