To find the chemical formula of an ionic compound, the first step is to find the charge of the 2 ions. As given already, the charge of sodium ion is 1+, and carbonate ion has a charge of 2-. We can picture it like that: Sodium ion loses 1 electron and carbonate ion gains 2.
The next step is to find how the 2 ions can lose and gain electrons equally. In this case, since each Na ion only loses 1 electron, it cannot satisfy the need of one carbonate ion, since they need 2, not 1. Therefore, 2 Na ions can cover the need of one carbonate ion. So, the ratio of Na to CO3 ion should be 2:1.
Now just combine the 2 ions, positive one at the front, which makes it NaCO3, make sure you do not add the charge and notice that CO3 is a molecule itself so do not remove the 3. Now because the ratio is 2:1, so the final formula is Na2CO3, no need to add 1 if the ratio is 1.
Your answer should be Na2CO3.
