<P> In diagrams, contributing structures are typically separated by double - headed arrows (↔). The arrow should not be confused with the right and left pointing equilibrium arrow (⇌). All structures together may be enclosed in large square brackets, to indicate they picture one single molecule or ion, not different species in a chemical equilibrium . </P> <P> Alternatively to the use of resonance structures in diagrams, a hybrid diagram can be used . In a hybrid diagram, pi bond that are involved in resonance are usually pictured as curves or dashed lines, indicating that these are partial rather than normal complete pi bonds . In benzene and other aromatic rings, the delocalized pi - electrons are sometimes pictured as a solid circle . </P> <P> The ozone molecule is represented by two resonance structures . In reality the two terminal oxygen atoms are equivalent and the hybrid structure is drawn on the right with a charge of − ⁄ on both oxygen atoms and partial double bonds with a full and dashed line and bond order 1 ⁄ . </P> <P> For hypervalent molecules such as xenon difluoride, the rationalization described above can be applied to generate resonance structures to explain the bonding in such molecules . This has been shown by quantum chemical calculations to be the correct description instead of the common expanded octet model . </P>

Which molecule or ion needs only two resonance structures to describe its bonding