Finding Formal Charge
Formal charge indicates which Lewis Structure (when there are several resonance structures) is the most favorable or predominant form. To calculate formal charge for each atom in a Lewis Structure we use a simple equation. The structure with formal charges closest to zero is considered the best/predominant structure. Formal charge assumes the bonds are entirely covalent (oxidation number assumes they are entirely ionic).
Calculating Formal Charge
To calculate formal charge we use this equation:
V = number of valence electrons for neutral elementN = number of non-bonding valence electrons around the element in the Lewis Structure
B = number of bonding valence electrons around the element in the Lewis Structure
Note: you may see a slightly different version of this equation, but they all give the same answer.