In which of the following pairs are both the ions coloured in...

$\mathrm{Ti}^{3+}=1 s^2, 2 s^2 2 p^6, 3 s^2 3 p^6 3 d^1\mathrm{Sc}^{3+}=$1 s², 2 s² 2 p⁶, 3 s² 3 p⁶ (Unpaired electron absent in d-orbital) $\mathrm{Cu}^{+}=$1 s², 2 s² 2 p⁶, 3 s² 3 p⁶ 3 d^{10} Thus, $\mathrm{Cu}^{+}$has completely occupied $d$-orbital. Hence, in the above ions, $\mathrm{Ni}^{2+}$ and $\mathrm{Ti}^{3+}$ ions are coloured ions in the aqueous solution because of the presence of unpaired electrons in the $d$-subshell. Related Theory Whenever light falls on the transition element compounds electrons absorb energy and excite. When these electrons de-excite they release visible light wavelength. That's why transition element compounds exhibit colour.