Account for the following :
(i) `NH_(3)` is a stronger base than `PH_(3)`
(ii) Sulphur has a greater tendency for catenation than oxygen.
(iii) Bond dissociation energy of `F_(2)` is less than that of `Cl_(2)`.

Explanation

(i) On moving frm nitrogen to phosphorus, i.e., down the group, the atomic size increases. As the size of the central atom incresases, the lone pair of electrons occupy a larger volume. Consequently, the electron density, and hence, the basic strength decrease. Thus, `NH_(3)` is a stronger base than `PH_(3)`.
(ii) The tendency for actenation depends upon the bond energy. The bond energy of a sulphur molecule is more than that of an molecule. Thus, the sulphur -sulphur bond strength is higher, and as a result the tendency of catenation is also higher. Sulphur shows catenation up to eight atoms.
(iii) Bond dissociation energy of fluorine.Also, the value of enthalpy of hydration of flurione is much higher than that of chlorine.
(i) The structure of nitric acid is
OR

The N-O bond has a double charactor. On the other hand, N-OH bond is a single bond. Since a double bond is shorter than a single bond ,the N-O bond (121 pm) is shorter than the N-OH bond (140 pm).
`SF_(4)` is easily hydrolysed whereas `SF_(6)` is not easily hydroysed. `SF_(6)` is an inery, non-toxic gas. This is a due to the presence of sterically protected six fluorine atoms. As a reusult, reactions like hydrolysis are not allowed to take place.
(iii) According to VSEPR theory, `XeF_(2)` has trigonal biptramidal geometry with `sp^(3)d` hybridisation. It has bond pairs and three lone pairs of electrons. The bond pairs occupy axial positions and the lone pairs occupy the equatorial positions. This is the most favourable arrangment as far as stability is concered. The two flourine atoms if placed at the equatorial positions will face minimum repulsion, and this wolud consequently increase the stability. Hence, the structure is linear.