Question

Assertion: Bond angle of $P{{F}_{3}}>PC{{l}_{3}}$ but bond angle of $PC{{l}_{3}}<\text{PB}{{\text{r}}_{3}}$
Reason: The bond angles show an increase in decreasing electronegativity of attached other atom on central atom but $P{{F}_{3}}\text{ p}\pi \text{-d}\pi $bonding results in an increase in bond angle.
(A) Both assertion and reason are correct and the reason is the correct explanation for the assertion.
(B) Both assertion and reason are correct but the reason is not the correct explanation for the assertion.
(C) Assertion is correct but the reason is incorrect
(D) Assertion is incorrect but the reason is correct

Answer 1

Hint: Bond angle of Phosphorus trifluoride is 97 degrees, Bond angle of phosphorous trichloride is 100 degrees and phosphorus tribromide had a bond angle of 101.5 degrees. As we move across the group, bond pair repulsion increases more as compared to lone pair-bond pair repulsion

Complete step by step solution:
Atomic number of phosphorus is 15. Electronic configuration of phosphorus is $1{{s}^{2}}2{{s}^{2}}2{{p}^{6}}3{{s}^{2}}3{{p}^{3}}$
As phosphorus forms, three bonds, three electrons from 3p orbital are involved in chemical bonding and two electrons from 3s are a lone pair.
Electronegativity of phosphorus is 2.1, Chlorine has electronegativity as 3.2 and for Fluorine, electronegativity is 4.

There are two types of electron repulsion: one is repulsion between Lone pair and bond pair
And second is repulsion between bond pair and bond pair.
The bond angle of Phosphorus trifluoride is 97 degrees, Bond angle of phosphorus trichloride is 100 degrees and phosphorus tribromide has bond angle of 101.5 degrees.
-Difference in electronegativity between phosphorus and chlorine is lower than that of between phosphorus and Fluorine, so in phosphorus trichloride Bond pair electrons move towards phosphorous which leads to greater repulsion experienced by bond pair-bond pair electrons so the bond angle is greater in phosphorus trichloride as compared to phosphorus trifluoride.
-As size of element increases, electron density also increases, so bond pair-bond pair repulsion also increases.
-As we move across the group, bond pair repulsion increases more as compared to lone pair-bond pair repulsion.
-So the bond angle of phosphorus trifluoride is less than phosphorus trichloride.

Hence, the correct answer is (D) Assertion is incorrect but reason is correct.

Note: There are two types of electron repulsion: one is repulsion between Lone pair and bond pair and second is repulsion between bond pair and bond pair. In phosphorus trichloride Bond pair electrons move towards phosphorus which leads to greater repulsion experienced by bond pair-bond pair electrons so the bond angle is greater in phosphorus trichloride as compared to phosphorus trifluoride.