OVERuse and UNDERuse of Formal Charge
(when the textbook fails)

March 04, 2024

And no, I’m not talking about errors, I’m talking about something way more subtle.

I recently came across a post in the AP Facebook group that illustrates one of my favorite mantras quite nicely, so I thought it was worth a quick blog post. Firstly the mantra.

Using a textbook as a guide as to what to teach in an AP course is a terrible idea, especially for the inexperienced

Don’t get me wrong, I’m all for textbooks as reference sources (I have hundreds of them in my possession), but you’d better not use them as a primary source of AP chemistry material. That’s a rookie error than can cost inexperienced teachers and their students dearly.

Now on to the recent example that neatly illustrates this.

The FB group question that was posed essentially asked that if, when constructing Lewis structures to show resonance, one should include all of the possible structures, even those with what we consider ‘unfavorable’ formal charge. The question asked about 2016, 2(e) specifically, but also referenced a textbook that offers the three Lewis structures below for nitric acid,

The analysis of those structures shows favorable (and identical) formal charge on structures I and II, and less favorable formal charge on structure III. In pure academic terms it is possible to construct an argument that structure III is part of the resonance set, even though it contributes little to the final hybrid, but I think that to suggest that a student should include III in the resonance set is terrible advice at AP.

The expected answer in AP exam will only involve the first two structures, and would never require the inclusion of III. This is literally what formal charge is for at the AP level, i.e., to allow you to completely disregard structures in order to simplify the process. I’ve written before (multiple times) about the overuse of FC (classically in this post from 2008, and the 2007, 6(c) the SO2 example), but this is an example of underuse!

Textbooks often fail to deal with the pragmatic nature of the AP exam itself, and offer arguments that although technically correct, are pragmatically useless to AP teachers and their students, and that only lead to confusion. Overthinking is a real problem here.

This is just one small example of where the textbook fails, and is a great reminder of how you should never, ever use a text to guide your AP teaching UNLESS it was written specifically for AP – and I’m not talking about all of the classic texts with a meaningless “AP Edition” splashed on the cover to generate sales. In every CB suggested textbook there are hundreds of irrelevant pages, and examples like the one above. Avoid them as a source of what to teach.

3 Comments

  1. Anonymous

    Off topic (but a College Board question):

    What is your take on AP Precalculus?

    Not a joke, they actually have this now … waiting to see when we get AP 9th grade biology and AP intro chemistry.

    Reply
    • Adrian

      I have no “take” on either the subject matter or the course because I know nothing about either, but as we all know the sole purpose of the CB AP program is to generate revenue, increase profits, and to grow. Trevor Packer is judged on his ability to lead an organization that does those things. As increasing pressure is placed upon AP enrollment from dual credit courses, the CB has to look for new revenue streams. It’s a very simple equation. This is why we already have “pre-AP chemistry” which bizarrely is not supposed to be an AP Chemistry prep course, rather it’s just the CB trying to get into the 8th, 9th and 10th grade market to sell more product. ZERO mystery here!

      Reply
      • Anonymous

        I didn’t even know that existed. Heck … sounds like when the Babylon Bee is prescient. 🙁

        To me, the cool thing about AP courses was always the driver to replicate a classic college course. It drove a level. That then benefited the students as a challenge. What Jaime Escalante liked about it.

        But it seems to be devolving more and more.

        I miss the 80s. (Better music also.)

        Reply

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