[dnbwise]

Some of the rules are that H is usually assigned +1 and O is usually assigned -2. An example of an exception would be H2O2, where O is assigned -1. My question is how do I determine exceptions to these rules?

[Donaldson Tan]

just memorise the exceptions. there are very few exceptions to remember.

an oxygen atom cant withdraw electron density from another oxygen atom because both atoms exhibit equal electronegativity. so when u have H-O-O-H, effectively, each oxygen atom is withdrawing electron density from each hydrogen atom. Hence, the oxidation number of O is -1, while that of H is +1.