Flexi Says: A series of rules have been developed to help us: For free elements (uncombined state), each atom has an oxidation number of zero. H2, Br2, Na, Be, K, O2, P4, all have oxidation number of 0. Monatomic ions have oxidation number equal to charge. Li+=+1, Ba2+=+2, Fe3+=+3, I−=−1, O2−=−2, etc. Alkali metal oxidation numbers = +1. Alkaline earth oxidation numbers = +2. Aluminum = +3 in all of its compounds. Oxygen’s oxidation number is -2 except when in hydrogen peroxide (H2O2), or a peroxide ion (O22-) where it is -1. Hydrogen’s oxidation number is +1, except for when bonded to metals as the hydride ion forming binary compounds. In LiH, NaH, and CaH2, the oxidation number is -1. Fluorine has an oxidation number of -1 in all of its compounds. Halogens (Cl, Br, I) have negative oxidation numbers when they form halide compounds. When combined with oxygen, they have positive numbers. In the chlorate ion (ClO3-), the oxidation number of Cl +5, and the oxidation number of O is -2. In a neutral atom or molecule, the sum of the oxidation numbers must be 0. In a polyatomic ion, the sum of the oxidation numbers of all the atoms in the ion must equal the charge on the ion.