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Oxidation Numbers (Oxidation States)

Key Concepts

  • An oxidation number (oxidation state) is the charge an atom would carry if the molecule or ion were completely ionic.

  • For a neutral molecule or compound, the sum of the oxidation numbers (states) for each element in the molecule equals 0.

  • For a charged molecule (ion), the sum of the oxidation numbers (states) for each element in the ion equals the charge on the ion.

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Rules for Assigning Oxidation Numbers

  1. Fluorine is assigned an oxidation number of -1 in compounds

  2. Oxygen is assigned an oxidation number of -2 in compounds

    Except in

    • Peroxides

    • Superoxides

    • Oxygen fluorides

  3. Hydrogen is assigned an oxidation of +1 in compounds

  4. Group 1 elements (Alkali Metals) are assigned an oxidation number of +1 in compounds

  5. Group 2 elements (Alkaline-earth metals) are assigned an oxidation number of +2 in compounds

  6. An atom of any element in the free state has an oxidation number of 0

  7. Any monatomic ion has an oxidation number equal to its charge

  8. The sum of the oxidation numbers of all the atoms in formula equals the electrical charge shown with the formula

Examples: Calculating an Oxidation Number (oxidation state)

  1. Find the oxidation number (state) of carbon in CO2

    • Write a general equation to calculate the oxidation number (state) for the unknown atom:
      charge on molecule = carbon's oxidation number + 2 x oxygen's oxidation number

    • Use the Rules above to list the oxidation numbers (states) for the molecule and each element:
      CO2 is a neutral molecule so its overall electrical charge is 0 (rule h above)
      The oxidation number (state) for oxygen is -2 (rule b above)
      let x be the unknown oxidation number (state) of carbon

    • Substitute in the values for the known oxidation numbers (states) in the general equation
      charge on molecule = carbon's oxidation number + 2 x oxygen's oxidation number
      0 = x + (2 x -2)

    • Solve the equation to determine the oxidation number (state) of the unknown atom:
      x (oxidation number of C) = 0 + 4 = +4
      The oxidation number (state) of carbon in CO2 is +4

  2. Find the oxidation number (state) for chromium in Cr2O72-

    • Write a general equation to calculate the oxidation number (state) for the unknown atom:
      charge on ion = 2 x chromium's oxidation number + 7 x oxygen's oxidation number

    • Use the Rules above to list the oxidation numbers (states) for the molecule and each element:
      Cr2O72- is an ion with an overall electrical charge of 2- (rule h above)
      The oxidation number (state) for oxygen is -2 (rule b above)
      let x be the unknown oxidation number (state) of chromium

    • Substitute in the values for the known oxidation numbers (states) in the general equation
      charge on ion = 2 x chromium's oxidation number + 7 x oxygen's oxidation number
      2- = 2x + (7 x -2)
      2- = 2x - 14

    • Rearrange the equation and solve it to determine the oxidation number (state) of the unknown atom:
      2x = 2- + 14
      2x = 12
      x = 12 ÷ 2 = +6
      The oxidation number (state) of chromium in Cr2O72- is +6


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