In the laboratory, sodium chloride is made by burning the sodium in the atmosphere of chlorine which is yellow in colour. The cause of yellow colour is:

Options

A. Presence of `Na^(+)` ions in the crystal lattice
B. Presence of `CI^(-)` ions in the crystal lattice
C. Presence of electron in the crystal lattice
D. Presence of face centered cubic crystal lattice

Answer

To determine the cause of the yellow color in sodium chloride (NaCl) when sodium is burned in the presence of chlorine, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Reaction**:
- When sodium (Na) is burned in chlorine (Cl2), sodium chloride (NaCl) is formed. The chemical reaction can be represented as:
\[ 2 \text{Na} + \text{Cl}_2 \rightarrow 2 \text{NaCl} \]

2. **Formation of Defects in NaCl**:
- Upon heating sodium chloride in the presence of excess sodium, sodium atoms can deposit on the surface of the NaCl crystals. This process can lead to the formation of defects in the crystal lattice of NaCl.

3. **Formation of F-Centers**:
- When sodium deposits on the surface, it can lead to the creation of vacancies in the NaCl lattice. Specifically, when a sodium atom loses an electron, it becomes a Na⁺ ion, leaving behind a vacancy (missing Na⁺ ion) in the lattice.
- The electron that is lost by sodium can occupy this vacancy, forming what is known as an F-center (color center).

4. **Absorption of Light**:
- The F-center contains an unpaired electron that can absorb visible light. This absorption of light is what leads to the yellow color observed in the sodium chloride crystals.
- The specific wavelengths of light absorbed by the F-center correspond to the yellow region of the visible spectrum.

5. **Conclusion**:
- The yellow color in sodium chloride is primarily due to the presence of electrons trapped in F-centers, which absorb visible light.

### Final Answer:
The cause of the yellow color in sodium chloride is the presence of electrons in the F-centers of the crystal lattice.

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