Oxidation States
Find the oxidation states of all elements in a compound or ion with a step-by-step solution.
Write the ion charge as 2-, 3+ or -, leave it empty for a molecule. For example, SO4 with the charge 2-. Write formulas in Latin letters starting with a capital: H2SO4, Ca(OH)2, CuSO4·5H2O (water of crystallization after a dot or *).
The oxidation state calculator determines the oxidation states of all elements in a substance or ion and writes them above the symbols. The solution shows which rules were used and how the unknown state was found.
Rules for oxidation states
- An element in an elementary substance has oxidation state 0: O2, Fe, S8.
- The oxidation states of all atoms add up to 0 in a molecule and to the charge in an ion.
- Fluorine is always −1; alkali metals +1; group 2 metals +2; aluminum +3.
- Hydrogen is +1, in metal hydrides (NaH, CaH2) −1.
- Oxygen is −2, in peroxides (H2O2, Na2O2) −1, in OF2 +2.
- In binary compounds the more electronegative element takes its lowest (negative) state.
For salts with two unknown elements (Fe2(SO4)3, NH4NO3) the calculator splits the compound into known ions.
Example. K2Cr2O7: 2 · (+1) + 2x + 7 · (−2) = 0, x = +6.
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