Classification of Elements and Periodicity in Properties - Result Question 11
####11. In which of the following options the order of arrangement does not agree with the variation of property indicated against it?
[2016]
(a) $Al^{3+}<Mg^{2+}<Na^{+}<F^{-}$(increasing ionic size)
(b) B $<$ C $<N<O$ (increasing first ionisation enthalpy)
(c) I $<Br<F<Cl$ (increasing electron gain enthalpy)
(d) $Li<Na<K<Rb$ (increasing metallic radius)
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Solution:
- (b) The correct order is $B<C<O<N$
Generally ionisation energy increases across a period. But here first I.E. of O is less than the first I.E. of N. This is due to the half-filled $2 p$ orbital in $N(1 s^{2}, 2 s^{2}, 2 p^{3})$ which is more stable than the $2 p$ orbital in $O(1 s^{2}, 2 s^{2}, 2 p^{4})$.
(c) The correct order of electron affinity is I $<Br<F<Cl$
Halogens have high electron affinities which decreases on moving down the group. However, fluorine has lower value than chlorine which is due to its small size and repulsion between the electron added and electrons already present.
