How Do I Assign Oxidation Numbers

The following guidelines will help you assign oxidation numbers. There are essentially two rules  

The oxidation number of any element, in its elemental form, is zero. 

The oxidation numbers in any chemical species must sum to the overall charge on the species. That is, oxidation numbers must sum to zero for any molecule and must sum to the charge on any polyatomic ion. The oxidation number of a monatomic ion is equal to the charge on the iom 

In addition to these two rules, it is necessary to know the elements that always, or nearly always, have the same oxidation number. Table 4.6 lists elements whose oxidation numbers are reliable, in order of decreasing reliability. 

To determine oxidation numbers in a compound or a polyatomic ion, you must use a stepwise, systematic approach. Draw a circle under each element s symbol in the chemical formula. Then draw a square under each circle. In the circle, write the oxidation number of the element in the square, write the total contribution to charge by that element. Start with the oxidation numbers you know, and use them to figure out the ones you don t know. Here is an example  

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