Meaning of molecular polarity
Molecular polarity describes whether a molecule has a nonzero net electric dipole moment. A molecule is polar if the vector sum of its bond dipoles (and any lone-pair contributions to electron density) produces a nonzero net dipole; it is nonpolar if those dipoles cancel by symmetry.
A compact way to express the rule is: \[ \vec{\mu}_{\text{net}} = \sum_i \vec{\mu}_i \] If \(\vec{\mu}_{\text{net}} = \vec{0}\), the molecule is nonpolar; otherwise it is polar.
Reliable procedure for deciding polar vs nonpolar
- Draw the correct Lewis structure (connectivity and lone pairs matter).
- Determine the molecular geometry (shape), not just electron-domain geometry.
- Mark bond dipoles toward the more electronegative atom (direction of electron density shift).
- Check symmetry and add vectors: symmetric shapes with identical surrounding atoms often cancel; asymmetric shapes often do not.
Apply the procedure to common molecules
The list below tests molecular polarity using standard shapes and symmetry: CO2, H2O, BF3, NH3, CH4, and SO2.
| Molecule | Lewis/VSEPR shape (molecular geometry) | Are individual bonds polar? | Do dipoles cancel? | Molecular polarity | Key reason |
|---|---|---|---|---|---|
| CO2 | Linear (O=C=O) | Yes (C–O) | Yes | Nonpolar | Two equal C–O dipoles are opposite along a line → net \(\vec{\mu}=\vec{0}\). |
| H2O | Bent (two lone pairs on O) | Yes (O–H) | No | Polar | Bent geometry prevents cancellation; dipoles add to a nonzero net dipole. |
| BF3 | Trigonal planar | Yes (B–F) | Yes | Nonpolar | Three identical bond dipoles separated by 120° cancel by symmetry. |
| NH3 | Trigonal pyramidal (one lone pair on N) | Yes (N–H) | No | Polar | Lone pair makes the shape pyramidal; dipoles do not cancel. |
| CH4 | Tetrahedral | Effectively very weak (C–H) | Yes | Nonpolar | Highly symmetric tetrahedral arrangement cancels dipoles (net \(\vec{\mu}=\vec{0}\)). |
| SO2 | Bent (one lone pair on S in a trigonal-planar electron geometry) | Yes (S–O) | No | Polar | Bent geometry yields a nonzero vector sum of S–O dipoles. |
Visualization: vector cancellation vs non-cancellation
Final classification
Molecular polarity is controlled by both bond polarity and geometry: CO2, BF3, and CH4 are nonpolar because symmetry cancels dipoles, while H2O, NH3, and SO2 are polar because their shapes prevent complete cancellation.