Dashed-line formulas are a tool for drawing resonance
hybrids. These formulas differ from normal Lewis structures
in two ways: 1) dashed lines are used to show partial bonds, and
2) d- and d+
are used to show partial charges (d is
the Greek letter "delta" and is commonly used in science
and mathematics to indicate a fractional or partial quantity).
Ozone's dashed-line formula shows partial bonds and
A dashed-line formula is useful because it tells you
right away, "this molecule is a resonance hybrid." It
also shows you what parts of the electron pattern are delocalized.
Despite these advantages, organic chemists try to
avoid dashed-line formulas because they hide necessary information.
(Look again at ozone's dashed-line formula. This molecule contains
18 electrons, but can you easily find them? Can you find any atoms
with Lewis octets?)
Dashed-line formulas may be hard to read, but they
are easy to draw, especially if you have good drawings of the important
Refer to all of the important contributors, major
and minor, and draw:
- All atoms
- All localized bonds (using solid lines) - a localized
bond appears in every resonance form
- All localized lone pairs (using dots) - a localized
lone pair appears in every resonance form
- All localized formal charges (using +, +2, -,
-2, etc.) - a localized charge appears on the same atom
in every resonance form
- All delocalized bonds (using dashed lines)
- a delocalized bond appears in some resonance forms, but
- All delocalized formal charges (using d-
and d+) - a delocalized charge
appears in some resonance forms, but not all
Notice that delocalized
lone pairs are not included in dashed-line formulas. Their absence
helps explain why these formulas are hard to use for electron counting.
Also notice that special symbols are used only for
delocalized bonds and charges. Standard
Lewis structure symbols are used for the rest of the drawing.