Like we said before, oxidation numbers don't always correspond to real charges and therefore a list of rules will be necessary. Now we're going to say when different elements are in a compound, these specific rules will be used to calculate our oxidation numbers. Now here let's take a look at these specific oxidation number rules.
When it comes to group 1A elements, they will be plus one when connected to any other element. So that's something similar to charges of groups and we're going to see some similarities between oxidation numbers and charges, but then we're going to see a huge deviation from that group, 2A plus two when they're connected to another element. Fluorine, it's -1 when it's connected to any other element.
Now this is where things start to change. Hydrogen can be either plus one or -1. It's plus one when it's connected to non metals. For example, if hydrogen is connected to chlorine or hydrogen is connected to oxygen or it's connected to the non metal of nitrogen. In all these instances, since hydrogen is connected to a nonmetal, its oxidation number will be plus one. Now it's -1 when it's connected to a metal or boron. So for example NaH or CaH2 or BH3. In these cases hydrogen will be -1 for its oxidation number.
Now oxygen is even more varied. We're going to say here that oxygen, when it's not a peroxide or superoxide, its oxidation number is -2. When it's in its peroxide form, its oxidation number will be -1. Now what exactly is a peroxide? Well, a peroxide we're going to say is when you have two group 1A elements connected to two oxygens, for example H2O2 (2 hydrogens which are in Group 1A, 2 oxygens). This would be hydrogen peroxide, Li2O2 (lithium peroxide) or K2O2 (potassium peroxide). Now if oxygen is a superoxide, its oxidation number is -1/2.
What is a superoxide? A superoxide is when you have one group 1 element with two oxygens. So here we could have potassium superoxide, cesium superoxide, sodium superoxides. So just remember oxygen can vary. So just look, be on the lookout, do you have a superoxide or peroxide? If not, its oxidation number is -2.
And then finally, when we're talking about Group 7A, we're talking about chlorine, bromine and iodine. They are going to be minus one, except when they're connected to oxygen. In that case, we won't know what their new oxidation number will be, and we'll have to calculate it. So again, we use these specific oxidation number rules when we're talking about different elements connected together. So we're going to have to utilize them to determine the oxidation number of any element given to us within a compound.