Which of the following is a lewis acid ?

Options

A. `BF_3`
B. `NH_3`
C. `PH_3`
D. `SO_2`

Answer

To determine which of the given options is a Lewis acid, we need to understand the definition of a Lewis acid. A Lewis acid is a species that can accept an electron pair from a Lewis base.

### Step-by-Step Solution:

1. **Identify the Options**: We need to analyze the given chemical species to identify which one can act as a Lewis acid. The options provided in the video seem to include boron (B), nitrogen (N), phosphine (PH3), and sulfur dioxide (SO2).

2. **Analyze Boron (B)**:
- Boron has an electron configuration of 1s² 2s² 2p¹.
- In its ground state, boron has three valence electrons.
- It has a vacant p-orbital, which allows it to accept an electron pair.
- Therefore, boron is electron-deficient and can act as a Lewis acid.

3. **Analyze Nitrogen (N)**:
- Nitrogen has an electron configuration of 1s² 2s² 2p³.
- It has five valence electrons and a complete octet when it forms bonds.
- It typically donates electrons (as in NH3) rather than accepting them, making it a Lewis base, not a Lewis acid.

4. **Analyze Phosphine (PH3)**:
- Phosphine has a similar structure to ammonia but with phosphorus.
- Phosphorus has five valence electrons and can donate a lone pair of electrons.
- Thus, it is also a Lewis base and not a Lewis acid.

5. **Analyze Sulfur Dioxide (SO2)**:
- Sulfur has six valence electrons and can form bonds with oxygen atoms.
- It can expand its octet and has the ability to accept electron pairs, making it an electron-rich species.
- However, it is more commonly recognized as a Lewis base due to its ability to donate lone pairs.

6. **Conclusion**:
- Among the analyzed species, boron (B) is the only one that can accept an electron pair and is therefore classified as a Lewis acid.

### Final Answer:
**Boron (B) is a Lewis acid.**