Question

Predict which of the following will be coloured in aqueous solution? $T{i}^{3+},V^{3+},C{u}^{+},S{e}^{3+},M{n}^{2+},F{e}^{3+}$ and $C{o}^{2+}$. Give reasons for each.

Answer 1

Only those ions will be coloured which have incomplete d-orbitals. Ions which has complete or vacant d-orbitals are colourless.

$\begin{array}{cc}T{i}^{3+}=\left[Ar\right]3d^1& Purple\\ V^{3+}=\left[Ar\right]3d^2& Green\\ C{u}^{+}=\left[Ar\right]3d^{10}& Colourless\\ S{c}^{3+}=\left[Ar\right]3d^0& Colourless\\ M{n}^{2+}=\left[Ar\right]3d^5& Pink\\ F{e}^{3+}=\left[Ar\right]3d^5& Brown\\ C{o}^{2+}=\left[Ar\right]3d^7& Pink\end{array}$

As $S{c}^{3+}$ and $C{u}^{+}$ have $3d^0$ and $3d^{10}$ configuration in their valence shell so their aqueous solutions are colourless. All others, i.e., $T{i}^{3+},V^{3+},M{n}^{2+},F{e}^{3+}$ and $C{o}^{2+}$ are coloured in aqueous medium.