The correct order of thermal stability of hydrides of group `15` is

Options

A. `BH_(3)gtPH_(3)gtAsH_(3)gtBiH_(3)gtSbH_(3)`
B. `NH_(3)gtPH_(3)gtAsH_(3)gtSbH_(3)gtBiH_(3)`
C. `NH_(3)ltPH_(3)ltSbH_(3)gtAsH_(3)gtBiH_(3)`
D. `BiH_(3)gtSbH_(3)gtAsH_(3)gtPH_(3)NH_(3)`

Explanation

To determine the correct order of thermal stability of hydrides of Group 15 elements (which include nitrogen, phosphorus, arsenic, antimony, and bismuth), we need to analyze the hydrides formed by these elements:

1. **Identify the Hydrides**: The hydrides of Group 15 are NH₃ (ammonia), PH₃ (phosphine), AsH₃ (arsine), SbH₃ (stibine), and BiH₃ (bismuthine).

2. **Understand Thermal Stability**: Thermal stability of a hydride refers to its ability to withstand heat without decomposing. This is largely influenced by the strength of the X-H bond, where X is the central atom (N, P, As, Sb, Bi).

3. **Analyze the Bond Strength**: The bond strength of the X-H bond decreases as we move down the group due to the increase in atomic size and the corresponding decrease in bond overlap. As the size of the central atom increases, the bond length increases, leading to weaker bonds.

4. **Order of Atomic Size**: The atomic size increases from nitrogen to bismuth:
- N < P < As < Sb < Bi

5. **Order of Bond Strength**: Consequently, the bond strength decreases in the same order:
- NH₃ > PH₃ > AsH₃ > SbH₃ > BiH₃

6. **Thermal Stability Order**: Since thermal stability is inversely related to bond strength, the order of thermal stability of the hydrides will be the reverse of the bond strength:
- NH₃ > PH₃ > AsH₃ > SbH₃ > BiH₃

7. **Final Answer**: Therefore, the correct order of thermal stability of the hydrides of Group 15 is:
- NH₃ > PH₃ > AsH₃ > SbH₃ > BiH₃