What is the oxidation number or oxidation state for each of the atoms in the following substances? What I want to do is look at those elements for which I know the oxidation state and use the information I know about them to solve for the oxidation number of the unknown atom. In this case, I know that hydrogen has an oxidation number of plus 1 and oxygen is minus 2. For the moment, I'm going to say N has an oxidation number of x. So I can say plus 1 from the hydrogen, plus x for the nitrogen, plus 3 times minus 2 for the oxygen. And I know that the sum of those values must be equal to 0 because this is a neutral compound. So I have 1 plus x minus 6 equals 0. So I have x minus 5 equals 0. So x equals 5. So I'm able to determine that the oxidation state of nitrogen is plus 5. Now I can look at NO2 minus. Again, I don't know what nitrogen is, so we're going to call it y this time. And oxygen is minus 2. So, I say y plus 2 times minus 2 equals, and this time it's going to equal minus 1. Because now we have a charge on that ion. So the sum of the oxidation numbers must equal the charge on the ion. So I have y minus 4 equals minus 1. Therefore y equals plus 3. So I can write plus 3 as my oxidation state for nitrogen. Now, I'm going to look at my SF6. In this case, I'm going to say s has an oxidation state of z. I know that my fluorine has an oxidation state of minus 1. So I'm going to say that if I have z plus 6 times minus 1, it's going to be equal to 0 because I have a neutral compound, so the sum of the oxidation numbers must be 0. So z minus 6 equals 0. So z equals 6. So the oxidation state of sulfur is plus 6.