Multiple oxidation states

Let us look at some molecules in isolation. Ni­trogen can form compounds with oxygen as well as hydrogen. Ammonia consists of mole­cules in which a single nitrogen atom is at­tached to three hydrogen atoms. Nitrogen is said to have an oxidation number of —3 in am­monia. In another compound known as dinitrogen pentoxide, two nitrogen atoms are attached to five oxygen atoms. In this case, the nitrogen's oxidation number is +5. If an ele­ment can exist in many different oxidation states, it increases the number of possible combinations it can form with other atoms.

Atoms, elements, and molecules: Key chemical reactions 19

Thus, nitrogen can form a whole range of compounds with oxygen and hydrogen. In these compounds, nitrogen's oxidation state takes on all possible values between +5 (in dinitrogen pentoxide) and —3 (in ammonia), including 0 (in free nitrogen gas).

To demonstrate the point made earlier that an oxidation reaction need not involve oxygen, consider the behavior of molten sodium chlo­ride (common table salt). While bound to each other in this compound, sodium has an oxida­tion number of +1. Chlorine has an oxidation number of —1. Passing an electric current through the sodium chloride breaks up the compound into its separate elements—sodium and chlorine. The sodium atom goes through a reduction reaction. Its oxidation number de­creases from +1 to the neutral state, 0. How­ever, the chlorine atom goes through an oxida­tion reaction. Its oxidation number increases from —1 also to the neutral state, 0. An

oxidation-reduction reaction has occurred without involving oxygen.

A practical application of oxidation-reduc­tion reactions is in the area of electric cells or batteries. Combining certain known elements or some stable compounds produces a pre­dictable oxidation-reduction reaction. This re­action, in turn, generates a voltage, that is, electrical power. Such electricity is the basis of electric cells or batteries.