Determine the oxidation number (ON) of each element in the species
Use these rules:
Free elements have ON = 0 (e.g., O₂, Fe, Na)
Monatomic ions have ON equal to their charge (e.g., Cl⁻ is −1, Ca²⁺ is +2)
Sum of oxidation numbers in a neutral compound = 0
Sum of oxidation numbers in a polyatomic ion = overall ion charge
Group 1 metals (Li, Na, K, etc.) have ON = +1 (except hydrides with metals: still typically +1)
Group 2 metals (Mg, Ca, etc.) have ON = +2
Hydrogen usually has ON = +1 (−1 when bonded to metals in metal hydrides)
Oxygen usually has ON = −2 (except peroxides ON = −1, superoxides ON = −1/2, and with F in OF₂: oxygen ON = +2)
Halogens usually have ON = −1 (except when bonded to oxygen or more electronegative elements)
The most electronegative element in a compound typically has the negative ON (apply to ambiguous cases)
Assign known ON values to all elements except the one(s) you need to solve for
Set up the equation using:
Total ON = 0 for neutral compounds
Total ON = ion charge for ions
Solve for the unknown oxidation number(s)
Verify that the resulting ONs satisfy all constraints and charge balance