Electronic configuration of Cu(II) is 3d9 whereas that of Cu(I) is 3d10. Which of the following is correct?

Concept: Stability of Cu(I) vs Cu(II) in aqueous solution

Although $\text{Cu}^+$ has the stable $3d^{10}$ configuration, $\text{Cu}^{2+}$ is more stable in aqueous solution due to its much higher hydration enthalpy.

Disproportionation of Cu⁺: $2\text{Cu}^+(aq) \rightarrow \text{Cu}(s) + \text{Cu}^{2+}(aq)$

This reaction is spontaneous, confirming Cu⁺ is unstable in water.

Reason: The higher charge density of $\text{Cu}^{2+}$ leads to a much larger hydration enthalpy compared to $\text{Cu}^+$. This extra stabilisation energy more than compensates for the higher 2nd ionisation energy.

$\Delta H_{\text{hyd}}(\text{Cu}^{2+}) \gg \Delta H_{\text{hyd}}(\text{Cu}^+)$

Answer: Option (2) — Cu(II) is more stable