Which pair of molecules are polar species ?

Options

A. `CO_(2) and H_(2)O`
B. `BF_(3) and PCl_(3)`
C. `SO_(2) and SCl_(2)`
D. `CS_(2) and SO_(3)`

Explanation

To determine which pair of molecules are polar species, we need to analyze the polarity of each molecule based on their molecular geometry and electronegativity differences. A polar molecule has a net dipole moment that is not equal to zero, meaning that the distribution of electron density is uneven.

### Step-by-Step Solution:

1. **Identify the Molecules**: The given pairs of molecules are:
- Pair 1: CO2 and H2O
- Pair 2: BF3 and PCl3
- Pair 3: SO2 and SCl2
- Pair 4: CS2 and SO3

2. **Analyze Pair 1 (CO2 and H2O)**:
- **CO2**: Carbon dioxide has a linear structure (O=C=O). The dipoles from the two C=O bonds cancel each other out, resulting in a non-polar molecule (dipole moment = 0).
- **H2O**: Water has a bent shape due to the two lone pairs on oxygen. The dipoles do not cancel out, resulting in a polar molecule (dipole moment ≠ 0).
- **Conclusion**: Not both are polar; CO2 is non-polar.

3. **Analyze Pair 2 (BF3 and PCl3)**:
- **BF3**: Boron trifluoride has a trigonal planar structure. The dipoles from the three B-F bonds cancel out, making it non-polar (dipole moment = 0).
- **PCl3**: Phosphorus trichloride has a trigonal pyramidal shape due to one lone pair on phosphorus. The dipoles do not cancel, making it polar (dipole moment ≠ 0).
- **Conclusion**: Not both are polar; BF3 is non-polar.

4. **Analyze Pair 3 (SO2 and SCl2)**:
- **SO2**: Sulfur dioxide has a bent shape. The dipoles from the two S=O bonds do not cancel, resulting in a polar molecule (dipole moment ≠ 0).
- **SCl2**: Sulfur dichloride also has a bent shape. The dipoles from the two S-Cl bonds do not cancel, making it polar (dipole moment ≠ 0).
- **Conclusion**: Both SO2 and SCl2 are polar.

5. **Analyze Pair 4 (CS2 and SO3)**:
- **CS2**: Carbon disulfide has a linear structure (S=C=S). The dipoles cancel out, resulting in a non-polar molecule (dipole moment = 0).
- **SO3**: Sulfur trioxide has a trigonal planar structure. The dipoles from the three S=O bonds also cancel out, making it non-polar (dipole moment = 0).
- **Conclusion**: Not both are polar; both are non-polar.

### Final Answer:
The only pair of molecules that are both polar species is **SO2 and SCl2**.

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