Question

Assertion: The bond angle of $PB{r_3}$ is greater than that of $P{H_3}$, but bond angle of $NB{r_3}$ is less than that of $N{H_3}$.
Reason: Electronegativity of phosphorus atoms is less than that of nitrogen.
choose the correct statement:
(A) Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
(B) Both Assertion and Reason are correct but Reason is not the correct explanation for
Assertion.
(C) Assertion is correct but Reason is incorrect.
(D) Both Assertion and Reason are incorrect.

Answer 1

Hint: As we know that bond angle is the angle which is formed between three atoms or two bonds that originates from same atom in a covalent species and down the group bond angle decreases along with electronegativity which is basically the tendency of an atom to attract the electrons from shared pair.

Complete Step by step answer: We are already aware that in the periodic table, various trends are followed. Among these trends we will discuss two that are bond angle and electronegativity. Bond angle is the angle which is formed between three atoms or two bonds that originate from the same atom in a covalent species and electronegativity is basically the tendency of an atom to attract the electrons from a shared pair towards itself. And while moving down the group bond angle as well as electronegativity decreases.
We also know the bond angle in $PB{r_3}$ is ${101.5^\circ }$and the bond angle in $P{H_3}$ is ${93.2^\circ }$. So, as the size of the pendant atom increases down the group, bond angle also increases because of the greater steric interactions which prefer larger bond angles. The size of the atom is larger than the size of hydrogen thus the bond angle is larger in $PB{r_3}$ and less in $P{H_3}$.
Similarly, the bond angle in$NB{r_3}$ is less than that of $N{H_3}$ having bond angle of ${106.6^\circ }$is due to the larger size of bromine because bromine will feel a repulsion by the lone pair of electrons on the nitrogen atoms and thus bromine will move away from nitrogen which results in squeezing of bond angle.
Electronegativity decreases down the group because of the increase in size of elements and along a period, it increases because of an increase in the effective nuclear charge which is the charge experienced by the valence shell electrons. For the same reason we can say that the electronegativity of phosphorus atoms is less than that of nitrogen. Thus the second statement is correct.

Therefore the correct answer is (B).

Note: Always remember, bigger the size of the atom lesser will be the electronegativity and lesser the size of the atom more will be the electronegativity. Similarly as the size of the central atom increases, the bond angle tends and if the size of the pendant atom increases, bond angle also increases.