The correct order of bond angles (smallest first) in `H_2 S, NH_3, BF_3 and SiH_4` is

Options

A. `H_2S` lt `SiH_4` lt `NH_3` lt `BF_3`
B. `H_2S` lt `NH_3` lt `BF_3` lt `SiH_4`
C. `H_2`lt `NH_3`lt `SiH_4`lt`BF_3`
D. `NH_3` lt `H_2 S` lt `SiH_4` lt `BF_3`

Explanation

To determine the correct order of bond angles (from smallest to largest) in the molecules H₂S, NH₃, BF₃, and SiH₄, we will follow these steps:

### Step 1: Identify the central atom and its valence electrons
- **H₂S (Hydrogen Sulfide)**: Sulfur (S) is the central atom. It has 6 valence electrons.
- **NH₃ (Ammonia)**: Nitrogen (N) is the central atom. It has 5 valence electrons.
- **BF₃ (Boron Trifluoride)**: Boron (B) is the central atom. It has 3 valence electrons.
- **SiH₄ (Silane)**: Silicon (Si) is the central atom. It has 4 valence electrons.

### Step 2: Calculate the steric number for each molecule
The steric number is calculated as the number of valence electrons of the central atom plus the number of monovalent atoms attached to it.

- **H₂S**: Steric number = 6 (S) + 2 (H) = 8/2 = 4
- **NH₃**: Steric number = 5 (N) + 3 (H) = 8/2 = 4
- **BF₃**: Steric number = 3 (B) + 3 (F) = 6/2 = 3
- **SiH₄**: Steric number = 4 (Si) + 4 (H) = 8/2 = 4

### Step 3: Determine the hybridization and molecular geometry
- **H₂S**: Steric number 4 → Hybridization: sp³ → Geometry: Tetrahedral (with 2 lone pairs) → Bond angle: ~92° (due to lone pair repulsion)
- **NH₃**: Steric number 4 → Hybridization: sp³ → Geometry: Tetrahedral (with 1 lone pair) → Bond angle: ~107°
- **BF₃**: Steric number 3 → Hybridization: sp² → Geometry: Trigonal planar → Bond angle: 120°
- **SiH₄**: Steric number 4 → Hybridization: sp³ → Geometry: Tetrahedral (no lone pairs) → Bond angle: 109°

### Step 4: Arrange the bond angles in order from smallest to largest
Now we can summarize the bond angles:
- H₂S: ~92°
- NH₃: ~107°
- SiH₄: 109°
- BF₃: 120°

### Final Order of Bond Angles (Smallest First)
The correct order of bond angles from smallest to largest is:
1. H₂S (~92°)
2. NH₃ (~107°)
3. SiH₄ (109°)
4. BF₃ (120°)