Heuristic Shield

In the previous post I discussed the example of a bonfire to counter students’ misapplying electrostatics when explaining why cations are smaller than their neutral atoms. Why did students misapply electrostatics in the first place? Because all of us often fall back on heuristics to explain phenomena.

There’s however a problem with the bonfire example, so let me repeat the relevant portion of the quote (not mine): A possible analogy to teach the nuclear attraction for an electron is to say that it is similar to the heat one receives from a bonfire—this is dependent on how big the bonfire is, the distance one is away from the bonfire, and whether one is blocked (screened/shielded) from the bonfire, but is independent of how many people are present at the same distance away from the bonfire.

The problem I’ve been pondering is how students interpret the words blocked or screened or shielded. I certainly use the phrase electron-shielding when discussing this topic in G-Chem, and the textbooks use it too. But now I’m suspicious as to how students conceptualize this idea. When used as a heuristic, it’s hard to imagine that the word shield doesn’t invoke the picture of a shield. Bet you visualized one too!

This line of thought is fleshed out carefully by Taber (Chem. Educ. Res. Pract. 2003, 4, 149-169). In our effort to provide pictorial chemical shorthand for students, we come up with terminology such as electron-shielding. What we mean by this, and I will quote Taber’s precise language, is “give rise to a repulsive interaction that partially cancels the nuclear attraction”. But then Taber continues with a key observation, “… although once the shorthand is accepted the notion of shielding may become reified and used without conscious awareness of its derivation.”

Here’s the problem. While I may have been careful in the way I introduced this explanation (I’d like to think so, but pondering the issue gives me pause), it’s likely that my students reify the shield by imagining a literal shield when they trot it out as an explanation, i.e., electrons closer to the nucleus literally block electrons further away from ‘experiencing’ the positive nuclear charge. If anything, the bonfire explanation will strengthen such a notion – the folks closer to the fire literally block me from experiencing as much heat! An analogy to counter one problematic heuristic reinforces another problematic one.

One might argue that if a heuristic helps students ‘explain’ the correct observations, what harm is there? After all, they’re getting the chemistry right even if the underlying physics is wrong or misleading. But then what kind of an explanation is the literal shield that defies physics? A magical one? That’s what I will ponder next.