In an aqueous medium, the ion is thermodynamically unstable and undergoes a rapid disproportionation — d and f Block Elements Chemistry Question

💡 Solution & Explanation

While $Cu^+$ has a stable $d^{10}$ configuration in the gas phase, the $Cu^{2+}$ ion ($d^9$) has a much higher charge density. When placed in water, the massive release of hydration energy by $Cu^{2+}$ completely overcompensates for the second ionization energy, driving the disproportionation.