Dashedline 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 dashedline formula shows partial bonds and
partial charges:
A dashedline 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 dashedline formulas because they hide necessary information.
(Look again at ozone's dashedline formula. This molecule contains
18 electrons, but can you easily find them? Can you find any atoms
with Lewis octets?)
Drawing tips
Dashedline formulas may be hard to read, but they
are easy to draw, especially if you have good drawings of the important
resonance contributors.
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
not all
 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 dashedline 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.
