BASIC CHEM! How can some molecules containing polar covalent bonds be nonpolar?

A molecule being polar basically means the electrons are concentrated in a certain area, with another area lacking electrons. An example of a polar molecule is water, which has a partial positive charge near the hydrogen atoms and a partial negative near the oxygen atom. However, a molecule like CCl4, which looks polar if you just look at the EN values, is nonpolar.
This is because of the orientation. There is no net dipole, because the vectors of each C-Cl4 bond cancel each other out. Symmetry is a second characteristic to observe if you want to determine polarity. If the geometric center of the positive and negative charges coincide, the molecule is nonpolar.

The answer lies with the geometry of the molecule. If the centers of positive and negative poles coincide, then the molecule will be nonpolar even if it has polar bonds. Such is the case with methane, CH4.

Metanes molecular geometry is in the form of a perfect tetrahedron. The H-C bonds lie at 110 degrees from each other so that the geometric center of the positive and negative regions cancel each other out.

Go to this website for some good illustrations.

http://wwwarpe.snv.jussieu.fr/td_2_eng/geo.html

Hope this helps!