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The two main classes of molecules are polar molecules and nonpolar molecules. Some molecules are clearly polar or nonpolar, while many have some polarity and fall somewhere in between. Here's a look at what polar and nonpolar mean, how to predict whether a molecule will be one or the other, and examples of representative compounds.
Key Takeaways: Polar and Nonpolar
- In chemistry, polarity refers to the distribution of electric charge around atoms, chemical groups, or molecules.
- Polar molecules occur when there is an electronegativity difference between the bonded atoms.
- Nonpolar molecules occur when electrons are shared equal between atoms of a diatomic molecule or when polar bonds in a larger molecule cancel each other out.
Polar molecules occur when two atoms do not share electrons equally in a covalent bond. A dipole forms, with part of the molecule carrying a slight positive charge and the other part carrying a slight negative charge. This happens when there is a difference between the electronegativity of each atom. An extreme difference forms an ionic bond, while a lesser difference forms a polar covalent bond. Fortunately, you can look up electronegativity on a table to predict whether or not atoms are likely to form polar covalent bonds. If the electronegativity difference between the two atoms is between 0.5 and 2.0, the atoms form a polar covalent bond. If the electronegativity difference between the atoms is greater than 2.0, the bond is ionic. Ionic compounds are extremely polar molecules.
Examples of polar molecules include:
- Water - H2O
- Ammonia - NH3
- Sulfur dioxide - SO2
- Hydrogen sulfide - H2S
- Ethanol - C2H6O
Note ionic compounds, such as sodium chloride (NaCl), are polar. However, most of the time when people talk about "polar molecules" they mean "polar covalent molecules" and not all types of compounds with polarity!
When molecules share electrons equally in a covalent bond there is no net electrical charge across the molecule. In a nonpolar covalent bond, the electrons are evenly distributed. You can predict nonpolar molecules will form when atoms have the same or similar electronegativity. In general, if the electronegativity difference between two atoms is less than 0.5, the bond is considered nonpolar, even though the only truly nonpolar molecules are those formed with identical atoms.
Nonpolar molecules also form when atoms sharing a polar bond arrange such that the electric charges cancel each other out.
Examples of nonpolar molecules include:
- Any of the noble gasses: He, Ne, Ar, Kr, Xe (These are atoms, not technically molecules.)
- Any of the homonuclear diatomic elements: H2, N2, O2, Cl2 (These are truly nonpolar molecules.)
- Carbon dioxide - CO2
- Benzene - C6H6
- Carbon tetrachloride - CCl4
- Methane - CH4
- Ethylene - C2H4
- Hydrocarbon liquids, such as gasoline and toluene
- Most organic molecules
Polarity and Mixing Solutions
If you know the polarity of molecules, you can predict whether or not they will mix together to form chemical solutions. The general rule is that "like dissolves like", which means polar molecules will dissolve into other polar liquids and nonpolar molecules will dissolve into nonpolar liquids. This is why oil and water don't mix: oil is nonpolar while water is polar.
It's helpful to know which compounds are intermediate between polar and nonpolar because you can use them as an intermediate to dissolve a chemical into one it wouldn't mix with otherwise. For example, if you want to mix an ionic compound or polar compound in an organic solvent, you may be able to dissolve it in ethanol (polar, but not by a lot). Then, you can dissolve the ethanol solution into an organic solvent, such as xylene.
- Ingold, C. K.; Ingold, E. H. (1926). "The Nature of the Alternating Effect in Carbon Chains. Part V. A Discussion of Aromatic Substitution with Special Reference to Respective Roles of Polar and Nonpolar Dissociation; and a Further Study of the Relative Directive Efficiencies of Oxygen and Nitrogen". J. Chem. Soc.: 1310-1328. doi:10.1039/jr9262901310
- Pauling, L. (1960). The Nature of the Chemical Bond (3rd ed.). Oxford University Press. pp. 98-100. ISBN 0801403332.
- Ziaei-Moayyed, Maryam; Goodman, Edward; Williams, Peter (November 1,2000). "Electrical Deflection of Polar Liquid Streams: A Misunderstood Demonstration". Journal of Chemical Education. 77 (11): 1520. doi:10.1021/ed077p1520