Rules continued
continued e. In binary compounds, Group VA (15) elements have oxidation numbers of -3, unless combined with elements to their right in the periodic table.
Oxygen usually has an oxidation number of -2 in its compounds. There are some exceptions: a. Oxygen has an oxidation number of -1 in peroxides, which contain the O22- ion. b. Oxygen has an oxidation number of -1/2 in superoxides, which contain the O2- ion.