A resonance hybrid can be drawn using a dashed-line
formula, but these drawings make it hard to count electrons.
Chemists prefer to draw a resonance hybrid as a superposition
(or combination) of Lewis structures. The individual Lewis structures
that appear in one of these superposition drawings are called resonance
contributors or resonance forms.
The following drawing of ozone illustrates both drawing
methods. A dashed-line formula is shown on the left. A superposition
of two resonance contributors (formulas I and II) is shown on the
A superposition drawing always includes at least two
resonance contributors and a double-headed arrow connecting
pairs of contributors. This arrow is a special
symbol that means, "construct a superposition of the electron
patterns connected by this arrow."
Resonance forms have two flaws: they are deceptive
and they ... are ... deceptive. Resonance forms,
like normal Lewis structures, are always drawn using standard
symbols. The drawing does not identify delocalized
electrons or differentiate them from localized
electrons. This is deceptive. Also, a resonance form necessarily
presents a picture of the hybrid that is incomplete and incorrect.
This is certainly deceptive.
One way to generate the correct picture for a resonance
hybrid is to draw a dashed-line formula.
Alternatively, we can construct a superposition of
the electron patterns shown in each resonance contributor. For example,
we can find the charge on Oleft
by averaging the neutral Oleft
in I and the negative Oleft in
II. This gives a partial negative charge. In the same way, we can
tell that Oleft-Ocenter
are really connected by a partial double bond because the bond is
a superposition of the double bond in I and the single bond in II.
Just remember this: even though resonance contributors
look like Lewis structures, their status is different. A Lewis structure
shows what a normal molecule looks like, but a resonance contributor
does not. A resonance contributor is, by definition, a poor picture.
You can think of a resonance contributor as being like a black-and-white
photo. Useful, but misleading.
(the way I really look*)
*Photo credit: http://www.georgeclooney.org/html/PhotoIndex.html