The substance as hard as hard as diamond is

Options

A. `B_(3)N_(3)H_(6)`
B. `B_(2)H_(2)`
C. `(BN)_(n)`
D. `Na_(2)B_(4)O_(7)`

Answer

To determine the substance that is as hard as diamond, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Hardness of Diamond**:
- Diamond is known as one of the hardest natural materials. Its hardness is primarily due to the strong covalent bonds between carbon atoms in a tetrahedral lattice structure.

2. **Consider Other Hard Materials**:
- There are other materials known for their hardness, such as boron nitride (BN). It can exist in several structural forms, including hexagonal and cubic.

3. **Examine Boron Nitride**:
- Boron nitride can be found in two notable crystalline forms:
- **Hexagonal Boron Nitride (h-BN)**: This form is softer and is often used as a lubricant.
- **Cubic Boron Nitride (c-BN)**: This form is structurally similar to diamond and is known for its hardness, making it suitable for cutting tools and abrasives.

4. **Compare Structures**:
- The cubic form of boron nitride (c-BN) has a structure analogous to that of diamond. This similarity in structure contributes to its hardness.

5. **Conclusion**:
- Based on the structural properties and hardness comparison, the substance that is as hard as diamond is **Cubic Boron Nitride (c-BN)**.

### Final Answer:
The substance as hard as diamond is **Cubic Boron Nitride (c-BN)**.