15 Jun 2011

Constructing MO diagrams

In-Class Activity

Submitted by Adam R. Johnson, Harvey Mudd College

I use this in-class exercise after I have taught the students how to construct LGOs using the generator orbital technique.  The previous week, they do an in-class exercise on that topic, and this week, they use the LGOs from the previous week to construct MO diagrams.

File polyatomicMO_0.docx31.25 KB
Learning Goals: 

Given a centrosymmetric molecule, a student should be able to construct its simplified, σ-only MO diagram.

Related activities: 
Implementation Notes: 

The students work in groups of 2 to 3.  Sometimes I match up "weak" and "strong" students and sometimes I let them pick.  I haven't really noticed much difference.  I circulate and give the students hints, and each group puts its MO diagram up at the chalkboard.  If there is time, I have each group go through the construction of the MO diagram, but often I just hit the highlights.

Time Required: 
one 50 minute class period
Evaluation Methods: 

I have homework assignments on this exact topic the week after they do this exercise.  I also make sure they know that this WILL be on the midterm and final exam, and I don't disappoint.

Evaluation Results: 

All students have a complete (and correct) qualitative σ-only MO diagram for their group's molecule by the end of the class period.  I collect each group's MO diagrams and post either the student-generated answer key or my answer key (linked to this LO) after class. 

Some students do really well with this technique, and by the end of my course, I would say that almost all students are able to construct an MO diagram, especially if they are given central atom AOs with symmetry labels, and similarly labeled LGOs.

Creative Commons License: 
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