Question

What is the charge on a carbonate ion `(CO_3)``2-`?Compared to the number of protons, how many electrons does the carbonate ion have?

Answer 1

The carbonate dianion MUST have 2 more electrons than it does nuclear protons. Let's see if it does.

Explanation:

The number of nuclear protons are the number of positively charged particles contained in atomic nuclei. `Z`, the atomic number, `=6` for carbon, and `8` for oxygen. In the carbonate atom nuclei there are therefore 30 positively charged protons.

So how many electrons on carbonate? There are 6 from carbon, 24 from oxygen, PLUS 2 extra negatively charged particles from the formal charge on the anion. 32 electronic charges (`-32`) versus 30 protonic charges (`+30`). There is a formal charge of `-2` on the carbonate anions, the which we conceive to be on the oxygen atoms (the doubly bound oxygen is neutral, whereas each of the singly bound oxygen atoms bears a negative charge), to give `O=C(-O^-)_2`

Of course this assignment of charge is a formalism; and we would distribute the charge over the oxygen centres by resonance.