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