Which of the following properties decreases for `MH_3` on descending the group from `NH_3` To `BiH_3` ?

Options

A. Thermal stability
B. Reducing power
C. The ease of replacing hydrogen atoms by other groups such as CI, Me
D. The lone pair donating ability

Answer

To solve the question regarding which property decreases for \( MH_3 \) on descending the group from \( NH_3 \) to \( BiH_3 \), we need to analyze the properties of the hydrides of group 15 elements (Nitrogen, Phosphorus, Arsenic, Antimony, and Bismuth).

### Step-by-Step Solution:

1. **Identify the Hydrides**: The hydrides in question are \( NH_3 \) (Ammonia), \( PH_3 \) (Phosphine), \( AsH_3 \) (Arsine), \( SbH_3 \) (Stibine), and \( BiH_3 \) (Bismuthine).

2. **Consider Thermal Stability**: As we move down the group from \( NH_3 \) to \( BiH_3 \), the thermal stability of these hydrides decreases. This is because the bond strength between the central atom and hydrogen decreases as the size of the central atom increases, making the hydrides less stable.

3. **Analyze Reducing Power**: The reducing power of these hydrides generally increases down the group. This is due to the increasing size and decreasing electronegativity of the central atom, which allows for easier release of hydrogen.

4. **Evaluate Ease of Replacing Hydrogen**: The ease of replacing hydrogen atoms with other groups (like chlorine or methyl) tends to decrease down the group. This is because the larger central atoms are less effective at forming stable bonds with other groups.

5. **Assess Lone Pair Donating Ability**: The ability to donate lone pairs also decreases down the group. This is because the lone pairs on the central atom become less available for donation as the size of the atom increases.

6. **Conclusion**: The property that decreases as we descend the group from \( NH_3 \) to \( BiH_3 \) is **thermal stability**.

### Final Answer:
**Thermal stability decreases for \( MH_3 \) on descending the group from \( NH_3 \) to \( BiH_3 \).**