In this case, the ionic structures make only a small contribution to the resonance
hybrid; the bond is almost purely covalent. Moreover, the two ionic structures have
the same energy and make equal contributions to the hybrid; so the average charge
on each atom is zero. However, in a molecule composed of different elements, such
as HCl, the resonance has unequal contributions from the two ionic structures. The lower-energy ionic
structure is H Cl Because the chlorine atom has a greater attraction for electrons
than does the hydrogen atom, the structure with the negative charge on the Cl
atom makes a bigger contribution than H
. As a result, there is a small net
negative charge on the Cl atom and a small net positive charge on the H atom.
Here we see the limitations of using formal charge alone to determine the distribution
of electrons in a structure. The formal charge on each atom in HCl is zero.
The charges on the atoms in HCl are called partial charges. We show the partial
charges on the atoms by writing HßCl . A bond in which ionic contributions
to the resonance result in partial charges is called a polar covalent bond. All
bonds between atoms of different elements are polar to some extent. The bonds in
homonuclear (same-element) diatomic molecules and ions are nonpolar.
The two atoms in a polar covalent bond form an electric dipole, a partial positive
charge next to an equal but opposite partial negative charge. In the original