Carbon dioxide is nonpolar because its bonds are symmetrical, according to Elmhurst College's Virtual Chembook. The bonds between the carbon and two oxygen atoms are polar; however, the entire molecule is nonpolar because the partial charges cancel each other.
The bonds in carbon dioxide are polar because carbon and oxygen have a difference in electronegativity of more than 0.4. Each bond has a dipole moment, which means that the electrons are not shared equally. In carbon dioxide, the oxygen atoms are more electronegative, so the electrons spend more time on the side with the oxygen atoms. The entire molecule is nonpolar because the bonds are linear, and the dipole moments point in opposite directions. This linear configuration cancels out the dipole moments, leaving a net polarity of zero.