In the NaCl crystal, the coordination number of each ion is 6. How can the CN change to 8?

Options

A. By increasing temperature
B. By keeping it for some time
C. By increasing pressure
D. CN does not change by any way

Answer

To solve the question of how the coordination number (CN) of ions in a NaCl crystal can change from 6 to 8, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Coordination Number**:
- The coordination number refers to the number of nearest neighbor ions surrounding a particular ion in a crystal lattice. In NaCl, each Na⁺ ion is surrounded by 6 Cl⁻ ions and vice versa, giving a CN of 6.

2. **Concept of Coordination Number Change**:
- To increase the coordination number from 6 to 8, we need to allow more ions to surround each ion. This means that we need to create a situation where each ion can interact with 8 neighboring ions instead of just 6.

3. **Conditions for Change**:
- The change in coordination number can occur under certain conditions, primarily through changes in pressure or temperature. By increasing the pressure, the ions can be forced closer together, allowing for more ions to surround each ion.

4. **Increasing Pressure**:
- When pressure is increased, the ions in the crystal lattice are compressed. This compression can allow additional ions to fit into the space around each ion, thus increasing the coordination number to 8.

5. **Temperature Consideration**:
- Reducing the temperature can also contribute to a change in coordination number. Lower temperatures can lead to a decrease in thermal motion, allowing ions to settle into a more compact arrangement, potentially increasing the coordination number.

6. **Conclusion**:
- Therefore, the coordination number of NaCl can change from 6 to 8 by either increasing the pressure or reducing the temperature, with the most straightforward method being the increase in pressure.

### Final Answer:
The coordination number of each ion in NaCl can change from 6 to 8 by increasing the pressure or reducing the temperature.