Steve Marsden’s

We have already pointed out on several occasions that electrons in covalent bonds are not generally shared equally by the atoms involved. Certainly in the case of diatomic molecules there is equal sharing, but most atoms will not have the same electronegativity and thus there will be some kind of displacement of electron density.

These electron displacements create slight charge separations across covalent bonds. In the most extreme case we would be talking about an ionic bond, but our main attention here is on molecular substances.

The charge separations across covalent bonds are not integers but rather fractional charges. The result is called a bond moment and is sometimes represented by an arrow over the bond, pointing toward the more electronegative atom. Such a bond is called polar covalent. The word polar here is used in the sense of positive and negative poles. In a simple molecule with only one bond, the molecule itself is said to be polar or to possess a molecular or "net" dipole.