Assigning Oxidation Numbers

1. Atoms in their elemental state have an oxidation number of zero (i.e. silver metal, Ag^o, and Hydrogen gas, H₂^o, have oxidation numbers of 0).
2. Monoatomic ions have an oxidation number equal to their charge (i.e. Na⁺ and Cl^- have +1 and -1 oxidation numbers, respectively).
3. Compounds
   • Fluorine always has a -1 oxidation number.
   • Oxygen usually has a -2 oxidation number with the following exceptions:
     • Oxygen has a positive oxidation number in Oxygen-Fluorine compounds: OF₂, O₂F₂, O₃F₂.
     • Oxygen has a -1 oxidation number in peroxides (i.e. hydrogen peroxide, H₂O₂, Hydrogen's oxidation number is +1, Oxygen's oxidation number is -1).
   • Hydrogen has a +1 oxidation number except when bonded to a metal (i.e. the H in NaH has a -1 oxidation number, H^- is called the hydride ion).
4. Atoms with unknown or variable oxidation numbers
   • Use Steps 1 - 3 to assign the oxidation numbers of the "known" atoms.
   • Sum these oxidation numbers.
   • Subtract this sum from the charge of the species.
   • Divide by the number of atoms in the formula with an unknown oxidation number
   • Example: K₂Cr₂O₇ (Cr's oxidation number is "unknown").
     • Applying steps 1-3, K's oxidation number is +1 (K₂ is +2) and O's oxidation number is -2 (O₇ is -14).
     • The sum of K's and O's oxidation numbers is -12.
     • The overall charge of K₂Cr₂O₇ is zero.   ∴  0 - (-12) = +12
     • Division by the number of Cr atoms in K₂Cr₂O₇ gives +12 ÷ 2 = +6. The oxidation number of Cr is +6.