Which is in the decreasing order of boiling points of hydrides? 

Options

A. `NH_(3) gt PH_(3) gt AsH_(3) gt SbH_(3)`
B. `SbH_(3) gt AsH_(3) gt PH_(3) gt NH_(3)`
C. `PH_(3) gt NH_(3) gt AsH_(3) gt SbH_(3)`
D. `SbH_(3) gt NH_(3) gt AsH_(3) gt PH_(3)`

Answer

To determine the decreasing order of boiling points of the hydrides of Group 15 elements (VA group), we will analyze the hydrides: ammonia (NH₃), phosphine (PH₃), arsine (AsH₃), and stibine (SbH₃).

### Step-by-Step Solution:

1. **Understand the Factors Affecting Boiling Points**:
- The boiling point of a substance is influenced by the strength of intermolecular forces. Stronger intermolecular forces lead to higher boiling points.

2. **Identify the Types of Intermolecular Forces**:
- In ammonia (NH₃), hydrogen bonding occurs due to the presence of a highly electronegative nitrogen atom bonded to hydrogen. This results in strong intermolecular attractions.
- In phosphine (PH₃), arsine (AsH₃), and stibine (SbH₃), the primary intermolecular forces are van der Waals forces, which are generally weaker than hydrogen bonds.

3. **Compare the Molecular Sizes**:
- As we move down the group from NH₃ to SbH₃, the size of the molecules increases (N < P < As < Sb). Larger molecules have greater surface area, which increases van der Waals forces.

4. **Analyze the Boiling Points**:
- **Ammonia (NH₃)**: Due to hydrogen bonding, it has a higher boiling point than expected based solely on molecular size.
- **Phosphine (PH₃)**: Weaker van der Waals forces compared to NH₃, so it has a lower boiling point.
- **Arsine (AsH₃)**: Larger than PH₃, thus has stronger van der Waals forces, leading to a higher boiling point than PH₃.
- **Stibine (SbH₃)**: Largest molecule among the four, thus it has the strongest van der Waals forces and the highest boiling point.

5. **Establish the Order**:
- Based on the analysis, the order of boiling points from highest to lowest is:
- **SbH₃ > NH₃ > AsH₃ > PH₃**

### Final Answer:
The decreasing order of boiling points of the hydrides is:
**SbH₃ > NH₃ > AsH₃ > PH₃**