Understanding Oxidation Number and Oxidation State

Key Definitions

• Oxidation Number: Represents how many electrons an atom has gained or lost in a molecule.

  • Elements have an oxidation state of zero.
  • Atoms in ionic compounds are assigned a positive or negative oxidation state.
  • Organic and some covalent compounds do not have oxidation states assigned to atoms.
  • According to IUPAC: The oxidation state is the charge of an atom after ionic approximation of its heteronuclear bonds.

• Oxidation State: Simply the oxidation number of an atom. They are generally equivalent, except in specific exceptions.

Rules for Determining Oxidation Numbers

1. Fluorine: Always -1.
2. Oxygen: Generally -2, except:
   • With fluorine.
   • In peroxides or peroxy anions, where it is -1.
3. Elemental State: Oxidation state is zero (e.g., He, O₂, S₈).
4. Ionic Compounds:
   • Metal/more electronegative atom: Positive oxidation number.
   • Non-metal/less electronegative atom: Negative oxidation number.
5. Alkali Metals (Group 1): +1 (e.g., sodium, potassium).
6. Alkaline Earth Metals (Group 2): +2 (e.g., calcium, magnesium).
7. Hydrogen: +1 with non-metals, -1 with metals.
8. Halogens: -1 in binary compounds, except inter-halogen compounds.
9. Monoatomic Ion: Oxidation state equals the ion charge.
10. Polyatomic Ion: Sum of oxidation states equals the ion charge.
    • Example: Nitrate ion (NO₃⁻), nitrogen is +5.
    • Example: Chlorite ion (ClO₂⁻), chlorine is +3.

Examples and Fun Facts

• Elements with Multiple Oxidation States: Vanadium, manganese, and chromium have diverse stable oxidation states and colors.
• Highest Known Oxidation State: +9 in tetroxoiridium(IX) (IrO₄⁺).

Oxidation State of Sulfur in Tetrathionate Ion (S₄O₆²⁻)

• Two sulfurs: Oxidation state 0.
• Two sulfurs: Oxidation state +5.
• Average oxidation number: (0+0+5+5)/4 = 2.5.

This example illustrates the rare occurrence where oxidation number and oxidation state can differ.