7.2 Oxidation states

• 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.
• A negative number shows that the element has gained electrons and has therefore been reduced.
• The more positive the number, the the more the element has been oxidised. The more negative the number, the more it has been reduced.
• Oxidation state rules, in order of priority (from 4 down, these apply when in a compound):

  1. Uncombined elements = 0
  2. Monatomic ions = charge
  3. Compounds = overall charge
  4. Group 1 atoms = +1
  5. Group 2 atoms = +2
  6. Fluorine = -1
  7. Hydrogen = +1
  8. Oxygen = -2
  9. Chlorine = -1
• Disproportionation reactions are reactions in which an element is both oxidised and reduced at the same time. For example, when a Cl2 molecule reacts with water, one chlorine atom is reduced to hydrochloric acid, and the other atom is oxidised to chloric (I) acid.
• All disproportionation reactions involve changes in oxidation state, so they all count as redox reactions.