The following ion exhibits colour in aqueous solution ?

Options

A. `Sc^(3+)`
B. `Cu^(+)`
C. `Ni^(2+)`
D. `Zn^(2+)`

Answer

To determine which ion exhibits color in aqueous solution, we need to analyze the electron configurations of the given ions and identify the presence of unpaired d electrons. The presence of unpaired d electrons is crucial because they allow for d-d transitions, which are responsible for the color observed in transition metal ions.

### Step-by-Step Solution:

1. **Identify the Ions**: The ions we need to analyze are Scandium (Sc³⁺), Copper (Cu⁺), Nickel (Ni²⁺), and Zinc (Zn²⁺).

2. **Write the Electron Configurations**:
- **Scandium (Sc³⁺)**: Scandium has an atomic number of 21. The electron configuration for Sc is [Ar] 3d¹ 4s². For Sc³⁺, it loses three electrons (two from 4s and one from 3d), resulting in:
\[ \text{Sc}^{3+}: [Ar] \]
- **Copper (Cu⁺)**: Copper has an atomic number of 29. The electron configuration for Cu is [Ar] 3d¹⁰ 4s¹. For Cu⁺, it loses one electron from the 4s orbital, resulting in:
\[ \text{Cu}^{+}: [Ar] 3d^{10} \]
- **Nickel (Ni²⁺)**: Nickel has an atomic number of 28. The electron configuration for Ni is [Ar] 3d² 4s². For Ni²⁺, it loses two electrons (both from 4s), resulting in:
\[ \text{Ni}^{2+}: [Ar] 3d^{8} \]
- **Zinc (Zn²⁺)**: Zinc has an atomic number of 30. The electron configuration for Zn is [Ar] 3d¹⁰ 4s². For Zn²⁺, it loses two electrons from 4s, resulting in:
\[ \text{Zn}^{2+}: [Ar] 3d^{10} \]

3. **Check for Unpaired Electrons**:
- **Sc³⁺**: No unpaired d electrons (3d subshell is empty).
- **Cu⁺**: No unpaired d electrons (3d subshell is fully filled).
- **Ni²⁺**: Has unpaired d electrons (3d subshell has 8 electrons, which means there are unpaired electrons).
- **Zn²⁺**: No unpaired d electrons (3d subshell is fully filled).

4. **Conclusion**: The only ion that has unpaired d electrons and can exhibit color in aqueous solution is **Ni²⁺**. This is due to the presence of unpaired d electrons that allow for d-d transitions, which absorb certain wavelengths of light and thus produce color.

### Final Answer:
The ion that exhibits color in aqueous solution is **Ni²⁺**.

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