How would you explain that (i) non-stoichimetric NaCl is yellow, (ii) non-stoichiometric ZnO is yellow ?

Answer

To explain why non-stoichiometric NaCl and ZnO are yellow, we can break down the explanation into clear steps:

### Step-by-Step Solution:

1. **Understanding Non-Stoichiometry**:
- Non-stoichiometric compounds have a composition that deviates from the ideal stoichiometric ratio. In the case of NaCl and ZnO, this means that there are vacancies in the crystal lattice.

2. **Anionic Vacancies**:
- In non-stoichiometric NaCl and ZnO, there are vacancies where anions (Cl⁻ in NaCl and O²⁻ in ZnO) are missing. These vacancies are known as "anionic vacancies."

3. **Formation of F-Centres**:
- The anionic vacancies in these compounds lead to the formation of F-centres. An F-centre is a type of color center that occurs when an anion vacancy is occupied by an unpaired electron.

4. **Absorption of Visible Light**:
- The presence of unpaired electrons in the F-centres allows these compounds to absorb specific wavelengths of visible light. This absorption is what gives the compounds their color.

5. **Resulting Color**:
- For both non-stoichiometric NaCl and ZnO, the absorption of certain wavelengths of light results in the reflection of yellow light. Thus, both compounds appear yellow to the human eye.

### Final Explanation:
In summary, non-stoichiometric NaCl and ZnO are yellow due to the presence of anionic vacancies that create F-centres, where unpaired electrons absorb visible light, leading to the reflection of yellow light.

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