A molecule is polar if it has an overall net dipole moment. This depends on two factors:
- Whether the molecule contains polar bonds (ΔEN ≠ 0)
- Whether the molecular geometry allows dipoles to cancel
Polar vs Non-Polar Molecules
| Molecule | Polar Bonds? | Shape | Dipoles Cancel? | Overall Polarity |
|---|---|---|---|---|
| CO₂ | Yes (C=O) | Linear | Yes - symmetrical | Non-polar |
| BF₃ | Yes (B-F) | Trigonal planar | Yes - symmetrical | Non-polar |
| CCl₄ | Yes (C-Cl) | Tetrahedral | Yes - symmetrical | Non-polar |
| H₂O | Yes (O-H) | Bent | No - lone pairs | Polar |
| NH₃ | Yes (N-H) | Trigonal pyramidal | No - lone pair | Polar |
| CHCl₃ | Yes (C-H, C-Cl) | Tetrahedral | No - different atoms | Polar |
Decision Rule
The Symmetry & Dipole Test
Step 1: Are all terminal atoms identical? Step 2: Is the geometry symmetrical (no lone pairs on central atom)? If BOTH are yes → non-polar. If either is no → likely polar.
Examiner Trap
Explaining "Like Dissolves Like" in Mark Schemes
Polar molecules dissolve in polar solvents (water). Non-polar molecules dissolve in non-polar solvents (hexane). This is the principle behind chromatography separations (2.2.10).
AQA GCSE & IB Chemistry
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