Which one of the following is hardest compound of boron ?

Options

A. `B_(2)O_(3)`
B. BN
C. `B_(4)C`
D. `B_(2)H_(6)`

Explanation

To determine which compound of boron is the hardest, we can analyze the given options: B2O3 (Boron Trioxide), Bn (Boron Nitride), B4C (Boron Carbide), and B2S6 (Boron Sulfide).

### Step-by-Step Solution:

1. **Identify the Compounds**:
- B2O3: Boron Trioxide
- Bn: Boron Nitride
- B4C: Boron Carbide
- B2S6: Boron Sulfide

2. **Research the Hardness of Each Compound**:
- **B2O3**: This compound is primarily used in glass and ceramics but is not known for its hardness.
- **Bn (Boron Nitride)**: This compound exists in several forms, including cubic boron nitride (c-BN), which is known to be extremely hard, even harder than diamond.
- **B4C (Boron Carbide)**: This is a very hard material and is often used in abrasives and armor.
- **B2S6**: This compound is less common and does not exhibit notable hardness compared to the others.

3. **Compare Hardness**:
- Cubic boron nitride (c-BN) is recognized as one of the hardest materials known, even surpassing diamond in certain applications.
- Boron carbide is also hard but is generally considered less hard than cubic boron nitride.

4. **Conclusion**:
- Among the given options, the hardest compound of boron is **Bn (Boron Nitride)**, particularly in its cubic form.

### Final Answer:
The hardest compound of boron is **Bn (Boron Nitride)**.