An element ‘M’ reacts with chlorine to form a compound ‘X’. The bond angle in ‘X’ is 120° . What is M ?

In a molecule, if the bond angle has 120° then the hybridization of central atom should be ${\mathrm{sp}}^2$ . Hence, the element ‘M’ must be boron.

$\mathrm{B}+3\mathrm{Cl}\to {\mathrm{BCl}}_3$