In an ion, the sum of the oxidation states is equal to the overall ionic charge. Note that the sign of the oxidation states and the number of atoms associated with each oxidation state must be considered. In OH−, for example, the oxygen atom has an oxidation state of −2 and the hydrogen atom has an oxidation state of +1, for a total of (−2)+(+1)=−1.
What is the oxidation state of an individual sulfur atom in SO42−?
What is the oxidation state of each individual carbon atom in C2O42−?