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The aqueous solution containing which one of the following ions will be colourless -
[Atomic number : Sc = 21, Fe = 26, Ti = 22, Mn = 25]
[Atomic number : Sc = 21, Fe = 26, Ti = 22, Mn = 25]
A
$Fe^{2+}$
B
$Mn^{2+}$
C
$Ti^{3+}$
D
$Sc^{3+}$
Detailed Solution
The colour of transition metal ions in solution is due to d-d transitions, which need unpaired electrons in partly filled d orbitals.
$Sc^{3+}$: Sc is $[Ar]3d^1 4s^2$, so $Sc^{3+}$ is $3d^0$. With no d electrons, no d-d transition is possible, so its solution is colourless.
$Fe^{2+}$: $3d^6$, pale green.
$Mn^{2+}$: $3d^5$, very pale pink.
$Ti^{3+}$: $3d^1$, purple.
So the colourless solution contains $Sc^{3+}$.
$Sc^{3+}$: Sc is $[Ar]3d^1 4s^2$, so $Sc^{3+}$ is $3d^0$. With no d electrons, no d-d transition is possible, so its solution is colourless.
$Fe^{2+}$: $3d^6$, pale green.
$Mn^{2+}$: $3d^5$, very pale pink.
$Ti^{3+}$: $3d^1$, purple.
So the colourless solution contains $Sc^{3+}$.