What the calculator does
For a neutral compound or an ion, the calculator gives:
- the oxidation state of each element, with the reason;
- separate values when an element appears in two different ions, as in NH₄NO₃;
- averages, clearly labelled, for mixed-valence compounds such as Fe₃O₄;
- the constraint that remains when the formula alone cannot decide.
How to enter a formula
Write formulas as usual, with charges after ^: KMnO4, Cr2O7^2-, Fe^3+. Brackets and hydrates work: K4[Fe(CN)6], CuSO4·5H2O.
The rules, in priority order
- An uncombined element (Fe, O₂, S₈) is 0. A single-element ion carries its charge.
- Fluorine is always −1.
- Group 1 metals are +1 and group 2 metals are +2. Aluminium is +3, zinc +2 and silver +1.
- Hydrogen is +1 with non-metals and −1 with metals (in hydrides such as NaH).
- Oxygen is −2, except in peroxides (−1), superoxides (−1/2) and compounds with fluorine (positive).
- Halogens are usually −1, but not when combined with oxygen or a more electronegative halogen.
- The oxidation states add up to the overall charge.
When the rules contradict rule 7, as for oxygen in H₂O₂, the lower-priority rule gives way. The calculator explains which one and why.
Worked example: potassium permanganate, KMnO₄
- Potassium is a group 1 metal: +1.
- Oxygen is −2, four times: −8.
- Charge balance. The compound is neutral, so +1 + x + (−8) = 0.
- Solve. x = +7, so manganese is in its highest oxidation state, +7.
Common mistakes
- Assuming oxygen is always −2. Check for peroxides and fluorides.
- Forgetting the overall charge of an ion. In SO₄²⁻ the states must add up to −2, not 0.
- Treating oxidation states as real charges. They are a bookkeeping device for electrons.
- Reporting a fractional average as if every atom had it. Mixed-valence compounds contain atoms in different states.
Practical applications
- Balancing redox equations by matching electrons lost and gained.
- Naming compounds, such as iron(II) chloride versus iron(III) chloride.
- Predicting reactivity. Elements in high oxidation states, such as Mn(VII) and Cr(VI), are strong oxidising agents.
- Electrochemistry and corrosion, where changes in oxidation state drive cells and rusting.
Assumptions and limitations
The calculator works from the formula alone. That decides the oxidation states of most inorganic compounds and of common polyatomic ions. Where atoms of the same element are not equivalent, as in thiosulfate or organic molecules, it gives the average and says so. Where two different elements are both unknown, it gives the constraint rather than a guess. Unusual bonding, such as metal–metal bonds or non-innocent ligands, needs structural information beyond a formula.