Sat November 26, 2011 By: Ayush Singh

Explain VSEPR theory using example. Also using this theory explain how bond angleof group 15 hydrides decreases in the order NH3 PH3 AsH3 SbH3 BiH3

Expert Reply
Tue November 29, 2011
The premise of VSEPR is that the valence electron pairs surrounding an atom mutually repel each other, and will therefore adopt an arrangement that minimizes this repulsion, thus determining the molecular geometry.
The ammonia molecule (NH3) has three pairs of electrons involved in bonding, but there is a lone pair of electrons on the nitrogen atom. It is not bonded with another atom; however, it influences the overall shape through repulsions.
The N-H bond is short (1.015 Å) compared to the heavier analogs, and nitrogen is more electronegative than hydrogen, so the bonding pair will reside closer to the central atom and the bonding pairs will repel each other opening the H-N-H angle more than observed for PH3
that is how the bond angle of group 15 hydrides decreases.
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