WAEC Chemistry 2015 Paper

Concept:Transition metal ions are coloured in aqueous solution only when they have partially filled $d$-orbitals.Explanation:Colour in these ions arises from $d$-$d$ transitions, in which visible light is absorbed to excite electrons between $d$-orbitals.This is possible only when at least one $d$-orbital is partially filled.$\mathrm{Cr^{3+}}$ has the configuration $[Ar]3d^3$ and contains unpaired $d$-electrons.$\mathrm{Fe^{3+}}$ has the configuration $[Ar]3d^5$ and contains unpaired $d$-electrons.$\mathrm{Mn^{3+}}$ has the configuration $[Ar]3d^4$ and contains unpaired $d$-electrons.Therefore, all three ions appear coloured in aqueous solution.$\mathrm{Sc^{3+}}$ has the configuration $[Ar]3d^0$, meaning it has no $d$-electrons at all.With no $d$-electrons, no $d$-$d$ transition can occur.Thus, $\mathrm{Sc^{3+}}$ does not produce colour and remains colourless in aqueous solution.Answer:$\mathrm{Sc^{3+}}$ (Option D)