In case of PF3 the size of F is small and thus it approaches near to the central atom that is P.
as it approaches near to P there comes the repulsion between the bond pairs,
which widens the bond angle and makes it larger than in case of PCl3.
Here two types of forces are deployed
- Bond pair – lone pair repulsions
- Bond pair –bond pair repulsions.
In case of the PF3 , the bond pair- bond pair repulsions compensate the effect of lone pair-bond pair repulsions. Thus the bond angle is larger as it has to face lesser repulsion in case of P-F-P bond angle.
In case of PCl3 , the size of Cl is larger as compared to F. so in this case the bonds are longer and bond pairs have enough space between them. and the lone pair – bond pair repulsion plays a bigger role in deciding the bond angle . So the bond angle is decreased due to the compression of bonds by repulsion of lone pair. As shown below:-