Which one of the following is Lewis acid?

Options

A. `PCl_3`
B. `AlCl_3`
C. `NCl_3`
D. `AsCl_3`

Explanation

To determine which of the given compounds is a Lewis acid, we need to analyze the definitions and characteristics of Lewis acids and the structures of the compounds provided.

### Step-by-Step Solution:

1. **Understand the Definition of Lewis Acid**:
- A Lewis acid is defined as a substance that can accept an electron pair. This means it has a tendency to form bonds by accepting electrons.

2. **Analyze the Given Compounds**:
- The compounds provided are: PCl3, AlCl3, NCl3, and AsCl3.

3. **Draw Lewis Structures**:
- **PCl3**: Phosphorus (P) has one lone pair, and each chlorine (Cl) has three lone pairs. PCl3 does not have a strong tendency to accept electrons because it has a lone pair.
- **AlCl3**: Aluminum (Al) has no lone pairs, and each chlorine has three lone pairs. AlCl3 is electron-deficient and has a strong tendency to accept electrons, making it a Lewis acid.
- **NCl3**: Nitrogen (N) has one lone pair, and each chlorine has three lone pairs. Similar to PCl3, NCl3 does not have a strong tendency to accept electrons.
- **AsCl3**: Arsenic (As) has one lone pair, and each chlorine has three lone pairs. Like PCl3 and NCl3, AsCl3 does not exhibit strong Lewis acid behavior.

4. **Identify the Lewis Acid**:
- Among the compounds analyzed, AlCl3 is the only compound that can accept an electron pair due to its electron deficiency. Therefore, it is classified as a Lewis acid.

5. **Conclusion**:
- The correct answer is **AlCl3**.

### Final Answer:
**AlCl3 is the Lewis acid among the given options.**