Oxidation Number

Oxidation Number (or Oxidation State) refers to the charge that a particular atom is assigned in compounds. The easiest determination of oxidation number is for ionic substances. For example, a Group I metal (e.g., Na, K, Cs, etc.) all attain a charge of +1 when they become ions. Thus, the Oxidation Number for the Na ion would be +1. Therefore, following this pattern, the charge of any metal ion would be its Oxidation Number.  Likewise, the charge of any non-metal ion would be its Oxidation Number.

For non-ionic compounds, the Oxidation Number is determined following the rules listed below.

Rules on how to determine Oxidation Number for non-metals in covalent compounds and polyatomic ions follows:

1. All shared electrons in covalent bonds are assigned to the more electronegative element.
2. All lone electron pairs for an element are assigned to that element.
3. Add together the number of electrons for steps (1) and (2) above.
4. Compare the total number of electrons in step (3) to the Group Number of the element.

To determine the Oxidation Number for a non-metal in covalent compounds or polyatomic is the difference between the Group Number and the total number of electrons assigned to that atom.  For example, if you assign a total of eight (8) electrons to an oxygen atom (Group Number is 6), the Oxidation Number would be -2 (i.e., Group Number - Assigned Electrons: 6-8 = -2). Likewise, if the total number of assigned electrons is 3 for a carbon atom (Group Number is 4), the Oxidation Number would be +1 (Group Number - Assigned Electrons: 4-3 = +1). See the following table for these two examples.

 Use the following Electronegativity values when determining which atom shared electrons will be assigned to calculater Oxidation Number.

Remember to assign all covalent shared electrons to the element with a larger Electronegativity value. After assigning electrons based on Electronegativity value, determine how many (if any) lone pair electrons are on that atom, and add the two values together.  This value would be the total number of assigned electrons. Subtract the total assigned electrons from the Group Number to get the Oxidation Number for that particular atom.  Under most conditions, O is assigned an Oxidation Number of -2 and H is assigned an Oxidation Number of +1 (although exceptions can occur).