Gadolinium has a low value of third ionisation enthalpy because of

Solution & Explanation

\textbf{Answer:} (b) high exchange enthalpy. \textbf{Solution:} Gadolinium has the configuration $[\mathrm{Xe}]4\mathrm{f}{}^{7}5\mathrm{d}{}^{1}6\mathrm{s}{}^2$. On removing the third electron, $\mathrm{Gd}{}^{3+}$ attains the extra-stable half-filled $4\mathrm{f}{}^7$ arrangement. A half-filled $4\mathrm{f}{}^7$ subshell has a large exchange energy (high exchange enthalpy), which makes the formation of $\mathrm{Gd}{}^{3+}$ especially favourable. This is why the third ionisation enthalpy of Gd is unusually low. Hence option (b) is correct.