Formal charge is used to describe the hypothetical charge of an individual atom in a compound.
Lewis Dot structures are 2D diagrams that allow us to visualize the structure of molecular compounds (nonmetal + nonmetal). Recall that atoms in a molecular compound are neutral. So, keep in mind that the formal charge of an atom does not describe its true charge.
Formal charge is extremely important when it comes to Lewis Dot structures. It allows us to evaluate the stability of a structure and verify that it is correct.
F.C. = V.E. – (Bonds + Dots)
– F.C. = formal charge of the atom
– V.E. = valence electrons (of the element in its neutral, nonbonded state)
– Bonds = the # of bonds on that atom
– Dots = the # of lone pair electrons (non-bonding electrons)
Let’s do an example calculating formal charge ↴
The sum of the formal charges of each atom must add up to the overall charge of the molecule.
Let’s check this with the CO₃²⁻ example ↴
We have a carbon with an F.C. of 0, an oxygen with an F.C. of 0, and two oxygen with an F.C. of -1.
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