[JacksonM]

Hey guys i'm pretty new here. I'm taking General Chemistry 1 for the first time and stumbled upon something i'm having a little trouble with...

So we're doing redox reactions now and have to find oxidation numbers for elements but I have a problem. I can't figure out when O is -2 and when it's -1. I know most Oxygens are -2 except for the peroxides but i can't figure out by the molecular formula which ones are peroxides. The example in my book is BaO₂. What makes this Barium Peroxide and not Dioxide?

Thanks! 

[JacksonM]

Actually just figured out that it's because Barium is +2 therefore Oxygen must be -1 since there are two of them... but that brings me to another question. How do you determine which one holds and which one is changed? Why do you change Oxygen rather than say... O₂ is -4 so Ba must be +4?

For example. In Sulfate SO₄^2- my professor said there is 8- oxidation number for Oxygen so Sulfur must be 6+. So why is Sulfur changed this time and not Oxygen?

Sorry if these are simple questions. This section has been the only one so far that's really confusing me.

[Arkcon]

A peroxide contains oxygen bonded to itself in the molecule.  That's it.  It isn't always easy to tell from the molecular formula that the oxygens have bonded to themselves and not another atom, but sometimes it is possible to tell.