The sum of the oxidation numbrs must equal the overall charge.
Corollaries for rule #1 for oxidation numbers.
a)The oxidation number of a pure element is zero b)The oxidation numbers of a compund must sum to zero. c)The oxidation numbers of an ion must sum to the charge of the ion.
Rule #2 for assigning oxidaton numbers.
The last remaining unassigned element is assigned and oxidation number consistent with rule #1.
Rule #3 for assinging oxidation numbers.
Oxidation numbers for specific atoms in molecules and ions must be assinged using the following set of rules, in order of priority until one unassinged atom remains(see rule #2). a)The oxidation number of Group IA metals is always +1 in a compound. b)The oxidation number of Group IIA metals is always +2 in a compound. c)The oxidation number of F is always a -1 in a compound. d)The oxidation number of Al is always +3 in a compound. e)The oxidation number of H is usually +1 in a compound. (Exception: -1 in binary compounds with metals.) f)The oxidation number of O is usually -2 in a compound. (Exception: -1 in peroxides such as H2O2 in a compound.) g)In cases where two unassigned atoms remain and ALL previous rules have been applied, the element which is closer to the right and top of the periodic table has an oxidation number which matches its charge as an ion.