Question

In which of the following options order of arrangement does not match with the variation of property indicated against it?

• A. $A{l}^{3+}<M{g}^{2+}<N{a}^{+}<F^{-}$ (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)

Answer 1

The correct option is B $B<C<N<O$ (increasing first ionisation enthalpy)

Ionisation enthalpy of $B$ is less than ionisation enthalpy of $C$ but ionisation enthalpy of $N$ is greater than ionisation enthalpy of $O$ due to half-filled orbitals in $N$ which have extra stability. Removal of an electron from half-filled, stable $2p$ orbital requires more energy than removal of an electron from partially filled $2p$ orbital of $O$.

Electron gain enthalpy of $Cl$ is greater than electron gain enthalpy of $F$ as $Cl$ has bigger size and lesser electronic repulsion than $F$.