20. In which of the following options order of arrangement does not agree with the variation of property indicated against it?
(a) Al3+<Mg2+<Na+<F- (Increasing ionic size)
(b) B<C<N
(c) I<Br<Cl
(d) Li<Na<K
(b, c)
Due to greater stability of the half filled electronic configuration of nitrogen, its ionisation enthalpy is higher than that of oxygen. Thus, option (b) is incorrect.
Due to stronger electron-electron repulsions in the small size of fluorine the negative electron gain enthalpy of fluorine is lower than that of chlorine. Hence, option (c) is incorrect.
