# Oxidation numbers

The oxidation number is a positive or negative number assigned to an atom. For an ion, this is the same as the charge: the number of electrons lost (positive) or number of electrons gained (negative). In a covalent molecule, the oxidation number is assigned as if the shared electrons belonged entirely to the more electronegative atom. Oxidation number is *not* the same as charge; it is just a systematic way of counting electrons.

The oxidation number of an uncombined element like O_{2(g)} or Ca_{(s)} is always *zero*. For atoms that are part of compounds, refer to this table:

Atom | Oxidation No. |
---|---|

oxygen | −2 |

peroxide | −1 |

hydrogen | +1 |

hydride | −1 |

group 1A | +1 |

group 2A | +2 |

group 7A | −1 |

This helps, but it leaves out a lot of atoms. To determine their oxidation number, we need to use a bit of algebra. For a neutral compound, the subscripts multiplied by the oxidation numbers sum to zero. For polyatomic ions, the sum is the charge.

## Example

What is the oxidation state of nitrogen in hydrazine?

Hydrazine is N_{2}H_{4}, and we know that hydrogen in a molecule has an oxidation number of +1. This is a neutral compound, so the oxidation numbers must add up to zero. Mathematically, this means that

$2x+4(+1)=0\phantom{\rule{2em}{0ex}}\phantom{\rule{0.2778em}{0ex}}\u27f9\phantom{\rule{0.2778em}{0ex}}\phantom{\rule{2em}{0ex}}2x=-4\phantom{\rule{2em}{0ex}}\phantom{\rule{0.2778em}{0ex}}\u27f9\phantom{\rule{0.2778em}{0ex}}\phantom{\rule{2em}{0ex}}x=-2\text{.}$

The oxidation number of nitrogen in hydrazine is therefore −2.