Formal charge is a calculation that requires a correctly drawn lewis structure. If you have the correctly drawn lewis structure then the sum of the formal charge of all the atoms in a molecule or ion, should equal the charge on your molecule or ion.
- Formal Charge (element A)
#=#(valence electrons element A) #-#(number of bonds on element A + number of lone pairs on element A)
- To find valence electrons use the periodic table. The group number of the A group elements will be the same as the number of valence electrons.
- The number of bonds on element A and the number of lone pairs on element A come from the correctly drawn lewis structure.
Charge of ion or molecule =(formal charge(A)+formal charge(B)+formal charge(C)+...)