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In which of the following pairs are both the ions coloured in aqueous solution -
[At. No. : Sc = 21, Ti = 22, Ni = 28, Cu = 29, Co = 27]
[At. No. : Sc = 21, Ti = 22, Ni = 28, Cu = 29, Co = 27]
A
$Ni^{2+}$, $Ti^{3+}$
B
$Sc^{3+}$, $Ti^{3+}$
C
$Sc^{3+}$, $Co^{2+}$
D
$Ni^{2+}$, $Cu^+$
Detailed Solution
Transition metal ions are coloured when they have unpaired electrons in partially filled d orbitals, so that d-d transitions can absorb visible light.
Ions with $d^0$ or $d^{10}$ configuration have no d-d transitions and are colourless.
$Sc^{3+}$: $3d^0$, colourless.
$Ti^{3+}$: $3d^1$, one unpaired electron, coloured (purple).
$Ni^{2+}$: $3d^8$, two unpaired electrons, coloured (green).
$Co^{2+}$: $3d^7$, three unpaired electrons, coloured (pink).
$Cu^+$: $3d^{10}$, colourless.
Every pair containing $Sc^{3+}$ or $Cu^+$ has one colourless ion, so the pair in which both ions are coloured is $Ni^{2+}$ and $Ti^{3+}$.
Ions with $d^0$ or $d^{10}$ configuration have no d-d transitions and are colourless.
$Sc^{3+}$: $3d^0$, colourless.
$Ti^{3+}$: $3d^1$, one unpaired electron, coloured (purple).
$Ni^{2+}$: $3d^8$, two unpaired electrons, coloured (green).
$Co^{2+}$: $3d^7$, three unpaired electrons, coloured (pink).
$Cu^+$: $3d^{10}$, colourless.
Every pair containing $Sc^{3+}$ or $Cu^+$ has one colourless ion, so the pair in which both ions are coloured is $Ni^{2+}$ and $Ti^{3+}$.