Question

Explain why bond angle of $N{H}_3$ is greater than $N{F}_3$ while bond angle of $P{H}_3$ is less than that of $P{F}_3$.

Answer 1

Both $N{H}_3$ and $N{F}_3$ are pyramidal in shape with one lone pair on $N$. However $F$ has larger electronegatively than $H$, The electron pair is more towards $F$ in $N{F}_3$. Hence repulsion between bond pairs in $N{F}_3$, is less than $N{H}_3$. As a result the bond angle decreases to $102.4°$, whereas in $N{H}_3$ it decreased to $107.3°$ only.

$P{H}_3$ and $P{F}_3$ are also pyramidal in shape with one lone pair on $P$. But $P{F}_3$ has great bond angle than $P{H}_3$. This is due to resonance in $P{F}_3$, leading to partial double bond character as shown in figure 2.

As a result the repulsion between $P-F$ bonds are large and hence the bond angle is large.