7.2 Oxidation states

• In an ionic compound, the oxidation state tells us how many electrons it has lost or gained, compared with the element in its uncombined state
• In a molecule, the oxidation state tells us about the distribution of electrons between elements of different electronegativity
• The more electronegative element is given the negative oxidation state
• Every element in its uncombined state has an oxidation state of zero
• • A positive number shows that the element has lost electrons and has therefore been oxidised
  • For example, Mg2+ has an oxidation state of +2
• • A negative number shows that the element has gained electrons and has therefore been reduced
  • For example Cl- has an oxidation state of -1
• • The more positive the number, the more the element has been oxidised
  • The more negative the number, the more it has been reduced
• The oxidation state of hydrogen in a compound is +1, except in metal hydrides e.g NaH where it is -1
• The oxidation state of group 1 elements in a compound is always +1
• The oxidation state of group 2 elements in a compound is always +2
• The oxidation state of aluminium in a compound is always +3
• The oxidation state of oxygen in a compound is -2

  except in peroxides where it is -1

  and in the compound OF2 where it is +2
• The oxidation state of Fluorine in a compound is always -1
• The oxidation state of chlorine in a compound is -1

  except in compounds with F and O where it has positive values
• The sum of all the oxidation states in a compound is zero since all compounds are electrically neutral
• The sum of the oxidation states of a complex ion, such as NH4+ or SO42- equals the charge on the ion