Chemistry 121 (003) - Fall, 2000 - Dr. Amar

Oxidation Numbers
(©2002, François G. Amar, All rights reserved)
     The oxidation number represents the limiting ionic contribution to bonding. 
For simple ionic crystals like NaCl, the oxidation state of each atom is the 
charge on the atom. The following RULES allow you to determine oxidation 
states for any atom in any compound.
     If two rules are in conflict, the lower-numbered rule takes precedence.
 
0. The sum of the oxidation numbers is 0 for a neutral compound. 
The sum of the oxidation numbers is equal to the overall charge of a 
molecular ion. 
 
1. The oxidation number of any atom in its elemental form is 0. 
Examples: H2 (g), Fe (s), Ar (g), Hg (l).
 
2. The alkali metals always have the oxidation state of +1.
 
3. Fluorine always has an oxidation state of -1.
 
4. Hydrogen is in the +1 oxidation state when bonded to a non-metal (like F or O). 
Hydrogen is in the -1 oxidation state when bonded to a metal (like Li).
 
5. Oxygen almost always has the oxidation number -2. 
Important exceptions are the peroxides like hydrogen peroxide (H2O2) 
and the superoxides.

 

Beyond these strict rules, it is useful to know a few facts and trends

  1. Carbon can have oxidation numbers ranging from -4 to +4 (CH4 to CO2)
  2. Nitrogen can have oxidation numbers from -3 to +5 (NH3 to NO3-).
  3. Halogens (except for fluorine) can have oxidation states of -1 to +7 (HCl to ClO4-).
  4. Sulfur can have oxidation states between -2 and +6 (SH2 to SO42-).