The appearance of colour in solid alkali metal halides is generally due to

Options

A. F - centres
B. Schottky defect
C. Frenkel defect
D. Interstitial positions

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks about the reason behind the appearance of color in solid alkali metal halides.

2. **Identifying Key Concepts**: The key concepts to consider are defects in the crystal structure of solids, particularly in ionic solids like alkali metal halides. The options provided include:
- F centers
- Schottky defect
- Frenkel defect
- Interstitial positions

3. **Explaining F Centers**: F centers are defects in ionic crystals where an anion vacancy is occupied by an unpaired electron. This occurs when an electron from a nearby ion moves to fill the vacancy, resulting in a color change due to the presence of this free electron.

4. **Role of Electrons in Color Appearance**: The unpaired electron in the F center can absorb energy and get excited to a higher energy level. When it returns to its ground state, it emits energy in the visible spectrum, which is perceived as color.

5. **Examples of Colors in Alkali Metal Halides**: Different alkali metal halides exhibit different colors due to the presence of F centers. For example, lithium chloride (LiCl) appears pink, and potassium chloride (KCl) appears lilac.

6. **Conclusion**: Based on the explanation of F centers and their role in imparting color to solid alkali metal halides, the correct answer to the question is that the appearance of color in solid alkali metal halides is generally due to **F centers**.

**Final Answer**: The appearance of color in solid alkali metal halides is generally due to **F centers**.

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