Predict which of the following will be coloured in aqueous solution: $Sc^{3+}$, $Fe^{3+}$, $Ti^{4+}$, $V^{3+}$ (Atomic nos. of Sc, Fe, Ti and V are 21, 26, 22 and 23 respectively)

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Accepted Answer

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> [!TLDR]
> Colour in transition-metal ions arises from d–d electronic transitions, which require *partly-filled* d-orbitals; ions with a $d^0$ configuration have no such transition available and are colourless.
>
> Work out each ion's d-electron count:
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> - **$Sc^{3+}$** (Z = 21): Sc is $[Ar]3d^1 4s^2$; removing 3 electrons ($4s^2$, $3d^1$) gives $Sc^{3+} = [Ar]3d^0$ — **no d electrons**.
> - **$Ti^{4+}$** (Z = 22): Ti is $[Ar]3d^2 4s^2$; removing all 4 gives $Ti^{4+} = [Ar]3d^0$ — **no d electrons**.
> - **$Fe^{3+}$** (Z = 26): Fe is $[Ar]3d^6 4s^2$; removing 3 electrons ($4s^2$, one $3d$) gives $Fe^{3+} = [Ar]3d^5$ — **5 unpaired d electrons**.
> - **$V^{3+}$** (Z = 23): V is $[Ar]3d^3 4s^2$; removing 3 electrons ($4s^2$, one $3d$) gives $V^{3+} = [Ar]3d^2$ — **2 unpaired d electrons**.
> …

Q.Predict which of the following will be coloured in aqueous solution: $Sc^{3+}$ , $Fe^{3+}$ , $Ti^{4+}$ , $V^{3+}$ (Atomic nos. of Sc, Fe, Ti and V are 21, 26, 22 and 23 respectively)

Concept understanding — Colour of Transition Metal Ions

The intuition: a window that absorbs some colours

The precise mechanism: d–d transition

Why not all d-block ions are coloured

Q.Predict which of the following will be coloured in aqueous solution: Sc3+Sc^{3+}Sc3+, Fe3+Fe^{3+}Fe3+, Ti4+Ti^{4+}Ti4+, V3+V^{3+}V3+ (Atomic nos. of Sc, Fe, Ti and V are 21, 26, 22 and 23 respectively)

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Q.Predict which of the following will be coloured in aqueous solution: $Sc^{3+}$ , $Fe^{3+}$ , $Ti^{4+}$ , $V^{3+}$ (Atomic nos. of Sc, Fe, Ti and V are 21, 26, 22 and 23 respectively)

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