Account for the following :
(i)The experimentally determined `N-F` bond length in `NF_3` is greater than the sum of the single bond covalent radii of `N` and `F`.
(ii) `Mg_3 N_2` when reacted with water gives `NH_3` but `HCl` is not obtained from `MgCl_2` on reaction with water at room temperature.
(iii) `(SiH_3)_3 N` is a weaker base than `(CH_3)_3 N`.

Explanation

(i) Nitrogen and fluorine are small atoms, and electron density is high. When the bond is formed, the bond pairs suffer repulsions, therefore the bond length slightly increases. Therefore, the experimentally determined `N-F` bond length in `NF_3` is greater than the sum of the single bond covalent radii of `N` and `F`.
(ii) `Mg_3N_2 + 6H_2 O rarr 3 Mg(OH)_2 + 2NH_3`
Magnesium chloride does not get hydrolysed, being a salt of strong acid and strong base. So, `NCl` is not obtained. `Mg_3 N_2` when reacted with water gives `NH_3`.
(iii)
`N(SiH_3)_3` is trigonal planar. Nitrogen atom shows `sp^3`- hybridisation. The structure is stabilised by the donation of a lone pair of electrons from nitrogen into vacant d-otbitals of silicon. Therefore, lone pair is not available for donation and therefore it is less basic than `N(Me)_3`.