Question

The molecule having smallest bond angle is:
(A) $NC{l}_3$
(B) $AsC{l}_3$
(C) $SbC{l}_3$
(D) $PC{l}_3$

Answer 1

Hint: Bond angle depends upon the electronegativity of the central atom. The decrease in electronegativity of an atom decreases the bond angle of its bond with other atoms.

Complete step by step solution:
Let us first learn about the concepts of electronegativity and bond angle.
Electronegativity- It is the tendency of an atom to attract a pair of electrons towards itself. Fluorine is the most electronegative metal. Electronegativity trend across the group- N, As, P and Sb are the elements of group XV. The electronegativity decreases down the group as we move from top to bottom. This is because the energy levels in an atom increases down the group. Hence, the electrons in the outer shells are loose and nucleus cannot attract them. Thus, due to the increase in distance between the nucleus and the electrons of the atom, electronegativity decreases. Hence, from N to Sb, the electronegativity tends to decrease.
-Bond angle- The angle between the two bonds emerging from the same atom which are covalent bonds is known as bond angle. The lone pair of electrons always tend to repel the bonded pair of electrons. Thus, the bonds move slightly inside which decreases the bond angle. This proves the fact that when electronegativity decreases, bond angle too decreases. Thus, as above illustration the bond angle trend for the XV group elements is as follows;
$$NC{l}_3>PC{l}_3>AsC{l}_3>SbC{l}_3$$
Hence, the smallest bond angle is present in $SbC{l}_3$.

Therefore, option (C) is correct.

Note: The bond angle, electronegativity and atomic radius trends are inter-related to each other. Do not get confused within those respective trends.