Q73 / 200 · NEET 2010 (Screening) · Chemistry

Which of the following ions will exhibit colour in aqueous solutions ?

  • A
    $\mathrm{La}^{3+}(\mathrm{Z}=57)$
  • Correct answer
    B
    $\mathrm{Ti}^{3+}(\mathrm{Z}=22)$
  • C
    $\mathrm{Lu}^{3+}(\mathrm{Z}=71)$
  • D
    $\mathrm{Sc}^{3+}(\mathrm{Z}=21)$

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.

Hence, the correct answer is option: 'B'.

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