Writing Resonance Structures

We're going to make a lot of use of resonance to show how delocalization of electrons influences chemistry. This page presents a few rules to keep in mind when drawing the structures.

To start, we need a valid Lewis structure for the species we are examining.

Without changing the number of unpaired electrons (and switching between ions and radicals always does this, so we avoid doing it), use curly arrows to move one or two pairs of electrons to create another valid Lewis structure.

Here are some things to keep in mind when deciding which electron pairs to move.

Here is an example, in which we write the resonance structures for the species in the middle:

Some general rules:

Finally, although formal charges on individual atoms may change from structure to structure, the net charge on the structure MUST remain the same. Otherwise, you are creating or destroying electrons, and Mother Nature doesn't like that.

Notice here that the starting structure has formal charges that add up to zero; so do both resonance structures that we drew.

Here's another example:

Here, the original structure is on the left, and we generated the third structure from the second. One cannot know ahead of time whether all structures can be generated from the original or from each other, or both. One simply keeps trying, until no new structures seem to appear.

Species with unpaired electrons can have resonance structures also.

Here are some exercises to use to practice:

Remember that none of the structures we draw has any objective reality; we would have to draw the molecular orbitals derived from systems of overlapping p orbitals to achieve that.

[My thanks to my former colleague at Kent State University, Professor Emeritus John Gordon, for the outline of these ideas.]


This page last modified 10:32 AM on Tuesday July 3rd, 2012.
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