A nonpolar molecule has no separation of charge, so no positive or negative poles are formed. In other words, the electrical charges of nonpolar molecules are evenly distributed across the molecule. Nonpolar molecules tend to dissolve well in nonpolar solvents, which are frequently organic solvents.
In a polar molecule, one side of the molecule has a positive electrical charge and the other side has a negative electrical charge. Polar molecules tend to dissolve well in water and other polar solvents.
There are also amphiphilic molecules, large molecules that have both polar and nonpolar groups attached to them. Because these molecules have both polar and nonpolar character, they make good surfactants, aiding in mixing water with fats.
Technically, the only completely nonpolar molecules consist of a single type of atom or of different types of atoms that display a certain spatial arrangement. Many molecules are intermediate, neither completely nonpolar nor polar.
What Determines Polarity?
You can predict whether a molecule will be polar or nonpolar by looking at the type of chemical bonds formed between the atoms of the elements. If there is a significant difference between the electronegativity values of the atoms, the electrons won't be shared equally between the atoms. In other words, the electrons will spend more time closer to one atom than the other. The atom that is more attractive to the electron will have an apparent negative charge, while the atom that is less electronegative (more electropositive) will have a net positive charge.
Predicting polarity is simplified by considering the point group of the molecule. Basically, if the dipole moments of a molecule cancel each other out, the molecule is nonpolar. If the dipole moments don't cancel out, the molecule is polar. Not all molecules have a dipole moment. For example, a molecule that has a mirror plane won't have a dipole moment because the individual dipole moments can't lie in more than one dimension (a point).
Nonpolar Molecule Examples
Examples of homonuclear nonpolar molecules are oxygen (O2), nitrogen (N2), and ozone (O3). Other nonpolar molecules include carbon dioxide (CO2) and the organic molecules methane (CH4), toluene, and gasoline. Most carbon compounds are nonpolar. A notable exception is carbon monoxide, CO. Carbon monoxide is a linear molecule, but the electronegativity difference between carbon and oxygen is significant enough to make the molecule polar.
Alkynes are considered nonpolar molecules because they don't dissolve in water.
The noble or inert gases are also considered nonpolar. These gases consist of single atoms of their element, such as argon, helium, krypton, and neon.