The hard rules left ambiguities. The combinations below also mathematically satisfy the formula charge using common oxidation states.
Note: Formal oxidation numbers are a bookkeeping convention. Results for exotic compounds may be ambiguous.
Oxidation Number Rules
1. Hard Rules (Highest Priority)
Free Element: Any element in its elemental form (e.g. Fe, O2, S8) has oxidation number 0.
Monatomic Ion: Equals its charge exactly (e.g. Na+ → +1).
Fluorine: Always -1 in every compound.
Group 1 (Alkali Metals): Always +1 in compounds (Li, Na, K, Rb, Cs).
Group 2 (Alkaline Earth): Always +2 in compounds (Be, Mg, Ca, Sr, Ba).
Aluminum: Always +3 in default compounds.
2. Conditional Rules
Hydrogen: Default +1. Except in metal hydrides (bonded to less electronegative metals), where it is -1.
Oxygen: Default -2.
Peroxides (e.g. H2O2): -1
Superoxides (e.g. KO2): -1/2
Ozonides (e.g. KO3): -1/3
Oxygen fluorides: +2 (OF2) or +1 (O2F2)
Halogens (Cl, Br, I): Default -1. Except when bonded to oxygen or a lighter halogen (e.g. ClO4-), then they are positive.
Chalcogens (S, Se, Te): Default -2 only when bonded solely to less electronegative elements (e.g. sulfides). Unfixed otherwise.
3. Universal Constraints
Charge Balance Solver: For remaining unknowns, the sum of (coefficient × oxidation number) over all atoms must equal the overall charge. The solver tries common real-world states first.
Electronegativity Tie-Break: If ambiguous, the more electronegative element receives the negative oxidation number.
Mixed-Valence / Alloys: Known mixed-valence compounds (Fe3O4) split across states. Pure metal alloys (Intermetallics) have physically undefined oxidation states.