Introduced to this in AP Chemistry
What is it?
Formal charge is an electron bookkeeping method used to evaluate non-equivalent Lewis Structures
Formal charge must be calculated for each atom in the structure.
The sum of the formal charges must equal the overall charge of the species.
Formula
FC = (# valence electrons) - (# of bonds) - (# of non-binding electrons)
Rules
In AP Chemistry formal charges are used to evaluate non-equivalent Lewis Structures
Best structures have charged as close to 0 as possible
Structures where all the formal charges are 0 are preferred to structures with non-zero formal charges.
Structures with smaller formal charge magnitudes are preferred to structures with larger formal charge magnitudes.
Negative formal charges should be placed on electronegative elements
If negative formal charges are present in the structure, the preferred structure has the negative formal charge on the most electronegative atom in the structure.