Question

Dipole-Dipole Forces Question?
What factors determine if a molecule is polar?

Answer 1

Electronegativity and molecular shape. For a bond in a molecule to be polar, the two elements need to have different electronegativities, so that the electrons are pulled more toward one atom than the other. If you don't have polar bonds (which is rare in compounds), then there's no hope of having a dipole on the molecule. Symmetrical molecules like H2, Cl2 or O2 are non-polar, because the two atoms forming the bond have identical electronegativities (of course!).

In compounds, if you have polar bonds, you don't necessarily have a dipole for the molecule... CO2 is a good example. The bonds are polar (because O is more electronegative than C), but they're exactly 180 degrees apart. So the dipole in one direction (to the right, for example) is exactly cancelled out by the dipole to the left. You don't have a direction you can point in that says "this direction is more negative or positive than another". A bent molecule like water IS polar, however, because the bonds are not 180deg apart, and the dipoles don't cancel.

Answer 2

http://www.answers.com/Q/How_do_you_determine_if_a_molecule_is_polar_or_non-polar

This website is also useful too.