oxidation number

1) a. Since the oxygen is just as an element, according to the definition of oxidation number, its oxidation number should be zero. b.For oxide, the oxidation no is -2. For the peroxide such as Na2O2, however, its oxidation no is -1. Thus b is true. Additional information: For superoxide such as KO2, the oxidation number is -0.5.
Calculation: Na2O2 => (2)(+1)+ 2x = 0; KO2=> (+1)+2x = 0
2)Yes, since at first, the sodium is a free element. However, after the reaction, sodium combines with the hydrogen atom to give sodium hydride. The H-atom combined with sodium indicates hydrogen gaining.
I hope you will find them helpful.

In the first question, there is a misunderstanding.
In choice (a), if oxygen means the gas, of course the oxidation number will be zero. But if it means the species O, which is only an atom, the oxidation number can be -2 ,-1or 0. For example, the oxidation number of O(oxygen atom) in H2O2 is -1 rather than -2. So the choice (a) would be correct.
In choice (b), oxide ion which is only an ion contains exactly 2 more electrons, it carries 2 negative charge,so the oxidation number surely be -2, but not -1. Therefore choice (b) is incorrect.

In the second question, this redox equation can be explained in terms of both sodium and hydrogen gaining or losing of electrons, as well as increasing and decreasing of oxidation number.
First, in the equation, sodium loses 1 electron and hydrogen gains 1 electron, this is only the electrons transformation. As sodium loses electron, it is oxided and hydrogen is reduced.
If we explain by using the concept of oxidation number, the O.N. of sodium increases from 0 to +1, it is thus oxided. On ther other hand, the O.N. of hydrogen decreases from 0 to -1, it is therefore reduced.

Actually, I think both of the statement are incorrect.
First, oxygen(O2) has a covalent bond between each oxygen molecule, there will be no electrons can be transferred, and also oxygen atoms can not be transferred any more.
Therefore, I think oxygen will have an oxidation number 0.
Secondly, normal oxide ion will have oxidation number equal to -2.
Therefore, I think both 1 and 2 statement are wrong.


Next, I think this redox equation can be explain by gaining or losing hydrogen because ....Sorry, I have forgotten the reasons.
However, it is still a redox reaction, since there is transfer of electrons from hydrogen to sodium.

I hope my anwser can fulfill your needs.