6.1.1—Oxidation numbers of elements in compounds

Oxidation number is the charge an atom would have if bonding electrons were assigned to the more electronegative atom. It is a bookkeeping tool for identifying oxidation and reduction.

Use rules such as elemental substances = 0, monoatomic ions = ionic charge, oxygen usually = −2 and hydrogen usually = +1, then make the total match the species charge.

In H₂SO₄, 2(+1)+S+4(−2)=0, so sulfur has oxidation number +6. In sulfate, the same bookkeeping must total −2.

Oxidation number is not always a real ionic charge. It is an assigned value that helps compare electron loss and gain in covalent compounds.