The distinction between "localized" and
"delocalized" only exists when we describe electron motion
using Lewis' bonding theory.
According to this theory, localized electrons
exhibit normal behavior. A localized lone pair remains
close to one atom. A localized bond pair travels
between two atoms.
Resonance hybrids necessarily
contain some "abnormal" electrons. A lone pair may seem
to have some bonding characteristics; instead of sticking near one
atom, it visits two atoms. A bond pair may appear to move between
two different pairs of atoms. These electrons step outside the boundaries
that Lewis' theory has set for them, and we consider them to be
The easiest way to spot delocalized electrons is to
compare electron locations in two resonance forms. If a pair appears
in one place in one form, and in a different place in another form,
the pair is delocalized.
You can see delocalized behavior in resonance forms
I and II below. Both forms contain two delocalized electron
pairs. Can you find them? What kind of electron pairs are delocalized
in each structure?
(Answer: these forms contain one delocalized bond
pair and one delocalized lone pair.)
Delocalized bonds and delocalized charges
The "delocalized" label also applies to
bonds and charges.
A delocalized bond is a bond that appears in
some resonance forms, but not others. Resonance form I contains
2 localized bonds and 1 delocalized bond.
A delocalized charge is a formal charge that
appears on one atom in some resonance forms and on other atoms in
other forms. Ozone's negative charge is delocalized over
the two end O, while the positive charge is localized on the center