Determine the oxidation number for the indicated element in each of the following substances: (a) S in SO2
So here we have to determine the oxidation number of sulphur and sulphur dioxide. Remember, oxygen can have different types of oxidation numbers. So if oxygen is a peroxide, it's oxidation number will be negative one. Remember peroxides have the formula X2O2 where X represents a group IA element. So examples would be hydrogen peroxide or sodium peroxide. If oxygen is a super oxide, it's oxidation number will be negative one-half. Remember, super oxides have the formula of XO2, again X represents a group IA element. So for example you could have potassium super oxide, or sodium super oxide. If it is not a peroxide or a super oxide it will be negative two. So oxygen here is negative two.
We don't have a rule for sulphur, so sulphur here will be X, now let's just solve for X. So X plus the two oxygens that we have, each one with a negative two for oxidation number, equals the charge of my compound. Here the compound has no charge here. If it had a charge you would see maybe a plus one or a plus two or minus two, but we don't see anything there so the charge is equal to zero. So X minus four equals zero, X equals plus four. So the oxidation number of sulphur here would be plus four.