Oxidation number is a device used to track electron ownership in redox reactions.

The oxidation number of an atom in a compound or ion is the charge that atom would have if all of the electrons in each bond were assigned to the more electronegative atom.

  • For monatomic ions the charge is equal to the oxidation number.
    Thus the oxidation number of zinc in Zn2+ is +2.

  • Because in the elemental form, there is no difference in electronegativity between bonded atoms, the atoms in an element are assigned an oxidation number of 0.
    Thus the oxidation number of Cu in Cu(s) is 0.
    Similarly the oxidation number of H in H2 is zero.

In redox reactions the oxidation number of BOTH the oxidant and reductant change.

  • The oxidation number of the oxidant (Cu2+ above) becomes more negative
  • The oxidation number of the reductant (Zn above becomes more positive.
Dr Sheila Woodgate
© The University of Auckland 2002