In which of the following set of compounds , bond angle remains constant for all members ?

Options

A. `NH_(3) , PH_(3) , AsH_(3)`
B. `PF_(3) , PCl_(3) , PBr_(3)`
C. `OH_(2) , OF_(2) , O Cl_(2)`
D. `BF_(3) , BCl_(3) , BBr_(3)`

Answer

To determine which set of compounds has a constant bond angle for all members, we need to analyze the hybridization and the presence of lone pairs in each option. Here’s a step-by-step solution:

### Step 1: Understand the Concept of Bond Angles
The bond angle in a molecule is influenced by its geometry, which is determined by the hybridization of the central atom and the presence of lone pairs. Different hybridizations have characteristic bond angles:
- **sp** hybridization: 180°
- **sp²** hybridization: 120°
- **sp³** hybridization: 109.5°

### Step 2: Analyze Each Option
We will analyze the given options one by one to check for hybridization and lone pairs.

#### Option 1: NH3, PH3, ASH3
- **Central Atom**: Varies (N, P, As)
- **Hybridization**: All are sp³ (Z = 4)
- **Lone Pairs**: Each has 1 lone pair.
- **Bond Angle**: Due to the presence of lone pairs, the bond angles will not be constant (less than 109.5°).

#### Option 2: PF3, PCl3, PBr3
- **Central Atom**: Phosphorus (P)
- **Hybridization**: sp³ (Z = 4)
- **Lone Pairs**: Each has 1 lone pair.
- **Bond Angle**: The bond angles will vary due to the presence of lone pairs.

#### Option 3: H2O, OF2, OCl2
- **Central Atom**: Oxygen (O)
- **Hybridization**: sp³ (Z = 4)
- **Lone Pairs**: Each has 2 lone pairs.
- **Bond Angle**: The bond angles will not be constant due to the presence of lone pairs.

#### Option 4: BF3, BCl3, BBr3
- **Central Atom**: Boron (B)
- **Hybridization**: sp² (Z = 3)
- **Lone Pairs**: No lone pairs.
- **Bond Angle**: All have a bond angle of approximately 120° due to sp² hybridization, which remains constant.

### Step 3: Conclusion
After analyzing all options, we find that the only set of compounds where the bond angle remains constant for all members is **Option 4: BF3, BCl3, BBr3**.

### Final Answer
The correct option is **Option 4**.

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