Which of the following is the correct order of bond-angle ?

Options

A. `NH_(3) lt CH_(4) lt C_(2)H_(2) lt H_(2)O`
B. `C_(2)H_(2)lt NH_(3) lt H_(2)Olt CH_(4)`
C. `NH_(3) lt H_(2)Olt CH_(4) lt C_(2)H_(2)`
D. `H_(2)Olt NH_(3) lt CH_(4) lt C_(2)H_(2)`

Answer

To determine the correct order of bond angles for the given species (NH3, CH4, C2H2, and H2O), we will analyze each molecule step by step.

### Step 1: Analyze NH3 (Ammonia)
1. **Valence Electrons**: Nitrogen has 5 valence electrons, and each hydrogen has 1 valence electron.
2. **Bonding**: Nitrogen forms 3 bonds with hydrogen using 3 of its valence electrons, leaving 1 lone pair.
3. **Steric Number**: Total = 3 bond pairs + 1 lone pair = 4.
4. **Hybridization**: sp³.
5. **Geometry**: Tetrahedral, but due to the lone pair, the shape is pyramidal.
6. **Bond Angle**: The ideal tetrahedral angle is 109.5°, but due to lone pair-bond pair repulsion, the angle is reduced to approximately 107°.

### Step 2: Analyze CH4 (Methane)
1. **Valence Electrons**: Carbon has 4 valence electrons, and each hydrogen has 1 valence electron.
2. **Bonding**: Carbon forms 4 bonds with hydrogen using all its valence electrons.
3. **Steric Number**: Total = 4 bond pairs + 0 lone pairs = 4.
4. **Hybridization**: sp³.
5. **Geometry**: Tetrahedral.
6. **Bond Angle**: The bond angle is 109.5° (ideal tetrahedral angle).

### Step 3: Analyze C2H2 (Ethyne)
1. **Valence Electrons**: Each carbon has 4 valence electrons, and each hydrogen has 1 valence electron.
2. **Bonding**: Each carbon forms one sigma bond with hydrogen and a triple bond with the other carbon, which counts as one bond pair.
3. **Steric Number**: Total = 2 bond pairs + 0 lone pairs = 2.
4. **Hybridization**: sp.
5. **Geometry**: Linear.
6. **Bond Angle**: The bond angle is 180°.

### Step 4: Analyze H2O (Water)
1. **Valence Electrons**: Oxygen has 6 valence electrons, and each hydrogen has 1 valence electron.
2. **Bonding**: Oxygen forms 2 bonds with hydrogen using 2 of its valence electrons, leaving 2 lone pairs.
3. **Steric Number**: Total = 2 bond pairs + 2 lone pairs = 4.
4. **Hybridization**: sp³.
5. **Geometry**: Tetrahedral, but the shape is bent due to lone pairs.
6. **Bond Angle**: The ideal angle is 109.5°, but due to lone pair-lone pair and lone pair-bond pair repulsion, the angle is reduced to approximately 104.5°.

### Step 5: Summarize Bond Angles
- NH3: 107°
- CH4: 109.5°
- C2H2: 180°
- H2O: 104.5°

### Step 6: Order the Bond Angles
From the calculated bond angles, we can arrange them in increasing order:
1. H2O (104.5°)
2. NH3 (107°)
3. CH4 (109.5°)
4. C2H2 (180°)

### Final Answer
The correct order of bond angles is: **H2O < NH3 < CH4 < C2H2**.

---