What does electronegativity have to do with bond polarity?
Electronegativity is an element's attraction towards electrons in a covalent bond. Polar bonds result from that uneven sharing of those electrons.
Think of it as a custody arrangement. For an H-H bond, the atoms have the same electronegativity value so the electrons in the covalent bond are shared evenly (i.e. 50-50 custody). For an H-F bond, F is more electronegative than H and pulls harder on those electrons so the electron density is centered closer to the F atom than the H atom. (i.e. week/weekend split in custody).
F has the highest electronegativity and there is a periodic trend in these values. As a result, the polarity of two bonds can be compared by looking at the differences in their electronegativity.
- H-F difference in electronegativity = 1.9
- O-F difference in electronegativity = 0.5
So, H-F is the more polar bond. When you differences in electronegativity greater than ~2, you have ionic compounds where the electrons are being transferred, not shared as they are in covalent bonds.