Oxidation Number
The oxidation number (or oxidation state) is a hypothetical charge assigned to an atom in a molecule or ion, assuming all bonds are fully ionic and electrons are transferred to the more electronegative atom. It is a bookkeeping tool used to track electron transfer in redox reactions, with defined rules for elements, ions, and compounds. Oxidation numbers help identify which species is oxidized (loses electrons, oxidation number increases) and which is reduced (gains electrons, oxidation number decreases) in a reaction.
Oxidation Number Rules and Examples
| Rule | Condition | Oxidation Number | Example |
|---|---|---|---|
| Free element | Atom in pure elemental form | 0 | O₂, Fe, Na |
| Monatomic ion | Single-atom ion | Equal to charge | Na⁺ = +1, Cl⁻ = −1 |
| Oxygen in compounds | Most compounds | −2 | H₂O, CO₂ |
| Oxygen in peroxides | Peroxide ion O₂²⁻ | −1 | H₂O₂, Na₂O₂ |
| Hydrogen in compounds | With nonmetals | +1 | HCl, H₂O |
| Hydrogen in metal hydrides | With active metals | −1 | NaH, CaH₂ |
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Related Terms
Oxidizing Agent
An oxidizing agent (oxidant) is a substance that accepts electrons from another species during a redox reaction, thereby causing that other species to be oxidized while the oxidizing agent itself is reduced. Common oxidizing agents include oxygen (O₂), hydrogen peroxide (H₂O₂), potassium permanganate (KMnO₄), and chlorine (Cl₂). Oxidizing agents are critical in combustion, corrosion, bleaching, respiration, and industrial synthesis processes.
Reducing Agent
A reducing agent (reductant) is a substance that donates electrons to another species in a redox reaction, causing that species to be reduced while the reducing agent itself is oxidized. Strong reducing agents include hydrogen gas (H₂), carbon (C), metals such as sodium and iron, and compounds like sodium borohydride (NaBH₄). Reducing agents are essential in metallurgy for extracting metals from ores, in organic synthesis, and in biological processes such as the Calvin cycle.
From Latin "oxidatio" (the process of combining with oxygen) and "numerus" (number). The concept was systematized in the early 20th century as chemists needed a formal method to track electron bookkeeping in reactions beyond simple oxygen-transfer processes.