The HNH angle value is higher than HPH, HAsH and HSbH angles. Why ?
In all the hydrides of group 15 elements, the central atom is sp3- hybridized. Three of the four sp3-orbitals form three E—H, s-bonds while the fourth contains the lone pair of electrons as shown below:
Since the lone pair-bond pair repulsions are stronger than the bond pair-bond pair repulsions, therefore, in NH3 the bond angle decreases from 109° – 28’ to 107.8°. As we move from N to P to As to Sb, the electronegativity of the central atom goes on decreasing. As a result, bond pairs of electrons, lie away and away from the central atom. In other words, force of repulsion between the adjacent bond pairs goes on decreasing and consequently the angles keep on decreasing from NH3 to SbH3. In other words, HNH bond angle is maximum (107.8°) followed by HPH (93.6°), followed by HAsH (91.6°), and HSbH (91.3°).
