How many crystalline allotropic forms of carbon are known ?

Explanation

To determine how many crystalline allotropic forms of carbon are known, we can break down the information as follows:

### Step-by-Step Solution:

1. **Definition of Allotropes**: Allotropes are different forms of the same element that exist in the same physical state but have different physical properties while retaining the same chemical properties.

2. **Identify the Element**: In this case, we are focusing on carbon (C).

3. **List the Crystalline Allotropic Forms of Carbon**:
- **Diamond**: This is one of the most well-known forms of carbon. It is characterized by its hardness and is used in jewelry and cutting tools.
- **Graphite**: Another well-known form of carbon, graphite is soft and slippery, used in pencils and as a lubricant.
- **Fullerenes**: These are a class of carbon allotropes that include structures like C60 (Buckminsterfullerene), which resembles a soccer ball.
- **Carbon Nanotubes**: These are cylindrical structures made of carbon atoms arranged in a hexagonal pattern, known for their strength and electrical conductivity.

4. **Count the Allotropic Forms**: The four main crystalline allotropic forms of carbon are:
- Diamond
- Graphite
- Fullerenes (C60, C70, etc.)
- Carbon Nanotubes

5. **Conclusion**: Therefore, there are **four crystalline allotropic forms of carbon** known.

### Final Answer:
There are **four crystalline allotropic forms of carbon**: Diamond, Graphite, Fullerenes, and Carbon Nanotubes.

---