Why Oxidation Numbers Are Important

Processes such as the combustion of fuels, the refinement of ores into their component metals, and the corrosion of metals have been among the most familiar chemical reactions since ancient times. The nature of these reactions, however, remained a complete mystery throughout the Middle Ages and well into modern times. It was not until the discovery of oxygen in the 1770s that processes as familiar as burning wood could be explained. 

Rather quickly, chemists of the late 18th century were finally able to associate combustion processes with the reaction of fuels—wood, coal, peat, oil—with oxygen found in the atmosphere. Thus, gaining oxygen atoms came to be called oxidation and the loss of oxygen 

it was realized that combustion reactions and the reactions of metals are just examples of more general processes that do not necessarily require the presence of oxygen. The terms oxidation and reduction, therefore, have been generalized to include a broader class of chemical reactions. 

In a redox reaction the oxidation numbers of two elements will change—the oxidation number of one element will increase, and the oxidation number of another element will decrease. An increase in oxidation number is called oxidation, and a decrease is called reduction. Oxidation and reduction must occur in pairs one cannot occur without the other one occurring also. 

Elements in nature can be found in different oxidation states. For example, the mineral cuprite has the formula Cu O. By rule 6, copper is in the +1 oxidation state. Malachite, on the other hand, has the formula CUjCOjlOHjj. Given that carbon is in the +4 state, each oxygen is in the -2 state, and each hydrogen is in the +1 

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