Formal charge of FC is the difference between the number of valence electrons of each atom and the number of electrons the atom is associated with. Formal charge assumes any shared electrons are equally shared between the two bonded atoms.
Formal charge is calculated using the equation:
FC = eV - eN - eB/2
eV = number of valence electrons of the atom as if it were isolated from the molecule
eN = number of unbound valence electrons on the atom in the molecule
eB = number of electrons shared by the bonds to other atoms in the molecule
Formal Charge Example Calculation
For example, carbon dioxide or CO2 is a neutral molecule that has 16 valence electrons. There are three different ways to draw the Lewis structure for the molecule to determine formal charge:
- The carbon atom may be joined to both oxygen atom via double bonds (carbon = 0, oxygen = 0, formal charge = 0)
- The carbon atom may have a single bond with one oxygen atom and a double bond to the other oxygen atom (carbon = +1, oxygen-double = 0, oxygen-single = -1, formal charge = 0)
- The carbon atom may be joined to each oxygen atom via single bonds (carbon = +2, oxygens = -1 each, formal charge = 0)
Each possibility results in a formal charge of zero, but the first choice is the best one because it predicts no charge in the molecule. This is more stable and thus is most likely.
See how to calculate formal charge with another example problem.
Formal Charge Key Takeaways
- Formal charge (FC) is the electric charge of an atom in a molecule.
- It is calculated as the number of valence electrons minus half the number of electrons shared in a bond minus the number of electrons not bound in the molecule.
- Formal charge is used to estimate the way electric charge is distributed in a molecule.