Question

Which of the following ion is colourless inspite of the presence of unpaired electrons?

• A. \(La^{3+}\)
• B. \(Eu^{3+}\)
• C. \(Gd^{3+}\)
• D. \(Lu^{3+}\)

Hint:

Half-filled and fully-filled configurations often show no colour.

Answer 1

The Correct Option is C

Concept:
•Colour in ions arises due to \(f\text{-}f\) or \(d\text{-}d\) transitions
•Half-filled and fully-filled orbitals show very weak or no transitions

Step 1: Electronic configurations:
•\(La^{3+} : [Xe]\) (no unpaired electrons)
•\(Eu^{3+} : 4f^6\)
•\(Gd^{3+} : 4f^7\) (half-filled)
•\(Lu^{3+} : 4f^{14}\) (fully-filled)

Step 2:
•\(Gd^{3+}\) has 7 unpaired electrons (half-filled stable)
•No effective \(f\text{-}f\) transition $\Rightarrow$ colourless Final: \[ \Rightarrow Gd^{3+} \text{ is colourless despite unpaired electrons} \]