[Mikez]

I am having some trouble with this concept and the textbook that I have doesn't explain it (just says to memorize all the possibilities).  How is it possible for oxygen to be -2 and -1?

thanks for your help

[tamim83]

That pretty much means that oxygen can either gain 1 electron (-1) or gain two electrons (-2).  It is just that oxygen anions with a oxidation of -2 is more common. 

Much of the main group elements do not have many common oxidation states.  It is only when you get to transition metals and some lower period main group elements where you have multiple oxidation states.  And yeah, I suppose memorizing the common ones for some (not all, that would really bite) is a good idea.   

[sdekivit]

and peroxides !

[tamim83]

Oooh yeah peroxides.  Oxygen in peroxides have an oxidation state of -1/2. 

[Will]

Oooh yeah peroxides.  Oxygen in peroxides have an oxidation state of -1/2. 

I think oxygen in peroxides have an oxidation state of -1, and in superoxides it is -1/2.

[tamim83]

Yup, you are right will. 

[Borek]

I think oxygen in peroxides have an oxidation state of -1, and in superoxides it is -1/2.

Just don't forget it is only for accounting, no real physical meaning