This is a truly hands-on activity in which students manipulate paper cutouts of carbon atomic orbitals and oxygen group orbitals to identify combinations with identical symmetry and build the carbon dioxide molecular orbital diagram. The activity pairs well with the treatment of MO theory in Miessler, Fischer, and Tarr, Chapter 5. An optional computational modeling component can be added at the end.
After participating in this activity, students will be able to demonstrate the ways in which atmoic orbitals and group orbitals interact to form bonding and anti-bonding molecular orbitals. Students will also be able to use symmetry labels to predict orbital interactions and in the case of no interactions, will be able to identify non-bonding orbitals.
printer, scissors, envelopes
In my inorganic course, I follow Miessler, Fischer, and Tarr’s presentation of molecular orbitals for larger molecules. This activity is done in class after we have worked through the FHF– MO diagram (section 5.4.1). Students have already determined the symmetries of the group orbitals derived from the F atomic orbitals, and they will use these in the activity as we determine the MO diagram for carbon dioxide (section 5.4.2).
The oxygen atomic orbitals combine in the same way that the two F atomic orbitals did in the FHF– example. (See the solutions set of images for the correct labels.)
Before class, I print a set of carbon atomic orbitals and oxygen group orbitals for each student and cut them into pieces. (The carbon atomic orbitals will be individual s, px, py, and pz orbitals while the group orbitals from the oxygen atoms consist of pairs of atomic orbitals.) The orbitals are mixed up in an envelope for each student. Note that this implementation is easy for very small classes, but would need to be streamlined for larger classes. (Ask students to bring their own scissors? Cut the orbitals en masse with a paper cutter?)
I hand out the sets of orbitals and ask the students to:
- Organize your orbitals into “atomic orbitals” and “group orbitals”
- Label each orbital with the appropriate symmetry label – put the labels on the backside of the paper so they don’t distract you.
- How many total orbitals do you have?
- Identify ways in which your atomic and group orbitals can interact to form molecular orbitals – do this by placing the atomic orbital in the center of the group orbital and assessing whether or not the symmetries match.
- Check your symmetry labels (on the backside of the paper) to see whether or not the orbitals you matched together really do have the same symmetry.
After the students have had some time to move around their orbitals, we construct the CO2 MO diagram together on the board (see Figure 5.25 in the 5th edition of Miessler, Fischer, and Tarr). Part of the process involves recognizing the oxygen group orbitals that did not “find a match” with any of the atomic orbitals on carbon and assigning these as non-bonding orbitals. In the end, we count the molecular orbitals formed to make sure there are the same number as we started with (n = 12).
I also calculate the molecular orbital surfaces using Spartan and show these to the students after they have developed the MO diagram.
I have used this activity twice in my advanced inorganic course for juniors and seniors. In both cases, I allow the students to keep their set of orbitals for further study.
Students seem to enjoy getting a chance to manipulate the orbitals directly. One drawback is that the desks in the room I teach in have very small tables, so students don't have a lot of room to work with. Also, there isn't a way to "reverse" the sign of the orbitals in this activity, so the students don't actually visualize the anti-bonding interactions. (I suppose one could print one version of each atomic orbital on one side of the paper and the opposite version on the other side, then allowing students to flip their atomic orbitals over to get the anti-bonding interaction...that would be very cool.)
I haven't directly assessed this activity, but I often include questions on exams that ask students to draw the molecular orbitals, and I hope that this activity helps with the visualization.
Informally, I can say that some students light up at the chance to put down their pens and play with the oribtals.