Question

The HNH angle value is higher than HPH, HAsH and HSbH angles. Why? [Hint: Can be explained on the basis of sp³ hybridisation in NH₃ and only s−p bonding between hydrogen and other elements of the group].

Answer 1

Hydride NH₃ PH₃ AsH₃ SbH₃

H−M−H angle 107° 92° 91° 90°

The above trend in the H−M−H bond angle can be explained on the basis of the electronegativity of the central atom. Since nitrogen is highly electronegative, there is high electron density around nitrogen. This causes greater repulsion between the electron pairs around nitrogen, resulting in maximum bond angle. We know that electronegativity decreases on moving down a group. Consequently, the repulsive interactions between the electron pairs decrease, thereby decreasing the H−M−H bond angle.

Answer 2

In all these cases, the central atom is sp³ hybridized. Three of the four sp³ orbitals form three σ-bonds, while the fourth contains the lone pair of electrons. On moving down from N to Sb, the electronegativity of the central atom goes on decreasing. As a result of this, bond  pairs of electrons lie away and away from the central atom. This is because of the force of repulsion between the adjacent bond pairs goes on decreasing and the bond angles keep on decreasing from NH₃to SbH₃. Thus, bond angles are in the order:

HNH    >    HPH   > HasH    >  HSbH

(107.8º)       (93.6º)   (91.8º)       (91.3º)