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# Oxidation state

## Oxidation numbers in covalent molecules

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.

## Oxidation number of atoms in covalent molecules

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.