Oxidation numbers provides a means of keeping track of electrons in redox reactions. For some elements, the oxidation number is just another way of stating what its most stable cation or anion will be. In other cases, it is not so obvious, so we have rules. Rules for assigning oxidation numbers 1. The oxidation number for elements is always zero. For example, Na (s) , O 2(g) , C (s) all have zero oxidation numbers. 2. The oxidation number of monoatomic ions is the same as their charge. You already know this one. This means that for Na + , the oxidation number is +1 and for Cl-, the oxidation number is -1. 3. Oxygen is assigned a -2 oxidation number in covalent compounds. This refers to compounds such as CO , CO 2 , SO 2 , and SO 3 . There is an exception to this rule, and it is in peroxides, such as H 2 O 2 . Here, each O in the O 2 2- group has a -1 oxidation number. 4.
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