At this moment, I'm on balancing redox equations. I'm stuck on step one though... 1. Assign oxidation numbers to all atoms in the equation... S + HNO3 --> SO2 + NO + H2O This particular problem is an example in my Chem book so I already know the answers. I just don't understand why they are what they are. I understand this part: All free, uncombined elements have an oxidation number of zero. ...so that's simple...it's the rest I'm not getting. The first part that throws me is the HNO3 part. Why is N +5?