Copper (I) ions are not known in aqueous solution.

Answer

**Step-by-Step Solution:**

1. **Understanding the Cuprous Ion (Cu⁺):**
- The cuprous ion (Cu⁺) is the +1 oxidation state of copper. It is important to understand its behavior in aqueous solutions.

2. **Disproportionation Reaction:**
- In aqueous solutions, the cuprous ion tends to undergo a disproportionation reaction. This means that Cu⁺ ions can simultaneously oxidize and reduce themselves, leading to the formation of two different products.
- The reaction can be represented as:
\[ 2 \text{Cu}^+ \rightarrow \text{Cu}^{2+} + \text{Cu} \]
- Here, one Cu⁺ ion is oxidized to Cu²⁺, and another Cu⁺ ion is reduced to solid copper (Cu).

3. **Stability of Cupric Ion (Cu²⁺):**
- The cupric ion (Cu²⁺) is more stable in aqueous solution compared to the cuprous ion. This stability can be attributed to the hydration energy associated with Cu²⁺ ions.

4. **Hydration Energy Comparison:**
- Hydration energy is the energy released when ions are surrounded by water molecules. The hydration energy of Cu²⁺ is greater than that of Cu⁺.
- This greater hydration energy compensates for the energy required to remove an additional electron from the copper atom (the second ionization enthalpy).

5. **Conclusion:**
- Due to the disproportionation reaction and the greater stability of the cupric ion (Cu²⁺) facilitated by its higher hydration energy, cuprous ions (Cu⁺) are not stable and thus not commonly found in aqueous solutions.