Lesson 2: Oxidation Numbers

• Oxidation number

  The charge an atom has, (or would have if ionic), when the electrons are counted, according to a set of rules
• The sign (+ or - ) for the oxidation number is written first (unlike in ions, where the + or - is written after the number)
• Guidelines for oxidation numbers

  • Free elements = 0
  • Simple ions = same as ion charge
  • Neutral compounds = sum of oxidation numbers of all atoms/ions = 0
  • Polyatomic ions = sum of all oxidation numbers of atoms = charge on ion
• Priority list for oxidation numbers

  • F = always -1
  • Group 1 = always +1
  • Group 2 = always +2
  • H = usually +1 (except for NaH, H = -1)
  • O = usually -2 (except for H2O2: O = -1, and OF2: O= +2)
• Finding unknown oxidation numbers using known oxidation numbers

  Set up equation with known and unknown oxidation numbers

  2. Solve for the unknown oxidation number
• In molecular compounds that contain atoms not on the priority list, the more electronegative element is assigned the charge it would have if it was in an ionic compound
• If electronegativity values are the same, the element that more likely forms an anion is given priority
• Identifying redox reactions

  • Any reaction that involves an element as a reactant or product

  All synthesis, decomposition, single displacement and combustion reactions are redox

  Double replacement reactions are never redox reactions
• Using oxidation numbers to identify redox reactions

  The species that is oxidized will have an increase in oxidation number

  The species that is reduced will have a decrease in oxidation number

  If there is no change in oxidation number, the reaction is not a redox reaction