We calculate the formal charge of an atom in a molecule or polyatomic ions as follows: Formal Charge = (valence electrons of the "free" element) - (unshared electrons) - (bonds). We can double-check formal charge calculations by determining the sum of the formal charges for the whole structure. Identifying a formal charge involves: Determining the appropriate number of valence electrons for an atom – This can be accomplished by inspecting the Determining whether the atom exhibits the appropriate number of electrons – In the Lewis structure, determine whether.