(i) NH3 is stronger base than PH3. This is because the lone pair of electrons on N atom in NH3 is directed and not diffused as it is in PH3 due to larger size of phosphorus and hence more available for donation.
(ii) Sulphur has a greater tendency for catenation than oxygen because S-S bond is stronger than O – O bond due to less interelectronic repulsions.
(iii) Bond dissociation energy of F2 is less than Cl2 this is due to relatively large electron – electron repulsion among the lone pairs in F2 molecule where they are much closer to each other than in case of Cl2.