CO3^2- is called a carbonate ion. First we need to count the total number of valence electrons. We just add the valence electrons of each atoms involved.
C = 4
O = 6 x 3
e = 2 ---> we add electrons when the charge of the ion is negative, in
this case it is negative 2 so we add two electrons.
Next, we have to assign the central atom; In this case it is C.
Now we have to connect the atoms and form bonds. We can form one C=O and two C-O bonds.
O = C - O
Finally, we have to distribute the remaining electrons on C atoms provided that it follow the octet rule.
O = C - O :
'' | ''
: O :
CO3^2- has a geometry of trigonal planar and has a resonance. It has a bond angle of 180 degrees and the molecule is polar.