The following ion is colourless in aqueous solution ?

Options

A. `Mn^(2+)`
B. `Cu^(2+)`
C. `Ni^(2+)`
D. `Zn^(2+)`

Answer

To determine which of the given ions is colorless in aqueous solution, we need to analyze the electronic configurations of the ions provided: Mn²⁺, Cu²⁺, Ni²⁺, and Zn²⁺.

### Step-by-Step Solution:

1. **Identify the Ions**: The ions we are considering are Mn²⁺, Cu²⁺, Ni²⁺, and Zn²⁺.

2. **Understand Color in Transition Metal Ions**:
- Transition metal ions are colored due to the presence of unpaired electrons in their d-orbitals.
- Ions with incompletely filled d-orbitals exhibit color because they can undergo d-d transitions (electronic transitions between d-orbitals).

3. **Determine the Electronic Configurations**:
- Mn²⁺: The electronic configuration is Argon (Ar) 3d⁵ (5 unpaired electrons).
- Cu²⁺: The electronic configuration is Argon (Ar) 3d⁹ (1 unpaired electron).
- Ni²⁺: The electronic configuration is Argon (Ar) 3d⁸ (2 unpaired electrons).
- Zn²⁺: The electronic configuration is Argon (Ar) 3d¹⁰ (0 unpaired electrons).

4. **Analyze the Configurations**:
- Mn²⁺, Cu²⁺, and Ni²⁺ all have unpaired electrons, which means they can undergo d-d transitions and will be colored.
- Zn²⁺ has a completely filled d-orbital (3d¹⁰) with no unpaired electrons. Therefore, it does not undergo electronic transitions and is colorless in solution.

5. **Conclusion**:
- Among the given ions, the one that is colorless in aqueous solution is **Zn²⁺**.

### Final Answer:
**Zn²⁺ is colorless in aqueous solution.**