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 first need to understand the definition of a Lewis acid. A Lewis acid is defined as an electron-deficient species that has the ability to accept electron pairs.

Let's analyze each option step by step:

### Step 1: Analyze BF3
- **Structure**: BF3 has boron (B) at the center bonded to three fluorine (F) atoms.
- **Electron Count**: Boron has 3 bond pairs with fluorine and only 6 electrons in its valence shell (3 from the bonds with F).
- **Conclusion**: Since boron has an incomplete octet (less than 8 electrons), it is electron deficient and can accept an electron pair. Therefore, BF3 is a Lewis acid.

### Step 2: Analyze NH3
- **Structure**: NH3 has nitrogen (N) at the center bonded to three hydrogen (H) atoms.
- **Electron Count**: Nitrogen has 3 bond pairs and 1 lone pair, giving it a total of 8 electrons.
- **Conclusion**: NH3 has a lone pair that can be donated, making it a Lewis base, not a Lewis acid.

### Step 3: Analyze PH3
- **Structure**: PH3 has phosphorus (P) at the center bonded to three hydrogen (H) atoms.
- **Electron Count**: Phosphorus has 3 bond pairs and 1 lone pair, similar to NH3.
- **Conclusion**: Like NH3, PH3 can donate its lone pair, acting as a Lewis base rather than a Lewis acid.

### Step 4: Analyze SO2
- **Structure**: SO2 has sulfur (S) at the center bonded to two oxygen (O) atoms with a double bond to one O and a single bond to the other.
- **Electron Count**: Sulfur has 2 double bonds and 1 lone pair.
- **Conclusion**: The lone pair on sulfur can be donated, which means SO2 can also act as a Lewis base.

### Final Conclusion:
From the analysis:
- **BF3** is a Lewis acid.
- **NH3**, **PH3**, and **SO2** are Lewis bases.

Thus, the answer to the question "Which of the following is a Lewis acid?" is **BF3**.