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These slides provide an introduction to s-p mixing in diatomic molecular orbital diagrams appropriate for students in a general chemistry course.
In particular, my students were looking for a tool to help them remember which ordering of orbital energies to use. I've always thought that the Z ≤ 7 order (with s-p mixing) looks like a tree. In office hours, a student pointed out that the "standard" (Z ≥ 8) ordering of molecular orbitals looks like a light bulb. Thus, because "it's only Christmas sometimes," the MO diagram with s-p mixing--which looks like a Christmas tree--is only used in a few cases. Light bulbs are used most of the time, so the MO diagram that looks like a light bulb is used for most diatomics.
| Attachment | Size |
|---|---|
| Five Slides About_s-p Mixing .pptx | 161.37 KB |
A student should be able to determine which ordering of molecular orbitals to use in generating a MO diagram for homonuclear and heteronuclear diatomic molecules with and without s-p mixing.
A student should be able to qualitatively explain (1) why s-p mixing only occurs for some elements and (2) why s-p mixing increases the energy of the 2σ MO relative to the 1π MOs.
I teach primarily using chalk, with slides as needed, so I actually go back and draw the shapes (in colored chalk) over MO diagrams I've previously drawn on the board. The students are suprised and excited to see the shapes emerge.
Evaluation
Student learning is assessed using homework assignments and exams.
Relevant questions on exams are generally along the lines of "rank the [number of unpaired electrons in/bond strength of/bond length of] the following 3 molecules" or "give an example of a molecule [with unpaired electrons/that is diamagnetic/etc.]" Questions are short answer, so a justification and correct MO diagrams are required.
Students in my course love MO diagrams, and will almost always choose to draw MO diagrams when they have a choice of questions on an exam. They also have a very high rate of successfully answering these questions.
I build on this enthusiasm to briefly show MO diagrams for a few polyatomic molecules and a transition metal complex later in the course. Students find these a bit scary to look at, but are excited that they can at least somewhat interpret them, which they couldn't do at the beginning of the semester. (I show a MO diagram for water on the first day of the semester and they panic, but remember it later.)
The students like MO diagrams because there's a process to follow. The conceptual understanding of s-p mixing is a bit more challenging.
This is a great way to remind students about this ordering. I'm totally planning to use this in the fall! Thanks for sharing.