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) Cl atom has a larger size when compared to F atom (because as we move down in group VII, the atom size increases). So the bond strength of NCl3 is less, leading to it being endothermic, whereas the bond strength of NF3 is more, so it is exothermic.
ii) The five electron pairs around sulfur in SF4 attain a trigonal bipyramidal geometry in which one position is occupied by a lone pair. This lone pair finds a position that minimizes the number of repulsions it has with bonding electron pairs. It occupies an equatorial position with two repulsions. The bonded electrons occupy the axial and equatorial positions. The axial S-F bonds are bent slightly away from the lone pair. Hence, all the bonds in SF4 are not equivalent.