In which of the following pairs are both the ions coloured in aqueous solution- [At. No. : \(\mathrm{Sc}=21,…
In which of the following pairs are both the ions coloured in aqueous solution-
[At. No. : \(\mathrm{Sc}=21, \mathrm{Ti}=22, \mathrm{Ni}=28, \mathrm{Cu}=29, \mathrm{Co}=27\)]
\(\mathrm{Ni}^{2+}, \mathrm{Ti}^{3+}\)
\(\mathrm{Sc}^{3+}, \mathrm{Ti}^{3+}\)
\(\mathrm{Sc}^{3+}, \mathrm{Co}^{2+}\)
\(\mathrm{Ni}^{2+}, \mathrm{Cu}^{+}\)
Solution
$\mathrm{Ti}^{3+}=1 s^2, 2 s^2 2 p^6, 3 s^2 3 p^6 3 d^1$
$\mathrm{Sc}^{3+}=1 s^2, 2 s^2 2 p^6, 3 s^2 3 p^6$
(Unpaired electron absent in d-orbital)
$\mathrm{Cu}^{+}=1 s^2, 2 s^2 2 p^6, 3 s^2 3 p^6 3 d^{10}$
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.