.........#(O=)C(-OH)_2#, and so the structure of the base is...
#(O=)C(-OH)(-O^(-))# else #(O=)C(-O)_2^-#
In #CO_3^(2-)# there are #3xx6(O)+4(C)+2("negative charges")=24#, i.e. #12*"electrons pairs"# to distribute around 3 centres.
And given #3xx8+6=30# nuclear charges, i.e. the positively charged protons in the carbon and oxygen nuclei, and the 8 inner core (i.e. #1s^2# electrons on each atom), we has 32 electronic charges BUT ONLY 30 protonic, nuclear charges. The difference, #""^(-)32+""^(+)30=-2# clearly, and this carbonate has a formal charge of #-2#.
We would expect the #C=O# bonds to be the shortest, however, we would struggle to find an experimental regime which could interrogate the difference in #C-O# bonds lengths.........Simple ideas of resonance stabilization could account for approx. equal #C-O# bond lengths, somewhat shorter than an actual #C-O# bond, i.e. #<1.40xx10^-10*m#.
