Which out of the following pairs has dipole moment ?
(i) `BF_(3) and NCl_(3)` (ii) `H_(2)Oand BF_(3)` (iii) `CO_(2) and H_(2)S`

Answer

To determine which pairs have a dipole moment, we need to analyze the molecular geometry and electronegativity of the atoms involved in each pair. A dipole moment occurs when there is an uneven distribution of electron density, leading to a separation of charge.

### Step-by-Step Solution:

1. **Analyzing BF3 and NCl3:**
- **BF3 (Boron Trifluoride):**
- Geometry: Trigonal planar.
- Boron has three valence electrons and forms three bonds with fluorine.
- The dipoles from each B-F bond point towards the more electronegative fluorine atoms.
- Since the molecule is symmetric, the dipoles cancel each other out.
- **Result:** Dipole moment (μ) = 0.

- **NCl3 (Nitrogen Trichloride):**
- Geometry: Trigonal pyramidal (due to one lone pair on nitrogen).
- Nitrogen has five valence electrons and forms three bonds with chlorine.
- The dipoles from each N-Cl bond point towards the more electronegative chlorine atoms.
- The lone pair on nitrogen creates an asymmetry, so the dipoles do not cancel out completely.
- **Result:** Dipole moment (μ) ≠ 0.

**Conclusion for (i):** NCl3 has a dipole moment, while BF3 does not.

2. **Analyzing H2O and BF3:**
- **H2O (Water):**
- Geometry: Bent (due to two lone pairs on oxygen).
- Oxygen is more electronegative than hydrogen, creating a dipole towards oxygen.
- The bent shape means the dipoles do not cancel out.
- **Result:** Dipole moment (μ) ≠ 0.

- **BF3:** (already analyzed)
- **Result:** Dipole moment (μ) = 0.

**Conclusion for (ii):** H2O has a dipole moment, while BF3 does not.

3. **Analyzing CO2 and H2S:**
- **CO2 (Carbon Dioxide):**
- Geometry: Linear.
- Carbon forms double bonds with two oxygen atoms.
- The dipoles from each C=O bond point towards the oxygen atoms.
- The linear shape means the dipoles are equal in magnitude but opposite in direction, cancelling each other out.
- **Result:** Dipole moment (μ) = 0.

- **H2S (Hydrogen Sulfide):**
- Geometry: Bent (similar to H2O).
- Sulfur is less electronegative than oxygen but still creates a dipole towards sulfur.
- The bent shape means the dipoles do not cancel out completely.
- **Result:** Dipole moment (μ) ≠ 0.

**Conclusion for (iii):** H2S has a dipole moment, while CO2 does not.

### Final Summary:
- **Pairs with dipole moments:**
- (i) NCl3
- (ii) H2O
- (iii) H2S