Explain the following facts giving appropriate reason in each case:
(i) NF₃ is an exothermic compound whereas NCl₃ is not.
(ii)All the bonds in SF₄ are not equivalent.

(i) An exothermic compound is more stable than an endothermic compound. Exothermic compound means a compound formed from its elements by releasing heat energy. Since energy is released, it means that the compound formed is more stable.

As Cl lies below F in the periodic table, Cl is a larger atom than F. Now, as the difference in size between N and F is small, N-F bond is quite strong. So NF₃ is an exothermic compound whereas NCl₃ is an endothermic compound (as N-Cl bond is weak in nature).

(ii)

Due to the presence of five electron pairs around sulphur adopt trigonal bipyramidal geometry. Also, one position is occupied by a lone pair. This lone pair finds a position which tries to minimize the number of 90° repulsions it has with the bonding electron pairs as lesser the repulsions, more stable is the molecule. It occupies an equatorial position with two 90° repulsions. The bonded electrons occupy the axial and equatorial positions. As the axial S-F bonds are slightly bent away from the lone pair, we can conclude that all the bonds in SF₄ are not equivalent.