How do electronegativity values help determining the polarity of a bond?

Electronegativity is a measure of how well an atom in a molecule will attract electrons from a bond. The higher an atom's electronegativity, the better it is at attracting electrons from a bond and the more time the electrons will want to spend on that atom. To determine how polar a bond is, you need to look at the electronegativities of the two bonded atoms. The difference in electronegativity will indicate how polar the bond is: If you have a large electronegativity difference then one atom (the one with the higher electronegativity) will attract the electrons from the bond more, so the electrons will spend a disproportionate amount of time on that atom giving it a partial negative charge and the atom it's bonded to a partial positive charge. If you have a small electronegativity difference then neither atom will be able to draw the electrons away from the bond that well and so the bond will only be slightly polar or not polar at all if the electronegativities are the same.