19 Aug 2009

In-Class Review of Symmetry Operations and Point Groups

In-Class Activity

Submitted by Maggie Geselbracht, Reed College
Categories
Prerequisites: 
Corequisites: 
Course Level: 
Topics Covered: 
Description: 

This in-class activity was used on the first day of Advanced Inorganic Chemistry in lieu of lecture to review symmetry operations and point groups in small molecules.  The learning object was adapted to a small group discussion format from a fundamental quiz posted by Barbara Reisner (James Madison University) and a problem set question posted by Adam Johnson (Harvey Mudd College).

Learning Goals: 

At the end of this in-class activity, students will be able:

  • To recall the types of symmetry operations present in simple molecules including the nomenclature used for different types of mirror planes.
  • To identify the distinguishing characteristics of each point group by thinking of a molecular example (preferably inorganic!) that belongs to that point group.

Implementation Notes: 

I let the students self-select into groups of 3-4 and provided them with a photocopy of a point group flowchart from an inorganic textbook.  Although I expect them ultimately to be able to work without the aid of a flowchart, this was a necessary “jog” to their memory on the first day of class.

Time Required: 
20-30 minutes
Evaluation
Evaluation Methods: 

I collected the group scribe sheets at the end of class to get a sense of how they did although this was not a graded activity.

Evaluation Results: 

The attached document provides a quick overview of the common problems that students had with this activity.  They generally did not get very far with the last question, thinking of molecular examples for common point groups.  In the future, I would either provide more time, pare down the list to 5 or 6 key point groups, or follow up the activity with a problem set question.

Creative Commons License: 
Creative Commons Licence

Comments

That is why I assign it as homework.  But I like your divvying up of teams into "scribe" "facilitator" "motivator" and "timekeeper."  I may use that format in the future.

Adam

I repeated this activity this spring, again on the first day of my Advanced Inorganic course.  The only change I made was to shorten the list of point groups on the last problem to: D5d, D5h, D2d, D2h, D3, C4v, C2h (or C3h), and Cs.  Alas, I still did not leave enough time!  I intended to follow-up with this question on their first problem set, but it was already too long.

This year, I had 3 groups of 3 students each.  I would say they did slightly better on the first two questions than the groups from 2 years ago.  I did point out to all the groups that naming the mirror planes in ethylene is tough because there is no principal rotation axis as they are all C2 axes.  Other than that issue, 1 group got everything correct, another group correctly answered the first question, but missed the rotational symmetry in the staggered form of ferrocene, and the final group missed key symmetry elements in all of the molecules except for NH3 and ran out of time for the ferrocene problem.

Once again, I used this activity to launch my Advanced Inorganic course. I used the shortened list of point groups for the last problem as described in my previous comment. I also "warned" the students in an e-mail the night before classes began that they might want to review symmetry before coming to class. I have appended the assessment results to the Word document attached above to include two years worth of data.

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