Formal charges help in the selection of the lowest energy structure from a number of possible Lewis structures for a given species. Generally the lowest energy structure is the one with the smallest formal charges on the atoms. Formal charge on an atom : = total no. of valence electrons-non-bonding electrons −21× bonding electrons. For Lewis structure of SO3 
Formal charge on S atom =6−0−21×12=0 Formal charge on three O atoms =6−4−21×4=0



