Predicting Molecule Polarity
- This classification table gives a good general understanding of predicting molecular dipole of some general molecular structures. However, one should not interpret it literally:
| Formula | Description | Example | |
| Polar | AB | Linear Molecules | CO |
|---|---|---|---|
| HAx | Molecules with a single H | HF | |
| AxOH | Molecules with an OH at one end | C2H5OH | |
| OxAy | Molecules with an O at one end | H2O | |
| NxAy | Molecules with an N at one end | NH3 | |
| Nonpolar | A2 | Diatomic molecules of the same element | O2 |
| CxAy | Most carbon compounds | CO2 |
- Determining the point group is a useful way to predict polarity of a molecule. In general, a molecule will not possess dipole moment, if the individual bond dipole moments of the molecule cancel each other out. This is because dipole moments are euclidean vector quantities with magnitude and direction, and a two equal vectors who oppose each other will cancel out.
Any molecule with an centre of inversion ( "i" ) or a horizontal mirror plane ( "σh ") will not possess dipole moments. Likewise, a molecule with more than one Cn axis will not possess dipole moment because dipole moments cannot lie in more than one dimension. As a consequence of that constraint, all molecules with D symmetry (Schönflies notation) will, therefore, not have dipole moment because, by definition, D point groups have two or multiple Cn axis.
Since C1, Cs,C∞h Cn and Cnv point groups do not have a centre of inversion, horizontal mirror planes or multiple Cn axis, molecules in one of those point groups will have dipole moment.
Read more about this topic: Chemical Polarity
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