oxidation numbers

• Oxidation number


  The charge of an atom in a compound
• Oxidation number of pure elements

  • Always zero
• Oxidation number of ions


  Equal to the charge of the ion
• Finding oxidation number of an element in a compound

  1. Determine electronegativity differences

  2. Assign charges based on electronegativity differences

  3. Balance the overall charge to be zero
• Fluorine always has an oxidation number of -1 in compounds
• Oxygen usually has an oxidation number of -2 in compounds, except in peroxides (-1) and superoxides (-1/2)
• Hydrogen has an oxidation number of +1 when bonded to non-metals, and -1 when bonded to metals
• Electronegativity increases from left to right and top to bottom on the periodic table
• Higher electronegativity


  Partial negative charge
• Lower electronegativity


  Partial positive charge
• The more electronegative element in a compound will have a negative oxidation number
• Oxidation state


  The charge on an atom in a compound
• Hydrogen has an electronegativity value of 2.1, sulfur is 2.5, and oxygen is 3.5
• Sulfur dioxide (SO2)


  Oxygen has the partial negative charge, sulfur has the partial positive charge
• Hydrogen sulfide (H2S)


  Hydrogen has the partial positive charge, sulfur has the partial negative charge
• In the periodic table, nitrogen, oxygen, and fluorine typically have oxidation states of -3, -2, and -1 respectively. Sulfur usually has an oxidation state of -2 if it is the more electronegative element.
• In H2S, hydrogen has an oxidation state of +1 and sulfur has an oxidation state of -2
• In SO2, sulfur has an oxidation state of +4 because it cannot be determined from the periodic table alone
• Finding the oxidation state

  1. Determine the more electronegative element

  2. Assign the appropriate oxidation state to the more electronegative element

  3. Use the neutral charge of the compound to solve for the oxidation state of the less electronegative element
• Ammonia (NH3)


  Nitrogen has an oxidation state of -3, hydrogen has an oxidation state of +1
• Nitrogen dioxide (NO2)

  Nitrogen has an oxidation state of +4, oxygen has an oxidation state of -2
• Methane (CH4)

  Carbon has an oxidation state of -4, hydrogen has an oxidation state of +1
• Carbon dioxide (CO2)


  Carbon has an oxidation state of +4, oxygen has an oxidation state of -2
• C3H8


  Carbon has an average oxidation state of -8/3, hydrogen has an oxidation state of +1
• Fe3O4


  Iron has an average oxidation state of +2.67, oxygen has an oxidation state of -2
• The average oxidation state can be a decimal value when there are multiple atoms of the same element with different oxidation states
• Sulfate ion (SO4^2-)


  Sulfur has an oxidation state of +6, oxygen has an oxidation state of -2
• Potassium chromate (K2CrO4)


  Potassium has an oxidation state of +1, chromium has an oxidation state of +6, oxygen has an oxidation state of -2
• Potassium bicarbonate (KHCO3)


  Potassium has an oxidation state of +1, hydrogen has an oxidation state of +1, carbon has an oxidation state of +4, oxygen has an oxidation state of -2
• Bromine chloride (BrCl3)


  Bromine has an oxidation state of +3, chlorine has an oxidation state of -1
• Iodine bromide (IBr5)


  Iodine has an oxidation state of +5, bromine has an oxidation state of -1
• The more electronegative element typically takes the natural oxidation state from the periodic table, the less electronegative element has its oxidation state determined