Absorbing some misconceptions

Chemistry topics that involve any aspect of quantum theory are often problematic for students. This may be due to the increased numerical demands from a topic that is primarily assessed through calculations. In addition, aspects of quantum theory are abstract and often counter-intuitive, placing additional strain on students developing an understanding beyond any numerical difficulties.

In this study, Nilüfer Kӧrhasan and Lu Wang shed some light on students’ understanding of the topic of atomic spectra, paying particular attention to the quantum mechanical context. An important aspect of this topic area is that a student’s understanding of atomic spectra is informed by two underlying models: atomic structure and the wave–particle duality of light.

Mental models

The authors conducted one-on-one interviews with a small group of students, during which a set of conceptual questions relating to atomic spectra were answered aloud by the students. These were recorded, and the data were coded and analysed to illuminate the underlying concepts that students use when explaining atomic spectra. More importantly, each student’s use, or omission of use, of these concepts allowed the authors to understand the mental model that a student used.

A mental model is the knowledge structures an individual holds about a specific concept that are used to construct their understanding.

This research identified six ideas relevant to students’ mental models of atomic spectra: bound electrons, discrete energy levels, electron transition, photon energy, spectral lines, and electron orbits. Different groupings of these ideas give rise to the four mental models that students use to make sense of atomic spectra in this research.

  1. The Scientific model: This represents the coherent use of all the identified concepts apart from electron orbits. It is the ‘correct’ mental model.
  2. The Primitive scientific model: This represents the coherent use of discrete energy levels, spectral lines and photon energy. Students who possessed this mental model would ignore the idea of electron transition when explaining spectral lines and considered the emission of a photon to be spontaneous and to occur when energy levels changed.
  3. The No photon model: This represents the coherent use of discrete energy levels, spectral lines and bound electrons. Students who possessed this mental model attributed a spectral line as being due to the energy of an electron, not a transition – ie they thought was the energy of the energy level and not a photon.
  4. The Orbit model: This is similar to the No photon model, except that students did not recognise discrete energy levels, but instead would invoke the idea of different orbits of varying distance from the nucleus.

Among several important conclusions from this study, two stand out. First, the concepts of electron transfer and photon energy are identified as ‘threshold concepts’ – concepts that open a door to a new way of thinking, enhancing the ability of learners to master a topic.

Second, one of the mental models, the Primitive scientific model, cannot be identified through assessment by calculations alone. This is because students will still be able to calculate photon energies based on them being equal to the difference in energy of the relevant energy levels, without considering any electrons. So, in a topic where calculation is the primary means of assessment, this mental model can linger unchallenged.

Teaching tips

Fortunately, there are some simple strategies that you can use to either minimise the construction of these alternative mental models or allow for the adoption of the correct one.

  • Ensure the interaction between the two separate concepts of atomic structure and the properties of light is understood before explaining how a photon and an electron interact to give rise to electron transitions. This should minimise students’ erroneous association of electronic energy levels directly with spectral lines.
  • It is the links between the ideas within the topic of atomic spectra that are important: incorrect or missing links lead to alternative models. Explicitly emphasise these links to help your pupils’ understanding. In fact, developing links between concepts can help to identify misconceptions in the isolated concepts themselves. The technique of concept mapping can really help with this.
  • Calculations are usually used to assess this topic, but we have seen this can lead to an alternative mental model remaining unnoticed. Shift the emphasis away from calculations by probing students’ understanding with more conceptual questions. Specifically, the researchers suggest asking questions like, ‘Why do lines in the emission and absorption spectra appear at the same wavelength?’ and, ‘Why are the lines coloured (in the visible region) on an emission spectrum, but dark on an absorption spectrum?’