An excess of potassium ions makes KCL crystals appear violet or lilac in colour since …………. .

Options

A. some of the anionic sites are occupied by an unpaired electron
B. some of the anionic sites are occupied by an pair of electrons
C. there are vacancies at some anionic sites
D. F - centres are created which impart colour to the crystals

Explanation

To understand why an excess of potassium ions makes KCl crystals appear violet or lilac in color, we can break down the explanation into several steps:

### Step-by-Step Solution:

1. **Understanding KCl Structure**: KCl (potassium chloride) forms a crystalline lattice structure where potassium ions (K⁺) and chloride ions (Cl⁻) are arranged in a regular pattern.

**Hint**: Remember that KCl is made up of alternating K⁺ and Cl⁻ ions in a 1:1 ratio.

2. **Exposure to Potassium Ions**: When KCl crystals are exposed to an excess of potassium ions, these ions can attach to the surface of the crystal lattice.

**Hint**: Think about how ions interact with each other in a crystal lattice.

3. **Anionic Vacancies Creation**: As potassium ions adhere to the surface, they attract the negatively charged chloride ions. This attraction can cause some chloride ions to leave their positions in the lattice, creating vacancies. These vacancies are known as anionic vacancies.

**Hint**: Consider how the movement of ions can create empty spaces in the lattice structure.

4. **Formation of F-Centers**: The chloride ions that leave behind their positions also leave behind unpaired electrons. These unpaired electrons occupy the anionic vacancies, forming what are called F-centers (color centers).

**Hint**: F-centers are crucial for understanding how color is imparted to the crystals.

5. **Color Absorption and Emission**: The unpaired electrons in the F-centers can absorb certain wavelengths of light. When they absorb light, they can become excited and then emit light of a specific wavelength when they return to their ground state. The specific wavelengths emitted correspond to the colors we observe.

**Hint**: Recall that different materials absorb and emit different wavelengths of light, leading to various colors.

6. **Resulting Color**: In the case of KCl, the presence of these F-centers leads to the characteristic violet or lilac color of the crystals.

**Hint**: Different salts have different colors due to the nature of their F-centers and the specific wavelengths they absorb and emit.

### Conclusion:
Thus, the excess of potassium ions leads to the formation of F-centers in KCl crystals, which are responsible for the violet or lilac color observed.

### Summary of Key Points:
- KCl has a crystalline lattice structure.
- Excess potassium ions create anionic vacancies.
- Unpaired electrons in these vacancies form F-centers.
- F-centers absorb and emit light, giving KCl its characteristic color.