Which of the following ions will give a colourless aqueous solution

Options

A. `Ni^(2+)`
B. `Co^(2+)`
C. `Cu_(2)^(2+)`
D. `Fe^(2+)`

Answer

To determine which of the following ions will give a colorless aqueous solution, we need to analyze the electronic configurations of the given ions and check for the presence of unpaired electrons in their d-orbitals. A colorless solution indicates that there are no unpaired electrons, as unpaired electrons are responsible for color due to d-d transitions.

### Step-by-Step Solution:

1. **Identify the Ions**: The ions we need to analyze are Ni²⁺ (Nickel), Co²⁺ (Cobalt), Cu²⁺ (Copper), and Fe²⁺ (Iron).

2. **Determine Electronic Configurations**:
- **Ni²⁺**: The electronic configuration of Ni is [Ar] 4s² 3d⁸. For Ni²⁺, we remove 2 electrons from the 4s orbital:
Ni²⁺: [Ar] 3d⁸
In 3d⁸, there are 2 unpaired electrons.

- **Co²⁺**: The electronic configuration of Co is [Ar] 4s² 3d⁷. For Co²⁺, we remove 2 electrons from the 4s orbital:
Co²⁺: [Ar] 3d⁷
In 3d⁷, there are 3 unpaired electrons.

- **Cu²⁺**: The electronic configuration of Cu is [Ar] 4s² 3d¹⁰. For Cu²⁺, we remove 2 electrons from the 4s orbital:
Cu²⁺: [Ar] 3d⁹
In 3d⁹, there is 1 unpaired electron.

- **Fe²⁺**: The electronic configuration of Fe is [Ar] 4s² 3d⁶. For Fe²⁺, we remove 2 electrons from the 4s orbital:
Fe²⁺: [Ar] 3d⁶
In 3d⁶, there are 4 unpaired electrons.

3. **Analyze Unpaired Electrons**:
- Ni²⁺: 2 unpaired electrons → colored solution
- Co²⁺: 3 unpaired electrons → colored solution
- Cu²⁺: 1 unpaired electron → colored solution
- Fe²⁺: 4 unpaired electrons → colored solution

4. **Conclusion**: None of the ions listed (Ni²⁺, Co²⁺, Cu²⁺, Fe²⁺) give a colorless solution since all have unpaired electrons. However, if we consider the possibility of zinc (Zn²⁺), which is not mentioned in the question but is often included in such discussions, we find:
- **Zn²⁺**: The electronic configuration of Zn is [Ar] 4s² 3d¹⁰. For Zn²⁺, we remove 2 electrons from the 4s orbital:
Zn²⁺: [Ar] 3d¹⁰
In 3d¹⁰, there are no unpaired electrons → colorless solution.

5. **Final Answer**: The ion that gives a colorless aqueous solution is **Zn²⁺** (if included), otherwise, none of the listed ions provide a colorless solution.