[dantheman683]

Hi I have a question on oxidation.When doing the oxidation number for certain questions, how do you figure out which element you are solving for? I already know that if there are only 2 elements (Oxygen and something else), you solve for the other element because Oxygen will be -2 most of the time. But let's say you have questions like these:
 
NiH^3
 
PCl^5
 
H^3PO^3
 
Which element do you solve for?

[Hunter2]

Right oxygen is -2, except in peroxy compunds  and compounds with flourine. Which numbers are here possible.

Which possibility has hydrogen for oxidation numbers?