Examining VSEPR though the Cross Cutting Concept of Patterns

student whiteboard about VSEPR before the lesson

Last year, I taught Valence Shell Electron Pair Repulsion (VSEPR) theory and then had the students complete the pHET simulation called Molecule Shapes. This year in effort to align to NGSS, I told the students we were going to look at the data available from the real molecules on the simulation and specifically look for patterns which is a cross-cutting concept; one of the three dimensions of the standards. The cross-cutting concept provides a lens for thinking about concepts in science.

Students had to look at the data and determine a pattern. Typically, I have taught VSEPR  and then had the students complete the simulation as a confirmatory activity to the information covered. Here, students started the topic by creating ideas for molecular shapes and an initial rationale. I initially thought all the students would create a similar claim, but was shocked at the variety that arose. It changed the focus of the lesson where no one complained about the shapes or angles, but were curious about making new shapes and seeing if they could debunk a fellow classmates initial claim. The following are a sample of boards that students generated about their patterns as well as a claim to explain how bond shape is determined to be the same.


   Figure 1 - Whiteboard #1


One group claimed that for molecules with no lone pairs, the bond angle is equal to three-hundred and sixty divided by the number of bond groups and that if the molecule has lone pairs the bond angle will decrease. If a molecule has no lone pairs, the molecule and electron geometry are different. See figure 1.


   Figure 2 - Whiteboard #2


A second group claimed if you increase the number of groups that are bonded and decrease the number of lone pairs of electrons surrounding the given molecule then you will increase the bond angle. See figure 2.

Another group claimed that as the number of lone pairs increases the bond angle decreases. See figure 3.


   Figure 3 - Whiteboard #3


Following the board creation students presented their model and students had the opportunity to ask questions and change their initial model. After this, students completed a POGIL activity* on molecular shapes and then re-visited their boards. The POGIL activity helped explain the reasoning behind their claims so students were able to go back and fix their reasoning.

The two activities were sufficient for students to understand VSEPR without direct lecture and provided the students the lens of patterns to think about the topic. I felt the students were more curious to understand the reasoning behind the bond angles and shapes than in previous years.

*The POGIL acivity mentioned above is called "Molecular Geometry". It is just one of the activities available in POGIL Activities for High School Chemistry, Trout, L. ed. Batavia, IL: Flinn Scientific, 2012.