What do you do when chemistry doesn’t match your concepts?


Perhaps my favourite phrase in the English language is ‘... but it’s a bit more complicated than that’. Anyone using this phrase is demonstrating their willingness to be clear and, most importantly, avoiding reductionism. 

But what excites me most about hearing this phrase while I’m learning about a topic is that it hints at deeper knowledge. More complicated? How? Why? What am I missing? It’s like catnip to a curious mind. This is the bait that chemistry hooked me with – from simplified bonding models through to Schrödinger’s equation, from optical activity through to symmetry operations and group theory.

What drove my curiosity is a simple ‘problem’ with teaching chemistry: it’s always more complicated than that. Each of my teachers had to make compromises in their explanations of the concepts I was learning to keep them within my grasp. The important point is that my teachers acknowledged these compromises. I didn’t labour under the misapprehension that I had the full picture and I had the incentive to learn more.

Caring about compromise

Recently on our blog, Eric Scerri has been writing about ideas in chemistry education that must die. This theme of compromise in teaching has arisen in discussion several times, whether in the neutrality of pH 7 or the application of Le Châtalier’s principle. While the level to which we teach the nuances of a topic might always be debated, I hope we can agree that students should always be aware of the limits of the models they have been taught.

But there is one occasion in my chemical education where I wasn’t told I was receiving an incomplete picture, and I remember it vividly. Learning about trends in the periodic table and the reactivity in elemental groups, my class was treated to (as most classes are) the joy of seeing lithium, sodium and potassium reacting in water. We were shown how the reactivity of the alkali metals increases as you proceed down the group, correlating with the decreasing ionisation energies.

Annoying nuance

In this issue of Education in Chemistry, Declan Fleming’s feature on the complicated reactivity of the alkali metals draws a much more detailed picture. Melting points, atomic masses and reaction dynamics all conspire to make the trend in reactivity less straightforward than I was taught. I’m proud to say this article is a significant contribution to the literature on the topic. You won’t find a more up to date and complete analysis anywhere else.

But how do you teach these subtleties? After all, teachers turn to the reactions of alkali metals to show trends in groups of elements. How can muddying the conceptual waters help? Perhaps Declan’s own example provides a solution. In addressing the lower than expected reactivity of rubidium and caesium, comparing the molecular mass of the heavier metals with lithium can give students a justification for the true reactivity trend while preparing them for future work in stoichiometry.

So I want to implore chemistry teachers everywhere to be unafraid of the limitations of your simplified models. Point them out, celebrate them, ask your students to speculate on them. As long as you can avoid nurturing misconceptions, the gaps in your explanations can be wonderful hooks for your students to dig deeper into chemistry.

But I realise it might be a bit more complicated than that.

Paul MacLellan, deputy editor