3.7 Resonance
Some molecules are not represented adequately by a single Lewis structure. For
example, consider the nitrate ion, NO3
, which, as potassium nitrate, is used in
fireworks and fertilizers. The three Lewis structures shown in (8) differ only in the
position of the double bond. All are valid structures and have exactly the same
energy. If one of the pictured structures were correct, we would expect the nitrate
ion to have two long single bonds and one short double bond, because a double
bond between two atoms is shorter than a single bond between the same types of
atoms. However, detailed measurements have shown that the bond lengths in a