⚗️ Oxidation Number Calculator – Determine Oxidation States Instantly
The oxidation number (also called oxidation state) of an atom in a chemical compound is a signed integer that represents the formal charge the atom would carry if all bonds in the molecule were completely ionic. Oxidation numbers are indispensable in chemistry for identifying which atoms are oxidised or reduced in a reaction, balancing redox half-equations, naming transition-metal compounds, and understanding electron-transfer processes in electrochemistry.
What Are the Standard Oxidation Number Rules?
Chemists apply a set of priority rules — strictly in order — to assign oxidation numbers without needing to draw Lewis structures:
| Priority | Rule | Example |
|---|---|---|
| 1 | Pure elements = 0 | O₂ → O = 0, Fe → Fe = 0 |
| 2 | Group 1 metals (Li, Na, K, Rb, Cs, Fr) = +1 | NaCl → Na = +1 |
| 3 | Group 2 metals (Be, Mg, Ca, Sr, Ba, Ra) = +2 | CaCO₃ → Ca = +2 |
| 4 | Fluorine = −1 (always) | OF₂ → F = −1 |
| 5 | Hydrogen = +1 (−1 in metal hydrides) | H₂O → H = +1; NaH → H = −1 |
| 6 | Oxygen = −2 (−1 in peroxides; +2 in OF₂; −½ in superoxides) | H₂O → O = −2; H₂O₂ → O = −1 |
| 7 | Halogens (Cl, Br, I) = −1 unless bonded to O or F | NaCl → Cl = −1; ClO₃⁻ → Cl = +5 |
| 8 | Sum of all oxidation numbers = net charge | SO₄²⁻: sum = −2 |
| 9 | Solve algebraically for the remaining element | H₂SO₄: S = +6 |
Worked Examples
H₂SO₄ — Sulfuric Acid (neutral)
H (×2):
+1 each → +2
O (×4):
−2 each → −8
S + 2 − 8 = 0
S = +6
KMnO₄ — Potassium Permanganate
K (×1):
+1 (Group 1)
O (×4):
−2 each → −8
+1 + Mn − 8 = 0
Mn = +7
SO₄²⁻ — Sulfate Ion (charge = −2)
O (×4):
−2 each → −8
S − 8 = −2
S = +6
Fe₃O₄ — Magnetite (mixed-valence)
O (×4):
−2 each → −8
3Fe − 8 = 0 → Fe = +8/3
Fe ≈ +2.67
Fe₃O₄ contains Fe²⁺ and Fe³⁺; +8/3 is the average.
Common Oxidation State Exceptions to Know
Several compounds deviate from the default rules. Knowing these exceptions is essential for correct oxidation-state assignments:
- Peroxides (H₂O₂, Na₂O₂, BaO₂): oxygen is
−1because of the O−O single bond. - Superoxides (KO₂, NaO₂): oxygen is
−½(average of the O₂⁻ radical). - Oxygen difluoride OF₂: oxygen is
+2because fluorine (more electronegative) takes the electrons. - Metal hydrides (NaH, CaH₂, LiH, AlH₃): hydrogen is
−1because the metal is less electronegative than hydrogen. - Halogens in oxoacids/oxoanions (ClO₃⁻, HClO₄, BrO₄⁻): the halogen carries a positive oxidation state (+3, +5, or +7) because oxygen pulls electron density away.
Why Oxidation Numbers Matter in Redox Chemistry
Oxidation numbers are the fastest way to identify oxidation (increase in oxidation number, loss of electrons) and reduction (decrease in oxidation number, gain of electrons) in any redox reaction. For example, in the reaction between iron and chlorine to form FeCl₃: iron goes from 0 to +3 (oxidised), chlorine goes from 0 to −1 (reduced).
In electrochemistry, oxidation numbers guide the writing and balancing of half-reactions. In naming inorganic compounds, they distinguish iron(II) sulfate (Fe²⁺, oxidation state +2) from iron(III) sulfate (Fe³⁺, +3) — both share the same empirical formula pattern but differ completely in properties and behaviour.
SO4 in the formula field and set the Net Charge to −2. For ammonium (NH₄⁺), enter NH4 with a charge of +1. The calculator then ensures the oxidation number sum equals the ionic charge rather than zero.Limitations of Algorithmic Assignment
The rule-based approach works for the vast majority of general-chemistry compounds. It has known limitations:
- Mixed-valence compounds like Fe₃O₄, Pb₃O₄, and Co₃O₄ contain the same element in two different oxidation states. The calculator correctly shows the fractional average and flags it with a warning.
- Complex coordination compounds such as K₃[Fe(CN)₆] involve ligands that can have non-standard charges. Assign ligand charges manually and enter the remaining ion's charge as the net charge to get the metal oxidation state.
- Organic compounds with many carbon environments require formal electronegativity-based methods beyond simple rules.