To determine which ion is least stable in an aqueous medium, we need to analyze the electronic configurations of the given ions and their potential to undergo oxidation.
### Step-by-Step Solution:
1. **Identify the Ions**: The ions we are considering are Fe²⁺, Co²⁺, Ni²⁺, and Mn²⁺.
2. **Determine Electronic Configurations**:
- **Fe²⁺**: The electronic configuration of Fe is [Ar] 4s² 3d⁶. For Fe²⁺, we remove two electrons from the 4s orbital, resulting in [Ar] 3d⁶.
- **Co²⁺**: The electronic configuration of Co is [Ar] 4s² 3d⁷. For Co²⁺, we remove two electrons from the 4s orbital, resulting in [Ar] 3d⁷.
- **Ni²⁺**: The electronic configuration of Ni is [Ar] 4s² 3d⁸. For Ni²⁺, we remove two electrons from the 4s orbital, resulting in [Ar] 3d⁸.
- **Mn²⁺**: The electronic configuration of Mn is [Ar] 4s² 3d⁵. For Mn²⁺, we remove two electrons from the 4s orbital, resulting in [Ar] 3d⁵.
3. **Assess Stability in Aqueous Medium**:
- **Fe²⁺**: Can be oxidized to Fe³⁺, which has the configuration [Ar] 3d⁵. The 3d⁵ configuration is half-filled, which is known for its stability. Thus, Fe²⁺ is less stable because it can easily oxidize.
- **Co²⁺**: Can be oxidized to Co³⁺, which has the configuration [Ar] 3d⁶. While this is a stable configuration, it is not as stable as a half-filled configuration.
- **Ni²⁺**: Can be oxidized to Ni³⁺, which has the configuration [Ar] 3d⁷. This is also stable but not as stable as half-filled configurations.
- **Mn²⁺**: Has a half-filled configuration (3d⁵), which is very stable and does not easily oxidize.
4. **Conclusion**: Since Fe²⁺ can easily oxidize to the more stable Fe³⁺ with a half-filled configuration, it is the least stable ion in aqueous medium. Therefore, the answer is **Fe²⁺**.
### Final Answer:
**Fe²⁺ is the least stable in aqueous medium.**
