Question

Among \( \mathrm{Cu^{2+}}, \mathrm{Fe^{3+}}, \mathrm{Ti^{3+}} \) and \( \mathrm{Zn^{2+}} \) ions, the number of ions that impart colour is _ _ _ _ _ _ _

Hint:

Ions with \(d^n\) where \(1 \le n \le 9\) are coloured; \(d^0\) and \(d^{10}\) are colourless.

Answer 1

Correct Answer: 3

Concept: Colour in transition metal ions arises due to \(d\text{-}d\) transitions, which occur only when the \(d\)-orbital is partially filled (not \(d^0\) or \(d^{10}\)).

Step 1: Electronic configurations: \[ \mathrm{Cu^{2+}} : [\mathrm{Ar}] \, 3d^9 \quad \text{(partially filled → coloured)} \] \[ \mathrm{Fe^{3+}} : [\mathrm{Ar}] \, 3d^5 \quad \text{(partially filled → coloured)} \] \[ \mathrm{Ti^{3+}} : [\mathrm{Ar}] \, 3d^1 \quad \text{(partially filled → coloured)} \] \[ \mathrm{Zn^{2+}} : [\mathrm{Ar}] \, 3d^{10} \quad \text{(fully filled → colourless)} \]

Step 2: Number of coloured ions = \(3\)