Which of the following compounds is not coloured ?

Options

A. 1. `Na[CuCl_4]`
B. 2. `Na_2[CdCl_4]`
C. 3. `K_4[Fe(CN)]_6`
D. 4. `K_3[Fe(CN)_6]`

Answer

To determine which of the given compounds is not colored, we need to analyze the electronic configurations of the elements in each compound and their oxidation states. The key concept here is that compounds with completely filled (D0 or D10) or completely empty d subshells are colorless, while those with incomplete d subshells (D1 to D9) are colored.

### Step-by-Step Solution:

1. **Identify the Elements and Their Oxidation States**:
- **Option 1**: Copper (Cu)
- Atomic number = 29
- Electronic configuration = [Ar] 3d^10 4s^2
- For Cu²⁺: 3d^9 4s^0 (incomplete d subshell)

- **Option 2**: Cadmium (Cd)
- Atomic number = 48
- Electronic configuration = [Kr] 4d^10 5s^2
- For Cd²⁺: 4d^10 5s^0 (completely filled d subshell)

- **Option 3**: Iron (Fe)
- Atomic number = 26
- Electronic configuration = [Ar] 3d^6 4s^2
- For Fe²⁺: 3d^6 4s^0 (incomplete d subshell)

- **Option 4**: Iron (Fe) again
- For Fe³⁺: 3d^5 4s^0 (incomplete d subshell)

2. **Analyze the d Subshells**:
- **Cu²⁺**: 3d^9 (incomplete, colored)
- **Cd²⁺**: 4d^10 (completely filled, colorless)
- **Fe²⁺**: 3d^6 (incomplete, colored)
- **Fe³⁺**: 3d^5 (incomplete, colored)

3. **Conclusion**:
- The only compound that is colorless is **Cadmium (Cd²⁺)** because it has a completely filled d subshell (4d^10).

### Final Answer:
**The compound that is not colored is Cadmium (Cd²⁺).**

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