Question

In solution Cu$^+$ ions are colourless while Cu$^{2+$ ions are coloured. Why?}

Hint:

Colour in transition metal ions arises due to d–d transitions. Ions with completely filled (d$^{10}$, d$^0$) configurations are colourless, while those with partially filled d-orbitals are coloured.

Answer 1

Case of Cu$^+$ (d$^{10}$ configuration):
- Cu$^+$ has electronic configuration [Ar] 3d$^{10}$.
- The d-orbitals are completely filled, so there are no vacant d-orbitals for d–d transitions.
- As no absorption of visible light takes place, Cu$^+$ solutions are colourless.
Case of Cu$^{2+$ (d$^9$ configuration):}
- Cu$^{2+}$ has electronic configuration [Ar] 3d$^9$.
- The d-orbitals are incompletely filled.
- d–d electronic transitions occur by absorbing light in the visible region.
- This absorbed light imparts a characteristic blue colour to Cu$^{2+}$ ions in solution.
\[ \boxed{\text{Hence, Cu$^+$ ions are colourless, Cu$^{2+}$ ions are coloured.}} \]