When NaCl crystal is heated in sodium vapors, then it attains yellow colour. It is due to

Options

A. electrons trapped in cation vacancies
B. F-centres, which is electron trapped in anion vacancy created by `Cl^(-)`
C. F-centres, which is cation trapped in cation vacancy created by `Na^(+)`
D. interstitial defect caused by external impurity

Explanation

To understand why NaCl crystals attain a yellow color when heated in sodium vapors, we can break down the process step by step:

### Step 1: Heating NaCl in Sodium Vapors
When NaCl (sodium chloride) crystals are heated in the presence of sodium vapors, sodium atoms are introduced into the environment of the NaCl crystal.

**Hint:** Consider what happens to the crystal structure when additional sodium is present.

### Step 2: Deposition of Sodium Atoms
The sodium atoms from the vapor phase get deposited on the surface of the NaCl crystal. This deposition alters the local environment of the crystal.

**Hint:** Think about how the introduction of sodium atoms affects the existing chloride ions in the crystal.

### Step 3: Chloride Ion Diffusion
As sodium atoms deposit on the surface, chloride ions (Cl⁻) from the NaCl lattice start to diffuse out of the crystal structure. This movement creates vacancies in the lattice.

**Hint:** Reflect on what happens to the charge balance in the crystal when chloride ions leave.

### Step 4: Formation of Electron Vacancies
When chloride ions leave the lattice, they leave behind vacancies. The sodium atoms that are deposited can lose an electron, which is then released into the crystal lattice.

**Hint:** Consider the role of electrons in maintaining electrical neutrality in the crystal.

### Step 5: Electron Occupying Vacancies
The released electron diffuses back into the lattice and occupies the position of the chloride ion vacancy. This electron is known as an "F-center."

**Hint:** Remember that F-centers are associated with color changes in ionic crystals.

### Step 6: Absorption and Emission of Energy
The F-center absorbs energy and can emit energy in the visible spectrum. This energy emission is what imparts the yellow color to the NaCl crystal.

**Hint:** Think about the relationship between energy absorption/emission and color perception.

### Conclusion
The yellow color observed in NaCl crystals heated in sodium vapors is due to the presence of F-centers, which are formed by electrons trapped in anionic vacancies created by the loss of chloride ions.

**Final Answer:** The yellow color is due to the presence of F-centers, which are electrons trapped in anionic vacancies created by the removal of chloride ions.

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