Question

The bond angles in $N{H}_3,N{F}_3$ and $NC{l}_3$ are in the order:

• A. ${\text{NCl}}_3>{\text{NH}}_3>{\text{NF}}_3$
• B. ${\text{NH}}_3>{\text{NCl}}_3>{\text{NF}}_3$
• C. ${\text{NF}}_3>{\text{NH}}_3>{\text{NCl}}_3$
• D. ${\text{NF}}_3>{\text{NCl}}_3>{\text{NH}}_3$

Answer 1

Answer: (A)

Comparison of NH₃ and NF₃ : Since N is more electronegative than hydrogen, shared pair of electrons of the N - H bond spent more time with nitrogen ( closer to nitrogen) giving a greater bond pair repulsion. On the other hand fluorine is more electronegative than nitrogen, shared pair of electrons of N - F bond spent more times with fluorine ( closer to fluorine ) giving less bond pair repulsion as compared to N - H bond. Although bond pair bond pair repulsion in both cases are less than lone pair - bond repulsion, contraction in bond angle is more in NF₃ due to smaller bond pair repulsion in this case than in NH₃. Hence bond angles are less than 109⁰28' in both cases but it is further less in NF₃



Comparison of NH₃ and NCl₃ : In NCl₃, chlorine is much larger in size than hydrogen, they comes closer and their valence shell electrons start repelling one another. Therefore, in NCl₃ the bond expand to some extent to compensate this repulsion:



Significant repulsion between the valence electrons of chlorine atom overcomes the lone pair bond - pair repulsion .



Hence, the overall order of bond angle is

NCl₃ > NH₃ > NF₃