29 Jun 2015

Gummies and Toothpicks Point Group Determination Activity

In-Class Activity

Submitted by Darren Achey, Kutztown University

In this activity, students will use gummies and toothpicks to construct models of molecules that will then be analyzed for their symmetry elements, and ultimately placed into the correct point group and the models can then be consumed.

File Assignment for Students39.23 KB
Learning Goals: 
  • Construct the molecules with the correct VSEPR geometry
  • Observe the symmetry elements present within each molecule
  • Classify each molecule with the correct point group
Equipment needs: 
  • (Relatively) round, small, and multicolored gummies
  • Toothpicks
Implementation Notes: 

I typically use this as an in-class activity (50 minutes) for a senior level, Advanced Inorganic Chemistry course that has between 10-15 students.  This activity is undertaken immediately after the symmetry elements are introduced and the students have had limited exposure to observing the symmetry elements themselves.  As such, the beginning is often a little slow as they gain confidence in classifying symmetry elements.

I allow the students to work in pairs or at most groups of three and have found this to be an effective way for students to show each other symmetry elements that their partner(s) do not see.  I also encourage students to consider not making a model for some of the molecules so that they can practice their visualization of the molecules in three dimensions.

The students usually cannot finish all of the molecules within the class period so the unfinished molecules become homework.  The final point groups are graded as a homework score for the course.

Alternatively, this exercise could also be completed in a laboratory session of approximately 2 hours.

Time Required: 
50 minutes in class, roughly another hour outside of class.
Evaluation Methods: 

This activity is graded as a homework assignment.  Full credit is awarded for the correct point group assignment.  If the student misses the point group assignment, partial credit is awarded for the symmetry elements the student found correctly, with an identification of where they deviated from the correct symmetry assignment.

In addition, the exam following this assignment typically has two unique (not one from this assignment) molecules that the students are asked to assign symmetry elements and the point group for. (These vary by semester to ensure that the test questions have not been passed down to the students from prior years)

Evaluation Results: 

Students tend to have trouble recognizing improper rotations without first seeing several examples of this.

Creative Commons License: 
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