Thinking about molecular structure – how novices and experts differ

A pair of hands, palms up

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Two enantiomers are mirror images of each other, like a pair of hands

Chemistry needs us to use spatial reasoning. Chemists interconvert 3D and 2D representations, visualise the effects of rotation or reflection, or even mentally manipulate objects when trying to assemble laboratory equipment. So, students learn a variety of visual/spatial strategies so they can operate in a chemistry environment. And it’s difficult.

‘What do you mean that bond is coming out of the board?’ ‘How can I look at that molecule from the other side? There isn’t anything on the back of the page …’ It doesn’t matter what level you teach, all chemistry teachers have to explain the intricacies of representing a 3D molecular structure in a 2D diagram.

You don’t always have to think in 3D

Think about trying to decide whether two different representations of butane show the same molecule or two enantiomers. Rotating one representation and checking whether it can be superimposed on the other would be one visual/spatial strategy.

However, analytic strategies can also successfully solve problems. Using the same situation, solve the problem without mental rotation by employing a rule such as: ‘if a carbon atom has two identical substituents, it cannot be a stereocentre.’

Analytic strategies are important as they reduce the cognitive load of solving problems that otherwise require visual/spatial thinking.

Getting analytic

A new study by Chryssa Tzougraki and her colleagues probes how students use visual/spatial and analytic strategies to understand molecular structure.

They investigated if there is a shift from using visual strategies to analytic strategies as expertise in organic chemistry develops.

The researchers used a Visual/Analytic Chemistry Task (VACT) specifically designed for this purpose. The VACT is a 30-minute paper and pencil test that asks students to compare 2D molecular structures of organic compounds.

Some questions can be solved by applying either visual/spatial or analytic strategies. And some questions require analytic strategies, where a visual strategy would result in an error.

The VACT test was taken by four groups of participants: secondary school students; 1st year undergraduate students; 3rd year undergraduate students; and secondary school chemistry teachers.

Two dimensions are enough for experts

The researchers found that more expert organic chemists use analytic strategies more. This is consistent with the wider view that learners across all disciplines undergo a systematic shift in thinking strategies as they develop expertise.

The results also indicate that adopting analytic strategies in organic chemistry is a challenging process. Because of this, fostering students’ analytic thinking is necessary as they develop.

Chryssa’s team also teased out some nuances in the way certain types of questions are approached. Students were more likely to answer incorrectly when the question contained a comparison of structures that were perceptually similar, compared to dissimilar structures. They conclude that perceptual similarity leads students to apply a visual/spatial strategy in error. For dissimilar structures, the participants were more likely to apply an analytic strategy.

This tendency was particularly noticeable in 3rd year university students who, in this study, had received many years of training in using analytic strategies. In other words, students jumped to the answer suggested by visual inspection of the question.

Teaching tips

  • If you’d like to try this with your own students, the authors can make the VACT available upon request. It is designed to take only 30 minutes.
  • Include explicit examples of questions where the answers suggested by visual inspection are incorrect and where only an analytic approach can lead to the correct solution. The VACT can be used for inspiration.
  • Use feedback and self-reflection exercises to encourage students to reflect specifically on which type of strategy they use.
  • Encourage students to use both visual/spatial and analytic strategies for each question.
  • Although experts are more likely to use the analytic approach, it doesn’t mean that is all we should teach. We still need to begin with the visual/spatial approach to ensure the concepts are properly understood. Later, we should do what we can to facilitate the transition.