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.
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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.
Sources
- 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
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