Allotrope of sulphur which is stable below `90^@ C` is ____.

Answer

To solve the question regarding the allotrope of sulfur that is stable below 90°C, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Allotropes**:
- Allotropes are different forms of the same element, where the atoms are bonded together in different ways. For sulfur, there are several allotropes, including rhombic (orthorhombic) sulfur, monoclinic sulfur, and others.

2. **Identifying the Allotropes of Sulfur**:
- The most common allotropes of sulfur are:
- Rhombic (or orthorhombic) sulfur (S8)
- Monoclinic sulfur (S8)
- Other forms exist at higher temperatures, such as S6, S7, and various larger ring structures (S9 to S20).

3. **Stability at Different Temperatures**:
- Rhombic sulfur (orthorhombic sulfur) is stable at room temperature and below 90°C.
- Monoclinic sulfur is stable at higher temperatures and converts to rhombic sulfur upon cooling.

4. **Conclusion**:
- Since the question specifically asks for the allotrope of sulfur that is stable below 90°C, the answer is **orthorhombic sulfur** (also known as alpha sulfur).

### Final Answer:
The allotrope of sulfur which is stable below 90°C is **orthorhombic sulfur**.

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