Why hydrogen has -1 oxidation number?

Answer 1

The oxidation state of oxygen is not -2, it is 0 - because oxygen is an element.

Oxide, however, has oxidation state of -2. This is because the anion is O2- and the oxidation state of ions is equal to the charge held by the ion.

The exception for oxide is that the oxidation state is -1 in peroxides e.g. H2O2 (hydrogen peroxide)

Answer 2

Hydrogen has only 1 valence electron. It can accept one more to get the electronic configuration of nearest noble gas. So, when hydrogen accepts one electron its oxidation number is -1.

Answer 3

Oxygen has 6 valence electrons. It needs to gain two more to complete octet. So oxygen has an oxidation number of -2 when it gains 2 electrons (as in oxide ion)

Answer 4

-2 is not possible but 1 is possible