## 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 O2(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 N2H4, 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 qquad => qquad 2x = -4 qquad => qquad x = -2.

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