In all the hydrides of Group 15 elements, the central atom is sp3 – hybridised. Three of the four sp3 – orbitals form three E – H (E stands for element of Group-15) σ – bonds while the fourth contains the lone pair of electron as shown below:
Since the lone pair repulsions are stronger than the bond pair-bond pair repulsions, therefore, the bond angle decreases from 109°28 to 107.8° in NH3. As we move from N to P to As to Sb, the atomic size goes on increasing. As a result, bond pairs of electrons lie always an away form the central atom. In other words, force of repulsion between the adjacent bond pairs goes on decreasing and consequently, the bond angles keep on decreasing from NH3 to SbH3.
