To determine which hydride is thermally more stable among NH3, PH3, AsH3, and BiH3, we can analyze the stability of hydrides of group 15 elements based on their atomic size and bond strength.
### Step-by-Step Solution:
1. **Identify the Group 15 Elements**:
The elements in group 15 include nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), and bismuth (Bi). The hydrides we are considering are NH3 (ammonia), PH3 (phosphine), AsH3 (arsine), and BiH3 (bismuthine).
2. **Understand the Trend in Atomic Size**:
As we move down the group from nitrogen to bismuth, the atomic size increases. This means that the distance between the nucleus and the outermost electrons increases.
3. **Analyze Bond Strength**:
The bond strength of the hydrides is influenced by the atomic size. As the atomic size increases, the bond length also increases, leading to a decrease in bond strength. This is because larger atoms form longer bonds that are easier to break.
4. **Compare the Hydrides**:
- NH3 has a small atomic size and strong N-H bonds.
- PH3 has a larger atomic size than NH3, resulting in weaker P-H bonds.
- AsH3 has even larger atomic size, leading to further reduction in bond strength.
- BiH3 has the largest atomic size, resulting in the weakest Bi-H bonds.
5. **Conclusion on Thermal Stability**:
Since stronger bonds correlate with greater thermal stability, NH3, with the strongest bonds among the listed hydrides, is the most thermally stable. Conversely, BiH3, with the weakest bonds, is the least thermally stable.
### Final Answer:
The thermally more stable hydride is **NH3 (ammonia)**.
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