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 \( H_2S \), \( NH_3 \), \( BF_3 \), and \( SiH_4 \), we can follow these steps:

### Step 1: Identify the Hybridization
1. **For \( H_2S \)**:
- Sulfur (S) has 2 hydrogen atoms bonded to it and 2 lone pairs.
- Steric number = Number of sigma bonds + Number of lone pairs = 2 + 2 = 4.
- Hybridization = \( sp^3 \).

2. **For \( NH_3 \)**:
- Nitrogen (N) has 3 hydrogen atoms bonded to it and 1 lone pair.
- Steric number = 3 + 1 = 4.
- Hybridization = \( sp^3 \).

3. **For \( BF_3 \)**:
- Boron (B) has 3 hydrogen atoms bonded to it and no lone pairs.
- Steric number = 3 + 0 = 3.
- Hybridization = \( sp^2 \).

4. **For \( SiH_4 \)**:
- Silicon (Si) has 4 hydrogen atoms bonded to it and no lone pairs.
- Steric number = 4 + 0 = 4.
- Hybridization = \( sp^3 \).

### Step 2: Determine the Molecular Geometry
1. **\( H_2S \)**: Bent shape due to 2 lone pairs.
2. **\( NH_3 \)**: Trigonal pyramidal shape due to 1 lone pair.
3. **\( BF_3 \)**: Trigonal planar shape.
4. **\( SiH_4 \)**: Tetrahedral shape.

### Step 3: Identify the Bond Angles
1. **\( H_2S \)**: The bond angle is approximately \( 104.5^\circ \).
2. **\( NH_3 \)**: The bond angle is approximately \( 107^\circ \).
3. **\( BF_3 \)**: The bond angle is \( 120^\circ \).
4. **\( SiH_4 \)**: The bond angle is \( 109.5^\circ \).

### Step 4: Order the Bond Angles
Now, we can arrange the bond angles in increasing order:
- \( H_2S \) (104.5°) < \( NH_3 \) (107°) < \( SiH_4 \) (109.5°) < \( BF_3 \) (120°).

### Final Answer
The correct order of bond angles is:
\[ H_2S < NH_3 < SiH_4 < BF_3 \]