The number of NaCl units present in a unit cell of NaCl are 

Options

A. 1
B. 2
C. 4
D. 8

Answer

To determine the number of NaCl units present in a unit cell of sodium chloride (NaCl), we need to analyze the arrangement of sodium ions (Na⁺) and chloride ions (Cl⁻) in the unit cell.

### Step-by-Step Solution:

1. **Identify the Unit Cell Structure**:
- Sodium chloride (NaCl) has a face-centered cubic (FCC) unit cell structure.

2. **Count the Na⁺ Ions**:
- In the FCC structure, Na⁺ ions occupy the body center and the face centers.
- **Body Center**: There is 1 Na⁺ ion at the body center, which contributes 1 whole ion to the unit cell.
- **Face Centers**: There are 6 face centers, and each face-centered Na⁺ ion contributes 1/2 of an ion to the unit cell. Therefore, the contribution from face centers is:
\[ 6 \text{ (face centers)} \times \frac{1}{2} = 3 \]
- **Total Na⁺ Ions**:
\[ 1 \text{ (body center)} + 3 \text{ (from face centers)} = 4 \text{ Na⁺ ions} \]

3. **Count the Cl⁻ Ions**:
- Cl⁻ ions are located at the corners and face centers.
- **Corners**: There are 8 corners in the unit cell, and each corner Cl⁻ ion contributes 1/8 of an ion to the unit cell. Thus, the contribution from corners is:
\[ 8 \text{ (corners)} \times \frac{1}{8} = 1 \]
- **Face Centers**: There are 6 face centers, and each face-centered Cl⁻ ion contributes 1/2 of an ion to the unit cell. Therefore, the contribution from face centers is:
\[ 6 \text{ (face centers)} \times \frac{1}{2} = 3 \]
- **Total Cl⁻ Ions**:
\[ 1 \text{ (from corners)} + 3 \text{ (from face centers)} = 4 \text{ Cl⁻ ions} \]

4. **Calculate the Number of NaCl Units**:
- Since there are 4 Na⁺ ions and 4 Cl⁻ ions in the unit cell, the number of NaCl units in the unit cell is:
\[ \text{Number of NaCl units} = \text{Number of Na⁺ ions} = \text{Number of Cl⁻ ions} = 4 \]

### Final Answer:
The number of NaCl units present in a unit cell of NaCl is **4**.

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