Which is the most thermodynamically stable allotropic form of phosphorus ?

Options

A. red
B. white
C. black
D. yellow

Explanation

To determine the most thermodynamically stable allotropic form of phosphorus, we need to analyze the different allotropes of phosphorus: white, red, black, and yellow phosphorus.

### Step-by-Step Solution:

1. **Understanding Allotropes of Phosphorus**:
- Phosphorus exists in several allotropic forms, primarily white, red, black, and yellow phosphorus. Each allotrope has different physical and chemical properties.

2. **Defining Thermodynamic Stability**:
- Thermodynamic stability refers to the stability of a substance in terms of its energy state. A thermodynamically stable substance is one that is least reactive and has the lowest energy state.

3. **Comparing the Allotropes**:
- **White Phosphorus**: This form is highly reactive and can spontaneously ignite in air. It has a tetrahedral structure and is not thermodynamically stable.
- **Red Phosphorus**: This form is less reactive than white phosphorus but still not as stable as black phosphorus. It has a network structure.
- **Black Phosphorus**: This is the least reactive form of phosphorus. It has an orthorhombic structure, which allows for the least angular strain, making it more stable.
- **Yellow Phosphorus**: This is similar to white phosphorus and is also highly reactive.

4. **Conclusion**:
- Among these allotropes, black phosphorus is the most thermodynamically stable form due to its orthorhombic structure, which minimizes angular strain and reactivity.

5. **Final Answer**:
- The most thermodynamically stable allotropic form of phosphorus is **black phosphorus**.