Final answer:
The correct symbol to use between resonance structures is the double-headed arrow (⇄ or ⇌), which denotes that the true structure of a molecule is an average of all the resonance forms.
Step-by-step explanation:
The symbol that should be used between two structures that represent resonance structures is the double-headed arrow, which is represented by ⇄ or ⇌. This symbol signifies that the actual electronic structure of a molecule is an average or a hybrid of all the resonance structures. It is not correct to use a single-headed arrow or any other symbol to denote resonance because it might imply a change or reaction rather than a delocalization of electrons. For instance, in the case of the ozone molecule (O3), the resonance structures can be depicted using a double-headed arrow to signify the partial character of the bonds between the oxygen atoms, which are neither pure single bonds nor pure double bonds. Such representations with double-headed arrows emphasize the concept that electrons in the molecule are delocalized and that the resonance forms illustrate the same molecular structure from different perspectives.