Key Concepts

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

Rules for Assigning Oxidation

Numbers

1. Fluorine is assigned an oxidation number of -1 in compounds
   HF oxidation number of fluorine = -1
2. Oxygen is assigned an oxidation number of -2 in compounds
   H₂O oxidation number of oxygen is -2

   Except

   • Peroxides: oxidation number of oxygen is -1
     H₂O₂ oxidation number of oxygen is -1
   • Superoxides: oxidation number of oxygen is -½
     KO₂ oxidation number of oxygen is ½
   • Oxygen fluorides: OF₂ oxidation number of oxygen is +2
     O₂F₂ oxidation number of oxygen is +1
3. Hydrogen is assigned an oxidation of +1 in compounds
   HCl oxidation number of hydrogen is +1
4. Group I elements (Alkali Metals) are assigned an oxidation number of +1 in compounds
   NaNO₃ oxidation number of sodium is +1
5. Group II elements (Alkaline-earth metals) are assigned an oxidation number of +2 in compounds
   MgBr₂ oxidation number of magnesium is +2
6. An atom of any element in the free state has an oxidation number of 0
   S₈ oxidation number of each sulfur atom is 0
7. Any monatomic ion has an oxidation number equal to its charge
   H^- oxidation number of hydrogen is -1
8. The sum of the oxidation numbers of all the atoms in a formula equals the electrical charge shown with the formula
   • The sum of the oxidation numbers of all the atoms shown in a formula of a compound is 0
     CO₂: let x be the unknown oxidation number of carbon
     0 = x + (2 x -2)
     So x (oxidation number of C) = 0 + 4 = +4
   • The sum of the oxidation numbers of all the atoms shown in the formula for a polyatomic ion or complex ion equals the electrical charge on the ion
     Cr₂O₇^2-: let y = unknown oxidation number of chromium
     -2 = 2y + (7 x -2)
     2y = -2 + 14
     2y = +12
     y (oxidation number of Cr) = 12 ÷ 2 = +6