Lewis Structures and Wild Geese

The title of this post attempts to link two disparate conundrums: the role of mnemonics in teaching chemistry and our expectations of students’ abilities to draw Lewis dot structures.

Memorization is not a popular topic among most teachers because of its low position in Bloom’s taxonomy. In chemistry, and certainly in organic chemistry, we discourage the substitution of memorization for understanding. However, students must memorize certain facts in order to function in a discipline and mnemonic devices can help them do this.

From my observations, mnemonics garner little respect. They do not populate textbooks with regularity as they lack scholarly panache. Perhaps instructors view them as a subspecies of academic dishonesty. They are often shared through informal channels. For example, a mnemonic I teach in general chemistry:

fails to show up in a Google search. I learned it from a student who wondered why I did not teach it. He had learned it from another instructor.

Mnemonics seem to vary in their merits. For example, “Mother Eats Peanut Butter Perhaps Her Hair Often Needs Degreasing” does not appeal to me as a means of memorizing the names of alkanes. Remembering methane, ethane, propane and butane and the subsequent Greek prefixes seems easier to me than remembering the mnemonic itself. At the same time, I still run through “Oh, my, such good apple pie, sweet as sugar” to confirm the number of carbons in a particular dicarboxylic acid (oxalic, malonic, succinic, etc.), and I teach this mnemonic to my students. So, what makes one mnemonic “acceptable” and another “unacceptable”? I’m not sure.

One mnemonic, which I first heard several years ago in Ann Taylor’s classroom at Wabash College, has proved its worth: HONC 1 2 3 4. It summarizes what I want from organic students when they approach Lewis structures.

In general chemistry, students learn an algorithm for obtaining a correct Lewis dot structure: count total valence electrons, arrange the atoms, form single bonds, place remaining electrons on outside atoms, etc. It works well and it avoids ugly attempts at trial and error. However, it takes a fair amount of time and doesn’t reflect how organic chemists think.

In organic chemistry, we simply want four bonds to carbon, three to nitrogen, two to oxygen and one to hydrogen in neutral structures. Hence, HONC 1 2 3 4. The numerals correspond to the number of bonds to each atom in a structure with no formal charges. This is the actual basis we use as experts when assessing structures for plausibility and assigning formal charges.

Joel’s forthcoming book summarizes the info in HONC, as shown in the table below:

Table 1-4: Most common number of covalent bonds
and lone pairs for selected atoms

This merits kudos, as it summarizes what students need to know. I refer to this table, mention HONC in lecture, and my students are empowered for the rest of the course. In addition, Joel’s textbook contains exercises that require students to apply this information.

HONC is not a panacea. I have to mention halogens and discuss exceptions to the octet rule (e.g., expanded valence and boron). I also still expect students to use the general chemistry algorithm in certain problems. However, HONC does give them a tool for quality control that they can carry with them easily throughout the course.

Moreover, it’s great for grading and lecturing. Write “HONC! HONC!” on an assignment in which a student shows five bonds to carbon? Make reference to a flight of geese flying over when faced with an incorrect structure on the board/screen? Students recognize the reference immediately.

— Steve Pruett

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