“A resonance arrow, specific
In theory portrayed, scientific:
See multiple structures
Advance, through their juncture,
Amount of chem info prolific.”
The 20 April 2023 Twitter limerick celebrated yet another type of arrow. This one is called the resonance arrow, and it communicates information about bonding in molecules.
“A resonance arrow, specific /
In theory portrayed, scientific…”
The resonance arrow is a single line with a double-headed arrow on either side: ↔
This arrow communicates information for a single molecular species (rather than a reaction or process), by acknowledging the specific concepts of resonance theory.
“See multiple structures /
Advance, through their juncture, /
Amount of chem info prolific.”
Ozone (O3) is an example of a molecule that exhibits resonance. I have shown below two Lewis structures that can be drawn for the molecule ozone (O3), connected by a resonance arrow. Structure 1 has a double bond (two lines) between oxygen A and oxygen B and a single bond (one line) between oxygen B and oxygen C. Structure 2 has a single bond between oxygen A and oxygen B and a double bond between oxygen B and oxygen C.
The true picture of ozone is that each of its two bonding regions (one between A and B; one between B and C) is essentially “one and a half“ bonds; in other words, we need to mentally average the two structures shown to truly understand what is going on.
Structures 1 and 2 are thus called resonance structures, and their “average” is called the resonance hybrid. Experimental support here includes the identical lengths for ozone’s two bonding regions and the fact that those bonds (128 picometers, or pm) are shorter than a single bond (143 pm) and longer than a double bond (121 pm) between oxygen atoms.
In other words, we have to acknowledge the “juncture” of “multiple structures,” to better understand a molecule that exhibits resonance. (This can all seem quite complex in its initial presentation! The verse, as always, gives an overview only.)
Chemphasis 