Oxidation Numbers

Firstly Happy New Year!!!

There is one week to go until you sit unit 1 of your chemistry A.S. so here are some brief notes on Oxidation numbers (for more detailed notes click here!)


The oxidation number of an element is worked out as either the actual charge on that element in an ion, or the charge it would have if the bonding electrons were given to the more electronegative element.

Rules for Evaluating Oxidation Numbers

  1. The oxidation number of an uncombined element is zero
  2. The oxidation numbers in a neutral compound add up to zero
  3. The oxidation numbers of the elements in an ion add up to the charge on that element
  4. All Group 1 elements have an oxidation number of +1 in their compounds and all group 2 elements are +2 in their compounds.
  5. Hydrogen is +1 in its compounds except when bonded to a metal where it is -1
  6. Fluorine is always -1 in its compounds
  7. Oxygen is always -2 in its compounds except when bonded to fluorine or in peroxides or superoxides

Quick Quiz

Fill in the form below with your answers to this question

Calculate the oxidation number of sulphur in

  1. S2
  2. SO2
  3. H2S
  4. SO42-
  5. S2O32-

Dont forget to include any signs!