Explain the following facts giving appropriate reason in each case :
(i) `NF_(3)` is an exothermic compound whereas `NCl_(3)` is not.
(ii) All the bonds in `SF_(4)` are not equivalent.

Explanation

(i) The high enthalpy of formation of `NCl_(3)`, is due to large difference in the size of the N and Cl atoms, as nitrogen belongs to second period and Cl atoms belongs to third period, this large difference makes the `N - Cl` bond weak and requires energy during its formation, so it is an endothermic compound. But in `NF_(3)`, since the size of the atoms is small, as both the atoms belong to second period, therefore, the `N - F` bond is strong and its formation is spontaneous, so it is an exothermic compound.
(ii) This is so because `SF_(4)` has see - saw shape and bonds are not equivalent on account of bond pair - lone pair repulsion . With S at the center , one of the three equatorial positions is occupied by a non bonding lone pair of electrons with two different types of F ligands, two axial and two equatorial. Axial Bonds suffer more repulsion than equatorial bonds as they are repelled by both bonding and non bonding electrons. Thus , the bonds are not equivalent in `SF_(4)`.