Among the three allotropes of an element, one is a good electrical conductor, the second one is known to be one of the hardest materials and the third one has a molecular form with an truncated icosahedral structure. The element is:

Options

A. boron
B. carbon
C. silicon
D. sulfur

Answer

To solve the question, we need to identify the element based on the characteristics of its allotropes provided in the question. Let's break down the information step by step:

### Step 1: Identify the Allotropes
The question mentions three allotropes of an element:
1. One is a good electrical conductor.
2. The second is known to be one of the hardest materials.
3. The third has a molecular form with a truncated icosahedral structure.

### Step 2: Analyze Each Allotrope
- **Good Electrical Conductor**: Among the allotropes, graphite is known for its ability to conduct electricity due to the presence of delocalized electrons in its structure.

- **Hardest Material**: Diamond is recognized as one of the hardest natural materials. Its tetrahedral structure gives it exceptional hardness.

- **Truncated Icosahedral Structure**: This structure is characteristic of Buckminsterfullerene (C60), which resembles a soccer ball and is a molecular form of carbon.

### Step 3: Identify the Element
Now, let's match these characteristics with the options given:
- **Boron**: Does not have allotropes that fit all three descriptions.
- **Carbon**:
- Graphite (good electrical conductor)
- Diamond (hardest material)
- Buckminsterfullerene (truncated icosahedral structure)
- **Silicon**: Does not have allotropes that fit all three descriptions.
- **Sulfur**: Does not have allotropes that fit all three descriptions.

### Conclusion
The element that fits all three descriptions is **Carbon**. Therefore, the answer is:

**Carbon**