Which of the following is a 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 each option based on the definition of a Lewis acid. A Lewis acid is a substance that can accept a pair of electrons.

### Step-by-Step Solution:

1. **Understanding Lewis Acids**:
- A Lewis acid is defined as an electron pair acceptor. This means it must have the ability to accept electrons from another species.

2. **Analyzing the Options**:
- **Option A: PCl3**
- Phosphorus (P) has 5 valence electrons and forms three bonds with chlorine (Cl), leaving it with 2 unshared electrons. Since it does not have a strong tendency to accept more electrons, it is not a Lewis acid.

- **Option B: AlCl3**
- Aluminum (Al) has 3 valence electrons and forms three bonds with chlorine. Aluminum has empty d orbitals, which allows it to accept additional electrons. Therefore, AlCl3 can act as a Lewis acid.

- **Option C: NCl3**
- Nitrogen (N) has 5 valence electrons and forms three bonds with chlorine, leaving it with one lone pair. NCl3 cannot accept more electrons because it does not have empty d orbitals and is already satisfied with its electron configuration.

- **Option D: AsCl3**
- Arsenic (As) is in the same group as phosphorus and has a similar electron configuration. AsCl3 has three bonds with chlorine and does not have a strong tendency to accept more electrons, similar to PCl3.

3. **Conclusion**:
- After analyzing all options, we find that only **AlCl3** can accept a pair of electrons and thus is classified as a Lewis acid. Therefore, the correct answer is **Option B: AlCl3**.