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Oxidation state

number that describes degree of oxidation of an atom in a chemical compound; the hypothetical charge that an atom would have if all bonds to atoms of different elements were fully ionic
Plutonium ions in several oxidation states

Oxidation state (or oxidation number) refers to the number of electrons added to or removed from an element when it forms a chemical compound. Sodium metal, for example, has an oxidation state of 0 in the elemental state. But when it gives up its one valence (outer) electron (symbolized by e), it becomes a sodium ion Na+ with an oxidation state of +1. Iron is another example. It can lose two electrons, forming an oxidation state of +2; or it can lose three electrons, forming an oxidation state of +3.

If elements gain electrons, they have negative oxidation states. If chlorine gains an electron, it forms a Cl ion (chloride) and has oxidation state of −1. If sulfur gains two electrons, it forms an S2− ion (sulfide) and has oxidation state of −2.

The Na+ ions and Cl ions then form a compound, NaCl, sodium chloride (table salt). Or the Fe2+ and S2− ions form FeS, iron sulfide.

All free elements have an oxidation state of 0.

  1. oxygen has an oxidation number −2 in most of its compounds except peroxides where it has an oxidation number −1.
  2. hydrogen has oxidation state +1 in most compounds except with electropositive elements like Na, where it has −1.
  3. alkali earth metals have oxidation state +2
  4. alkali metals have oxidation state +1
  5. oxidation states of all the atoms in a molecule add up to 0 and in an ion to the ion charge.

Examples: potassium permanganate with the formula KMnO4 has Mn in oxidation state +7, and CrO42− has Cr in oxidation state +6.