Assigning Oxidation Numbers

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Oxidation-reduction reactions (redox reactions) are reactions in which electrons are lost by an atom or ion in one reactant and gained by an atom or ion in another reactant. Although electrons are gained and lost in these reactions, the balanced equation for a redox reaction does not show the electrons that are being transferred. In order to tell whether a redox reaction has occurred or not, we need a way to keep track of electrons. The best way to do so is by assigning oxidation numbers to the atoms or ions involved in a chemical reaction.

Oxidation numbers are hypothetical numbers assigned to an individual atom or ion present in a substance using a set of rules. Oxidation numbers (or oxidation states as they are also called) can be positive, negative, or zero. It is VERY IMPORTANT to remember that oxidation numbers are always reported for one individual atom or ion and not for groups of atoms or ions.

The following rules are used to assign oxidation numbers. Chem 1115 students will have these rules available on exams. Chem 1215 students must memorize these rules.

Chem 1115

Chem 1215

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Oxidation Number Rules
  1. The oxidation number for an atom in its elemental form is always zero.
    • A substance is elemental if both of the following are true:
      • only one kind of atom is present
      • charge = 0
    • Examples:
      • S8: The oxidation number of S = 0
      • Fe: The oxidation number of Fe = 0
  2. The oxidation number of a monoatomic ion = charge of the monatomic ion.
    • Examples:
      • Oxidation number of S2- is -2.
      • Oxidation number of Al3+ is +3.
  3. The oxidation number of all Group 1A metals = +1 (unless elemental).
  4. The oxidation number of all Group 2A metals = +2 (unless elemental).
  5. Hydrogen (H) has two possible oxidation numbers:
    • +1 when bonded to a nonmetal
    • -1 when bonded to a metal
  6. Oxygen (O) has two possilbe oxidation numbers:
    • -1 in peroxides (O22-)....pretty uncommon
    • -2 in all other compounds...most common
  7. The oxidation number of fluorine (F) is always -1.
  8. The sum of the oxidation numbers of all atoms (or ions) in a neutral compound = 0.
  9. The sum of the oxidation numbers of all atoms in a polyatomic ion = charge on the polyatomic ion.

When assigning oxidation numbers to the elements in a substance, take a systematic approach. Ask yourself the following questions:

  1. Is the substance elemental?
  2. Is the substance ionic?
  3. If the substance is ionic, are there any monoatomic ions present?
  4. Which elements have specific rules?
  5. Which element(s) do(es) not have rules?
    • Use rule 8 or 9 from above to calculate these.

Example: Determine the oxidation number of each element in Na2SO4.

  1. Is the substance elemental?
    • No, there are three elements present.
  2. Is the substance ionic?
    • Yes, because metal + non-metal = ionic.
  3. If the substance is ionic, are there any monoatomic ions present?
    • Yes, the sodium ion (Na+) is monoatomic.
    • Therefore, the oxidation number of Na is +1.
  4. Which elements have specific rules?
    • Oxygen has a rule (-2 in most compounds).
    • Oxidation number of O = -2.
  5. Which element(s) do(es) not have rules?
    • S does not have a rule.
    • Use rule 8 (and a little simple algebra) to find the oxidation number of S.
      • Let S = oxidation number of sulfur.
      • According to rule 8:
        • (# Na) (Oxid. # of Na) + (# S)(Oxid. # S) + (# Oxygens) (Oxid. # of O) = 0
        • So: 2(+1) + S + 4(-2) = 0
      • Solve for S using algebra. S = +6.
      • Therefore, the oxidation number of S = +6

Example: Determine the oxidation number of each element in K2C2O4.

  1. Is the substance elemental?
    1. No, there are three elements present.
  2. Is the substance ionic?
    • Yes, because metal + non-metal = ionic.
  3. If the substance is ionic, are there any monoatomic ions present?
    • Yes, the potassium ion (K+) is monoatomic.
    • Therefore, the oxidation number of K is +1.
  4. Which elements have specific rules?
    • Oxygen has a rule (-2 in most compounds).
    • Oxidation number of O = -2.
  5. Which element(s) do(es) not have rules?
    • C does not have a rule.
    • Use rule 8 (and a little simple algebra) to find the oxidation number of C.
      • Let C = oxidation number of carbon.
      • According to rule 8:
        • (# K) (Oxid. # of K) + (# C)(Oxid. # C) + (# Oxygens) (Oxid. # of O) = 0
        • So: 2(+1) + 2C + 4(-2) = 0
      • Solve for C using algebra. C = +3.
      • Therefore, the oxidation number of C = +3

Practice Problems

Determine the oxidation number of each element in the following compounds:

  1. Ba(NO3)2
  2. NF3
  3. (NH4)2SO4

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