Identify the element and determine if it is in an element, compound, or ion
For a free element (single, uncombined atom), set oxidation number to 0
For a monatomic ion, set oxidation number equal to the ion charge
Use the rule that oxidation numbers of all atoms in a neutral compound sum to 0
Use the rule that oxidation numbers of all atoms in a polyatomic ion sum to the ion charge
Apply common electronegativity rules: more electronegative atoms are assigned negative oxidation states
Use fixed oxidation number rules:
Group 1 metals (Li, Na, K, etc.): +1
Group 2 metals (Mg, Ca, etc.): +2
Hydrogen: usually +1 (with nonmetals), usually −1 (with metals in metal hydrides)
Oxygen: usually −2 (except peroxides like H2O2 where −1, and with F where oxygen is positive)
Halogens (F, Cl, Br, I): usually −1 (except with more electronegative elements or in compounds like interhalogens)
Metals in compounds typically take values consistent with charge balance
Assign oxidation numbers to the known atoms using the rules above
Let the unknown oxidation number be a variable (x)
Use algebra to satisfy the sum rule (0 for neutral compounds, ion charge for ions)
Solve for x
Check that each atom’s oxidation number is consistent with the compound’s charge and the fixed-rule constraints