Question

Which of the following transition metal ions is not coloured?

• A. $ Cu^{+} $
• B. $ V^{3+} $
• C. $ Co^{2+} $
• D. $ Ni^{2+} $

Answer 1

Answer: (A)

Presence of unpaired electron is a necessity for transition metal ions to give colour.
The electronic configuration of given transition metal ions are as
${ }_{29} Cu ^{+} =[ Ar ] 3 d^{10} 4 s^{0}$ (no unpaired electron)
${ }_{23} V ^{3+} =[ Ar ] 3 d^{2} 4 s^{0} $ (two unpaired electrons)
${ }_{27} Co ^{2+} =[ Ar ] 3 d^{7} 4 s^{0} $ (three unpaired electrons)
${ }_{28} Ni ^{2+} =[ Ar ] 3 d^{8} 4 s^{0} $ (two unpaired electrons)
Thus, $Cu ^{+}$ion is colourless.