Bonds between atoms with differing electronegativity are polar: the
atom with the higher electronegativity has more electron density around it, and
thus a partial negative charge: the other atom has less electron density and
thus a partial positive charge. The only non-polar bonds are those between the
same atoms: H-H bonds, for example: all others are at least slightly polar, with
the polarity increasing as the electrnegativity difference increases.
The Chime models below give a graphical model of this phenomenom. The density
of dots around the nucleus indicates electron density: areas with higher
electron density have more dots. Compare the H2 molecule (nonpolar)
to the HF molecule (polar) to the LiF molecule (strongly polar) As the
electronegativity difference increases, more and more electron density moves to
the more electronegative atom, making the bond more polar.