[toadesque]

I was getting a little confused with this.

It says:

Draw the Lewis structures for these ions, and show which atom in each bears the formal charge.

So for this one:

CO3^-2 (carbonate ion)

I did the Lewis structure correctly but the formal charge part was confusing me.

First of all, how do I know which atom to do the formal charge for? How do I know whether to do it for carbon or oxygen?

I went and did it for Oxygen anyway and I ended up with "-1" (6 - (6+ 1/2(2)). So in order to determine whether the charge is positive or negative you just compare the number to the atom's valence electrons right? Oxygen has 6 valence electrons and the formal charge number was -1, so wouldn't that mean that Oxygen has a positive formal charge? But the solutions manual says that both of the Oxygens (the ones with single bonds) have a negative formal charge.

I don't have my textbook right now so I have to do this from memory

[Schrödinger]

Please refer to the image i have attached.

First of all, i think it is always better to calculate formal charge for all the atoms of the molecule. You will find that some carry formal charge = 0. Those need not be considered in the end while listing the atoms that carry formal charges.

I hope you know how to calculate formal charge(the formula). Both the single bonded Oxygens carry -1 formal charge and the other two atoms don't carry any formal charge.