A molecule can be determined to be polar or nonpolar in two steps, which means putting the molecule through the Lewis Structure to determine its shape and determine if that shape is symmetrical or nonsymmetrical. The Lewis structure is used to show where and how the various atoms that make up a molecule connect. The Lewis Structure can help give visual clues to the polarity of a molecule because polarity is normally found in atoms with uneven electron distribution.
Once the molecule being studied has been laid out in the Lewis Structure, it is used to determine the shape of the molecule. A molecule whose shape is not symmetrical is considered polar because it has the associated uneven electron distribution. The second step, determining the symmetry of a molecule, also allows it to be determined if the atoms on the outer edge of the molecule are the same. If the atoms on the outer edge are all the same, then the molecule is nonpolar. If the atoms on the outer edge are different, even if the shape of the molecule is symmetrical it can still be a polar molecule because the differences in the edge atoms cause the charge to be uneven. These outer molecules are why using both steps in the process are important.