[Anmoljathanna]

Hi I was recently studying and in my textbook I came across a question that," why is bond angle of PH₃ less than that of NH₃?"
The solution in the book said that ,"the decrease in bond angle is due to smaller bp-bp repulsion".

Now my thoughts are that how can force of repultion between two charged particles change by just changing the central atom?

I guessed I would find my answers in the vsepr theory as it talks about lp-lp, lp-bp and bp-bp repulsion.

Abbreviations
Lp- lone pair
Bp- bond pair

[orgopete]

Simple, since Coulombic forces are inverse square forces, an increase in distance will result in a weaker force. If we compare the bond length of aniline and a similar P-aromatic bond length, the distances will be about 139 pm vs 180 pm. I would argue the distance of the bonding electrons from each carbon should be similar, therefore the bonding electrons of phosphorous should be found at a greater distance from the nucleus than nitrogen. This greater distance should result in a weaker bond pair-bond pair repulsion.

One might also anticipate several effects upon the non-bonded electrons as well. Since the bonded electrons are further from one another, the non-bonded electrons can be pulled slightly closer to the nucleus. If they do, the nuclear repulsion to protons will be greater and phosphorous bases strength weaker or phosonium acid strength greater.