NCl3 is an endothermic compound while NF3 is an exothermic compound. explain

Explanation

In `NH_3` the nitrogen atom is `sp^3` -hybridized. Three of the four `sp^3`-orbitals form three N-H, `sigma`- bonds while the fourth contains the lone pair of electrons as shown in the Fig. Since the lone pair-bond pair repulsions are stronger than the bond pair-bond pair repulsions, therefore in NH, the bond angle decreases from `109-5^@` to `107-8^@`. As we move from N to Sb, the electronegativity of the central atom goes on decreasing and hence the bond pair of electrons lie away and away form the central atom. In other words, bond pair-bond-pair repulsion is maximum in `NH_3` and minimum in `SbH_3`. Consequently, HNH bond angle is maximum `(107-8^@)`, followed by HPH with bond angle `(93.6)^@`, HAsH with bond angle of `91.8^@` while HSbH bond anlge `(91.3^@)` is the minimum.