Determine oxidation states for atoms in compounds and ions. Identify redox changes and balance half-reactions visually.
This tool assigns an oxidation number to every atom in a formula or ion, then, for a full reaction, compares those numbers across reactants and products to flag which atoms were oxidized and which were reduced. It works from the conventional priority rules (fluorine is always -1, oxygen defaults to -2, hydrogen is +1 with nonmetals) rather than from actual electronegativity calculations, so results are rule-based. Users are most often surprised that atoms of the same element in one formula can carry different oxidation states in reality, while a rule-based tool reports a single averaged value per element unless the species is split into distinct bonding environments.
A concrete input and expected output from the current implementation.
Input
Zn + Cu^2+ -> Zn^2+ + Cu
Expected output
Zn: 0 -> +2 (oxidized, loses 2 e-); Cu: +2 -> 0 (reduced, gains 2 e-); net transfer: 2 electrons from Zn to Cu
Zn starts as the free element (oxidation state 0) and ends as Zn2+, so its number rises by 2; Cu2+ ends as the free element, so its number falls by 2, meaning the electrons Zn loses are exactly the electrons Cu gains.
Each atom's oxidation number is set using the standard hierarchy: free elements score 0, monatomic ions take their charge, fluorine is always -1, oxygen defaults to -2 (except in peroxides or OF2), hydrogen is +1 with nonmetals and -1 with metals, and the last unassigned element is solved so the formula's atoms sum to its net charge.
For a full reaction, the calculator assigns oxidation states to every atom on both sides, matches identical elements across reactants and products, and reports which ones changed value. It labels the increasing element oxidized and the decreasing one reduced, tallies electrons lost against electrons gained, and can display each half-reaction separately.
It returns one number per element for the whole formula unit, not a number per atom position. An element that appears once, such as Cu in CuSO4, gets that single atom's value; an element that appears more than once, such as S in Na2S4O6, gets one averaged value shared by all its atoms, so both nitrogens in N2O report the same value rather than per-site states.
It identifies which elements change oxidation state and separates the oxidation and reduction into half-reactions, which is the harder part of balancing. Whether it also completes full mass and charge balancing (adding H+, OH-, or H2O) depends on the specific mode you select on the page itself.
The example and behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.
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