Which of the following is the correct order of bond angle in `H_2S`, `NH_3`, `BF_3` and `SiH_4`?

Options

A. `H_2Slt NH_3lt BF_3lt SiH_4`
B. `NH_3lt H_2Slt SiH_4lt BF_3`
C. ` H_2Slt NH_3lt SiH_4lt BF_3`
D. ` H_2Slt SiH_4lt NH_3lt BF_3`

Answer

To determine the correct order of bond angles in the molecules \( H_2S \), \( NH_3 \), \( BF_3 \), and \( SiH_4 \), we will analyze the molecular geometry and the presence of lone pairs around the central atom in each molecule.

### Step-by-Step Solution:

1. **Identify the Molecular Geometry**:
- **\( H_2S \)**: The molecular geometry is bent due to the presence of two lone pairs on sulfur. The bond angle is approximately \( 104.5^\circ \).
- **\( NH_3 \)**: Ammonia has a trigonal pyramidal shape due to one lone pair on nitrogen. The bond angle is approximately \( 107^\circ \).
- **\( BF_3 \)**: Boron trifluoride has a trigonal planar geometry with no lone pairs on boron. The bond angle is approximately \( 120^\circ \).
- **\( SiH_4 \)**: Silane has a tetrahedral shape with no lone pairs on silicon. The bond angle is approximately \( 109.5^\circ \).

2. **Consider the Effect of Lone Pairs**:
- Lone pairs repel more than bonding pairs, which reduces the bond angles in molecules with lone pairs.
- In \( H_2S \), with two lone pairs, the bond angle is smaller.
- In \( NH_3 \), with one lone pair, the bond angle is slightly smaller than \( 109.5^\circ \).
- In \( SiH_4 \) and \( BF_3 \), there are no lone pairs, allowing for larger bond angles.

3. **Order the Bond Angles**:
- The bond angles from largest to smallest are:
- \( BF_3 \) (approximately \( 120^\circ \))
- \( SiH_4 \) (approximately \( 109.5^\circ \))
- \( NH_3 \) (approximately \( 107^\circ \))
- \( H_2S \) (approximately \( 104.5^\circ \))

4. **Final Order**:
- Therefore, the correct order of bond angles is:
\[ BF_3 > SiH_4 > NH_3 > H_2S \]

### Conclusion:
The correct order of bond angles in \( H_2S \), \( NH_3 \), \( BF_3 \), and \( SiH_4 \) is \( BF_3 > SiH_4 > NH_3 > H_2S \).