Chemistry - Complete d-block Question with Solution | TestHub

Key Idea:- Colour is obtained as a consequence of $d-d($ or $f-f)$ transition, and for $d-d$ (or $f-f)$ transition, presence of unpaired electrons is the necessary condition. $$\begin{array}{c}{\mathrm{La}}^{3+}(\mathrm{Z}=57)=[\mathrm{Xe}]4{\mathrm{f}}^{0}5{\mathrm{d}}^{0}6{\mathrm{s}}^0\\ \text{ (no unpaired electron) }\\ {\mathrm{Ti}}^{3+}(\mathrm{Z}=22)=[\mathrm{Ar}]3{\mathrm{d}}^{1}4{\mathrm{s}}^0\\ \text{ (one unpaired electron) }\\ {\mathrm{Lu}}^{3+}(\mathrm{Z}=71)=[\mathrm{Xe}]4{\mathrm{f}}^{14}5{\mathrm{d}}^{0}6{\mathrm{s}}^0\\ \text{ (no unpaired electron) }\\ {\mathrm{Sc}}^{3+}(\mathrm{Z}=21)=[\mathrm{Ar}]3{\mathrm{d}}^{0}4{\mathrm{s}}^0\end{array}$$ (no unpaired electron) Hence, only ${\mathrm{Ti}}^{3+}$ will exhibit colour in aqueous solution.