Formal charge question Focus 2 (7th Edition)

[Miriam Sheetz 2B]

How do you find the formal charge of C2O4 2- from focus 2 problem 1 in the 7th edition?

[EllerySchlingmann1E]

To find the formal charge for C2O4 2- you want to draw the Lewis structure as the book instructs, so with only a single bond between the carbons. This allows for each of the carbons to double bond with one oxygen and single bond with the other. The formal charge of oxygen when it is double bonded is 0 because it is left with 4 electrons in lone pairs and 4 involved in bonds. FC=valence electrons - (unbonded e + bonded e/2), therefore FC of double bonded oxygen is 6-(4+4/2)=0. The single bonded oxygens each have a formal charge of -1 because they have 6 unbonded e and only 2 bonded e: FC=6-(6+2/2)=-1. Lastly, the carbons have formal charge of 0 because all 4 of their valence electrons are involved in bonds. Therefore, the overall formal charge of the ion is -2 because of the two oxygens each with -1 formal charge.