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Oxidation state facts for kids

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Plutonium in solution
Plutonium ions in several oxidation states

In chemistry, the "oxidation state" (or "oxidation number") of an element is used to predict what sort of chemical compounds form.

For example, if you put hydrogen and oxygen together, they form H2O, and not H3O because the oxidation number of oxygen is −2, which means that we need two +1 hydrogens for everything to balance.

Oxidation states were used by Antoine Lavoisier to predict how oxygen reacted with certain things; hence the name. Lavoisier's theory can also be used to predict other reactions. Much later, it was realized that the substance, when oxidized, loses electrons. So the meaning was extended to include other reactions in which electrons are lost, regardless of whether oxygen was involved.

More abstractly, oxidation states can be thought of as a loss of electrons. For example, sodium metal, 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.

See also

Kids robot.svg In Spanish: Estado de oxidación para niños

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