Many substances contain bonds that are intermediate in character—between pure covalent and pure ionic bonds. Such polar bonds occur when one of the elements attracts the shared electrons more strongly than the other element. In hydrogen fluoride, for instance, the shared electrons are so much more attracted by fluorine than hydrogen that the sharing is unequal. (See Figure 1.)
Figure 1. Unequal sharing of electrons.
Due to the unequal sharing of the bonding electrons, the two atoms have fractional electrical charges, represented by the Greek letter delta (δ) in Figure 2.
Figure 2. Showing the partial charges of a polar bond.