Identify the element whose oxidation number you need
Use the following fixed rules first:
Free element (O₂, N₂, Fe, etc.): oxidation number = 0
Monoatomic ion charge = oxidation number (e.g., Na⁺ = +1, Cl⁻ = −1)
Group 1 metals (Li, Na, K, etc.): +1
Group 2 metals (Mg, Ca, etc.): +2
Fluorine (F): −1
Oxygen (O): typically −2 (except in peroxides and with F)
Hydrogen (H): typically +1 (except in metal hydrides where it is −1)
Apply special cases:
Peroxides (O₂²⁻): oxygen = −1
Superoxides (O₂⁻): oxygen = −1/2
Oxides of fluorine: oxygen oxidation number is positive (e.g., OF₂ gives O = +2)
For compounds, use charge balance:
Sum of oxidation numbers of all atoms in a neutral compound = 0
Sum of oxidation numbers of all atoms in a polyatomic ion = overall ion charge
Use algebra to solve for the unknown oxidation number:
Let the unknown oxidation number be x
Add known oxidation numbers from rules
Set the total equal to the compound/ion charge and solve for x
For compounds with multiple oxidation states:
Determine which oxidation number(s) satisfy the total charge balance
Verify the result is consistent with common oxidation states for that element
For elements not covered by fixed rules:
Assign oxidation numbers by charge balance and chemical reasoning (typical oxidation states)