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?

1 Answer
Dec 19, 2015

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


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.