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Oxidation state

In covalent molecules, the oxidation number of an atom indicates whether an atom gets a greater or smaller part of the electrons shared with other atoms in a covalent bond.

The oxidation number is a property of an atom within a compound. Single, unbounded atoms do not have oxidation numbers.

Changes in oxidation number help indicate whether an oxidation or a reduction has occurred:

  • An increase in the oxidation number of an atom indicates oxidation. The atom has lost control over electrons (is getting a smaller part of them) and has become more positively charged.
  • A decrease in the oxidation number of an atom indicates reduction. The atom has gained control over electrons and so has become more negatively charged.

The oxidation number of a covalent compound can be determined by adding the oxidation numbers of the constituent atoms.

If there is no superscript by a compound's molecular formula, the oxidation number for the compound is $$0$$.

The oxidation number of an oxygen atom is usually $$-2$$ while that of a hydrogen atom is usually $$+1$$.

Water ($$\ce{H2O}$$) has an overall oxidation number of $$0$$ (it has no superscript).

Oxygen has an oxidation number of $$-2$$ and each hydrogen has an oxidation number of$$+1$$.

Overall oxidation number of $$\ce{H2O}=1 + 1 - 2 = 0$$

Hydroxide ($$\ce{OH^-}$$) has an overall oxidation number of $$-1$$. Oxygen's oxidation number is $$-2$$ and hydrogen's is $$+1$$, so:

Overall oxidation number of $$\ce{OH^-}=-2 + 1 = -1$$

Examining oxidation numbers of covalent compounds is important for recognising whether or not a compound has been oxidised or reduced in a reaction.