Oxidation Number Calculator

Determine the specific oxidation state of an unknown central atom by balancing the overall charge of the polyatomic ion or molecule.

Overall Charge of Molecule/Ion

Sum of Known Oxidation States

Unknown Oxidation State

Chemical StateExtremely Oxidized (Strong Oxidizing Agent)

Calculated locally in your browser. Fast, secure, and private.

Tracking Electrons in Reactions

In chemistry, an oxidation number (or oxidation state) is a number assigned to an element in a chemical compound that represents the number of electrons lost or gained by an atom of that element.

While Formal Charge assumes perfect covalent sharing, Oxidation Numbers assume perfect ionic stealing—it pretends that the more electronegative atom in a bond takes 100% of the shared electrons.

This concept is absolutely critical for analyzing Redox (Reduction-Oxidation) reactions, which power batteries, rust iron, and metabolize food in the human body. By tracking how oxidation numbers change during a reaction, you can instantly see which chemical stole electrons and which one lost them.

The Golden Rules of Oxidation Numbers

To find an unknown oxidation state, you must rely on a strict set of hierarchical rules for the "known" elements:

Free Elements: Any element in its pure, uncombined state (Na, O₂, S₈) has an oxidation number of exactly 0.
Monatomic Ions: The oxidation number is equal to the charge of the ion (e.g., Na⁺ is +1, Cl⁻ is -1).
Fluorine: Always -1 in all compounds.
Oxygen: Almost always -2 (except in peroxides like H₂O₂, where it is -1, or when bonded to Fluorine).
Hydrogen: Usually +1 when bonded to nonmetals (like H₂O), but -1 when bonded to metals (like NaH).
The Sum Rule: The sum of all oxidation numbers in a molecule MUST equal the overall electrical charge of that molecule.

How to Calculate the Unknown

When dealing with a complex polyatomic ion (like Permanganate, MnO₄⁻), you will know the rules for Oxygen, but Manganese is a transition metal with unpredictable states. You must use algebra to solve for the unknown.

Oxidation State = Overall Charge - (Sum of Known States)

Where:

Oxidation State=

The unknown oxidation number

Overall Charge=

Charge of the entire molecule/ion

Known States=

Sum of oxidation numbers of all other atoms

Example Calculation: Permanganate (MnO₄⁻)

Find the oxidation number of Manganese (Mn) in the MnO₄⁻ ion.

Overall Charge: The ion has a charge of -1.
Known States: There are 4 Oxygen atoms. The rule says Oxygen is -2. So, $4 \times -2 = -8$ .
Calculate: $Oxidation State = -1 - (-8) = +7$ .

Therefore, Manganese is in a highly oxidized +7 state.

Frequently Asked Questions

A positive number indicates that the atom has mathematically 'lost' electrons to a more electronegative partner. A +7 oxidation state means the atom is electron-deficient, acting as if it has lost 7 valence electrons.

Yes, though it is relatively rare. Fractional oxidation numbers usually represent a mathematical average. For example, in the azide ion (N₃⁻), the three nitrogen atoms share a -1 charge, resulting in an average oxidation state of -1/3 per nitrogen atom.

Oxidation is the loss of electrons (the oxidation number becomes more positive). Reduction is the gain of electrons (the oxidation number becomes more negative). You can remember this using the acronym 'OIL RIG' (Oxidation Is Loss, Reduction Is Gain).

Transition metals have complex 'd-orbitals' that are very close in energy to their outermost 's-orbitals'. Because the energy difference is so small, they can lose a variable number of electrons depending on the specific chemical environment they are in.

Usually no. In ionic compounds like NaCl, the oxidation numbers (+1 and -1) perfectly match the physical charges of the ions. However, in covalent molecules like CO₂, the carbon has an oxidation state of +4, but it does not actually possess a physical +4 charge; it merely shares its electrons unequally.

Related Calculators in Chemistry & Materials Science