nitrate ion are all the same. At 124 pm, they are longer than a typical N£O double
bond (120 pm) but shorter than a typical NßO single bond (140 pm). The bond
order in the nitrate ion lies between 1 (a single bond) and 2 (a double bond).
Because all three bonds are identical, a better model of the nitrate ion is a blend
of all three Lewis structures, with each bond intermediate in properties between a
single and a double bond. This blending of structures, which is called resonance, is
depicted in (9) by double-headed arrows. The blended structure is a resonance
hybrid of the contributing Lewis structures. A molecule does not flicker between
different structures: a resonance hybrid is a blend of structures, just as a mule is a
blend of a horse and a donkey, not a creature that flickers between the two. The
three resonance structures in (9) do not actually exist as actual molecules; they are
simply a way of showing that the molecule is best regarded as a blend of structures
and that the electrons are spread out across the molecule.
Electrons that are shown in different positions in a set of resonance structures
are said to be delocalized. Delocalization means that a shared electron pair is distributed
over several pairs of atoms and cannot be identified with just one pair of
atoms. As well as delocalizing electrons over the atoms, resonance also lowers the
energy below that of any single contributing structure and helps to stabilize the
molecule. This lowering of energy occurs for quantum mechanical reasons. Broadly
speaking, the wavefunction that describes the resonance structure is a more accurate
description of the electronic structure of the molecule than the wavefunction
for any single structure alone, and the more accurate the wavefunction, the lower
the corresponding energy.
The following points will help you to write appropriate resonance structures
and to predict which are more important:
• In each contributing structure, the nuclei are in the same positions; only the
locations of lone pairs and bonding pairs are changed.
• Structures with the same energy (so called “equivalent structures”) contribute
equally to the resonance.
• Low-energy structures contribute more to the resonance mixture than highenergy
structures.
For example, although we might be able to write the two hypothetical structures
NNO and NON for the dinitrogen oxide (nitrous oxide) molecule, there is no
resonance between them, because the atoms lie in different locations.