Kekulè's Monkey in Anthracene
This Demonstration shows the four principal resonance structures of anthracene. To better understand how these structures are formed, they can be visualized as a sequence of rotations of the double bonds. As a double bond shared between two rings belongs to both, four different structures can be formed by rotating a shared double bond. Individual resonance structures are actually an abstraction, but these help us to understand the behavior of the molecule.[more]
The concept of resonance was first introduced by German organic chemist August Kekulè after his legendary dream about the ring structure of benzene . To introduce the concept of bond order: after choosing a certain C-C bond, assign the values 1 or 2 according to whether the bond is single or double; do this for all the resonance structures. The total is divided by the number of resonance structures to get the bond order.
The first three windows show how to find the resonance structures and the linking between them. The animation is controlled by the "run/stop" control.
The last window shows the four structures all together: the second and third have more rings than the others, giving them higher stability. Selecting a bond displays its bond order.[less]
Snapshot 1: first resonance structure of the anthracene; a rotation of the double bond gives the second resonance structure
Snapshot 2: the C-C bond 10a-8a has a bond order of 1.25 as it is a single bond in three separate structures, while in the second one it is a double bond
Snapshot 3: the C-C bond 8-7 has a bond order of 1.75 as it is a double bond in three separate structures, while in the first one it is a double bond
 Wikipedia. "Aromaticity." (Feb 12, 2020) en.wikipedia.org/wiki/Aromaticity.