Question

How would you determine the formal charges on each atom in the carbonate ion (`CO_3^(2-)`)?

Answer 1

`CO_3^(2-)` has `3xx6+4+2=24" valence electrons"` to distribute around 4 centres.

Explanation:

Clearly I got the number of electrons from the atomic numbers of each element, and added 2 electrons to represent the charge. The charge lies on the most electronegative atoms, i.e. the oxygen atoms.

A Lewis structure of `O=C(-O^-)_2` gives a neutral doubly oxygen, a neutral carbon, and 2 anionic oxygen atoms associated with 7 valence electrons. Of course, I can draw resonance structures to show the equality of the oxygen atoms.

See here for another example that demonstrates assignment of formal charge.