The CoolMolecules website offers an interesting way to approach the subject of multiple bonds (moving from sigma structures to Lewis structures). Multiple bond theory was developed primarily as a way to explain the uncharacteristically short bonds found in some molecules. Using the data in the CoolMolecules website, students can gain an accurate appreciation of multiple bonds by approaching the subject from the same angle: data first, theory second.

Note that we decided not to show double bonds in the molecules in the database, for three reasons:

  1. Multiple bonds are theoretical interpretations of the experimental data. We prefer, for the purposes of this website, to include as few of our interpretations of the data as possible so that the students can come to their own conclusions about bonding.
  2. We could find no suitable way to suitably depict molecules that have delocalized pi-electrons.
  3. Only sigma structures, not Lewis structures, are necessary for predicting the geometry around a central atom. Thus, no multiple bonds are needed for an initial prediction of molecular shape.

Along with the concept of double bonds comes the concept of resonance: the idea that some bonds have characteristics that are intermediate between single-bonds and double-bonds. As described in the second activity, below, the CoolMolecules website can also be used to demonstrate that molecules involving resonance have bonds that are intermediate in length between their cooresponding single and double bonds.

Suggested Activity #1

We suggest having students explore the "entry level" database, taking special note of variation in bond length between molecules. The "bonds" search was developed especially for this use. Once the "bonds" search is selected, the red histogram on the left can be used to narrow down the search to show only molecules that contain bond lengths within a certain range. It is difficult to see the pattern if the whole database is searched, so we suggest using the "entry level" database and restricting the search to CC, CN, or CO bonds.

Students should be making sigma structures of some molecules on the list, and a classroom discussion could follow about why some of the bonds are so much shorter than others. At that point, the octet rule for the central atom can be introduced, and students can learn to draw full Lewis structures. See the Construct a Lewis Structure web page for detailed, step-by-step information about converting sigma structures to Lewis structures.

Suggested Activity #2

Good examples of resonance in the database include:

Students should compare the bond distances in these compounds to other compounds that do not involve resonance. Some of these may be surprising if you don't realize that ions in the database are really in crystals along with cations, which probably aren't shown. The presence of cations can dramatically change bond lengths in nearby anions.