Which is a Lewis base ?

Options

A. `B_2H_6`
B. `LiAlH_4`
C. `AlH_3`
D. `NH_3`

Answer

To determine which substance is a Lewis base, we first need to understand the definition of a Lewis base. A Lewis base is defined as a substance that can donate an electron pair to form a covalent bond.

### Step-by-Step Solution:

1. **Understand the Definition of Lewis Base:**
- A Lewis base is an electron pair donor. It can donate a pair of electrons to a Lewis acid to form a coordinate covalent bond.

2. **Analyze the Given Options:**
- We have four options to consider: B2H6, LiAlH4, AlH3, and NH3.

3. **Evaluate Each Option:**
- **B2H6 (Diborane):**
- This compound is known to act as a Lewis acid because it can accept electron pairs. Therefore, it is not a Lewis base.
- **LiAlH4 (Lithium Aluminum Hydride):**
- This compound also acts as a Lewis acid, as it can accept electron pairs from other molecules. Hence, it is not a Lewis base.
- **AlH3 (Aluminum Hydride):**
- Similar to the previous two, AlH3 behaves as a Lewis acid and cannot be classified as a Lewis base.
- **NH3 (Ammonia):**
- Ammonia has a lone pair of electrons on the nitrogen atom, which it can donate. Thus, NH3 is a Lewis base.

4. **Conclusion:**
- Among the given options, NH3 is the only compound that donates an electron pair, making it the correct answer.

### Final Answer:
- The Lewis base among the options provided is **NH3**.