Which one of the following molecules hydrides acts as a Lewis acid ?

Options

A. `NH_(3)`
B. `H_(2)O`
C. `B_(2) H_(6)`
D. `CH_(4)`

Answer

To determine which of the given hydrides acts as a Lewis acid, we need to understand the definitions of Lewis acids and bases.

### Step-by-Step Solution:

1. **Define Lewis Acid and Lewis Base**:
- A **Lewis acid** is a chemical species that can accept an electron pair because it has an empty orbital.
- A **Lewis base** is a species that can donate an electron pair because it has a filled orbital.

2. **Analyze Each Option**:
- **Option 1: NH3 (Ammonia)**:
- NH3 has a lone pair of electrons on the nitrogen atom. It can donate this lone pair, making it a Lewis base, not a Lewis acid.

- **Option 2: B2H6 (Diborane)**:
- B2H6 is an electron-deficient molecule. It has empty p-orbitals on the boron atoms, allowing it to accept electron pairs. Therefore, B2H6 acts as a Lewis acid.

- **Option 3: H2O (Water)**:
- Water has lone pairs of electrons on the oxygen atom and can donate these pairs, making it a Lewis base.

- **Option 4: CH4 (Methane)**:
- Methane has no lone pairs and cannot accept any electron pairs, making it neither a Lewis acid nor a Lewis base.

3. **Conclusion**:
- Among the options analyzed, only B2H6 acts as a Lewis acid because it can accept an electron pair due to its empty orbitals.

### Final Answer:
**B2H6 (Diborane) acts as a Lewis acid.**

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