The concept of formal charge is a very useful one that is essentially a way of keeping track of electrons. The formal charge is the apparent electronic charge of each atom in a molecule, based on the electron-dot structure. Formal charges help assess resonance structures and molecular topology, and they are presented here as a simplified method of describing structures, just as the Bohr model is a simple method of describing electronic configurations in atoms. Both of these methods have limitations, and other approaches are more accurate, but they can be useful as long as their imperfections are kept in mind.
Determination of Formal Charge
This is done according to the following procedure:
1. Any unshared pairs of electrons belong to the atom on which the electrons are located.
2. Shared electron pairs are divided equally between the atoms sharing them.
3. The total number of electrons on an atom in a structure is the sum from steps 1 and 2.
4. Compare the total number of electrons that appear to be on each atom to the number of valence shell electrons that it normally has. If the number of electrons in the valence shell is greater than indicated in step 3, the atom appears to have lost one or more electrons and has a positive formal charge. If the number indicated in step 3 is larger than the number in the valence shell, the atom appears to have gained one or more electrons and has a negative formal charge.
5. Structures that have formal charges with the same sign on adjacent atoms will contribute little to the true structure.
6. The sum of formal charges on the atoms must total the overall charge on the species.
Application of Formal Charges
Formal charges can help in eliminating resonance structures expected to contribute very little to the electronic ground state of the molecule, and, in some cases, suggesting multiple bonds beyond those required by the octet rule. It is, however, essential to remember that a formal charge is only a tool for assessing the Lewis structures, not a measure of any actual charge on the atoms. The number of valence electrons available in a free atom of an element minus the total for that atom in the molecule—determined by counting lone pairs as two electrons and bonding pairs as one electron assigned to each atom—is the formal charge on the atom:
Charge on molecule or ion = sum of formal charges
Resonance structures that contribute more to the electronic ground state of the species generally (1) have smaller magnitudes of formal charges, (2) place negative formal charges on more electronegative elements (in the upper right-hand part of the periodic table), and (3) have smaller separation of charges. Examples such as SCN -, OCN -, and CNO—will illustrate the use of formal charges in describing the electronic structures.