Which of the following ions will exhibit colour in aqueous solutions ?
Which of the following ions will exhibit colour in aqueous solutions ?
- $\mathrm{La}^{3+}(\mathrm{Z}=57)$
- $\mathrm{Ti}^{3+}(\mathrm{Z}=22)$
- $\mathrm{Lu}^{3+}(\mathrm{Z}=71)$
- $\mathrm{Sc}^{3+}(\mathrm{Z}=21)$
Solution
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.
$\begin{gathered}
\mathrm{La}^{3+}(\mathrm{Z}=57)=[\mathrm{Xe}] 4 \mathrm{f}^0 5 \mathrm{~d}^0 6 \mathrm{~s}^0 \\
\text { (no unpaired electron) } \\
\mathrm{Ti}^{3+}(\mathrm{Z}=22)=[\mathrm{Ar}] 3 \mathrm{~d}^1 4 \mathrm{~s}^0 \\
\text { (one unpaired electron) } \\
\mathrm{Lu}^{3+}(\mathrm{Z}=71)=[\mathrm{Xe}] 4 \mathrm{f}^{14} 5 \mathrm{~d}^0 6 \mathrm{~s}^0 \\
\text { (no unpaired electron) } \\
\mathrm{Sc}^{3+}(\mathrm{Z}=21)=[\mathrm{Ar}] 3 \mathrm{~d}^0 4 \mathrm{~s}^0
\end{gathered}$
(no unpaired electron) Hence, only $\mathrm{Ti}^{3+}$ will exhibit colour in aqueous solution.
Asked in: NEET 2010 (Screening)
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