Bottom-Up Chemical Bonding

In my last post, I discussed how the octet rule is often used by students as an explanative rule for everything in chemical bonding, rather than as a rule-of-thumb. Today I will discuss an approach proposed by Nahum and co-workers proposing an alternative approach to teaching chemical bonding (Nahum, T. L.; Mamlok-Naaman, R.; Hofstein, A.; Kronik, L. Journal of Chemical Education 2008, 85, 1680-1685). The title of their article: A New “Bottom-Up” Framework for Teaching Chemical Bonding.

The authors contend that the difficulty with the traditional approach is to divide up different materials according to some common set of physical properties (e.g., low or high boiling points, ability to dissolve in water, ability to conduct electricity). These materials are classified as different “structures” of matter associated with different “types” of chemical bonds or forces. This seemingly clean classification into categories is illustrated in Figure 1 from their paper

Students like to have these clean classification schemes. It helps them study using a divide-and-conquer strategy, which makes sense from a cognitive load point-of-view. With this scheme in mind, the student will be able to trot out the following answers to “explain” the physical properties of different substances – as shown below from the authors’ contribution to Concepts of Matter in Science Education(mentioned in my previous post).

The problem here is that our seemingly “good” students might not actually understand at a fundamental level what they are saying. The authors refer to this as a pseudo-conception, i.e., “students use the right terms in the right context with no conceptual thinking or scientific understanding”. If pushed a little further to explain their answers, the (Un)Happy Atom story, starring the octet rule, is likely to make an appearance. As a teacher who encounters a relatively wide range of student interests and abilities, there’s a part of me that thinks this is okay if the student is not majoring in chemistry, biochemistry or some flavor of physics. However, for majors in my department, my lofty goal is that they learn how to reason chemically at a fundamental level – and not pseudo-chemically.

And those were the good students. The ones who seem to turn in nonsensical answers on our exams, are those who mix up their concepts. They think there is a black-and-white difference between ionic and covalent bonding and run into trouble when this is not the case. The concept of electronegativity gets misapplied all over the place as they grasp-at-straws for explanation. Hydrogen bonds are simply confusing, as are dipole-dipole interactions be they permanent, temporary, induced or whatever else was written down somewhere in their notes.

Nahum and colleagues propose an alternative approach to Chemical Bonding. Start with the Atom and build your way up. Here’s Figure 2 from their article.

Most modern chemistry textbooks already start with Atoms (i.e., Stage 1). “Atoms First” approach has been heavily touted by textbook publishers (this century) as new-and-improved. After that, however, the classifications of Figure 1 are used to organize the material in your typical textbook. Ionic compounds and ionic bonds lead off, followed by a morass of topics related to covalent bonds (Lewis dot structures, the octet rule, molecular orbitals, hybridization, molecular shape, etc). Metallic bonding gets short shrift with a simple delocalized electron cloud model that is not properly explained. Then it’s on to Intermolecular Forces with each category having its own imposed typology (dispersion forces, dipole-dipole interactions, hydrogen bonds, etc).

Instead of all this, Nahum and colleagues propose that Stage 2 should start with a generic energy curve illustrating what happens when two atoms approach each other, as shown in Figure 3 below. Note that no mention is made yet about what “types” of bonds these are or how strong they are. The point to hit home here is that stability is correlated with minimizing the energy.

The authors suggest emphasizing Coulomb’s Law to discuss the balance between attractive and repulsive energies. I’m less sure how to implement this. While standard textbooks use some variation of Coulomb’s Law to explain pretty much all the various bonds and forces in Figure 1, things are actually more complicated if you’re a quantum mechanic. Since chemical bonding and quantum mechanics is my area of expertise, I struggle with how to simplify things without leading students down the path to pseudo-conception, misconception or confusion. However, I agree with the authors that the generality of Figure 3 makes it particularly useful. It works for H₂, He₂, Li₂, F₂ and LiF. The equilibrium bond distances and the bond energies are different in each case, but the curve still works.

Stage 3 is where we dive into chemical bonds. The authors suggest starting with ionic bonds before moving on to covalent bonds, and then “once [these concepts] are internalized, we recommend stressing right away that the nature of most bonds is infact partly covalent and partly ionic, that is, polar…” They describe these two categories in terms of “charge sharing” and “charge transfer” although they don’t define either term. Partly, the idea is to illustrate that heteroatomic bonds are stronger than homoatomic bonds in general, although the authors point out that other factors also contribute to the bond energy. This is also where Electronegativity gets introduced. From there, the authors recommend discussing hydrogen bonds, followed by the Van der Waals force in the helium dimer before bringing up the interaction between diatomic molecules. The main point is to emphasize the “continuum” of interactions rather than the clean category approach of Figure 1.

In Stage 4, the main concept is Valency. Start with defining the valence shell, and then discuss periodic properties, before moving on to Lewis dot structures and the octet rule. Hopefully at this point, the students are immersed in ideas of energy stabilization that they indeed recognize the rule-of-thumb nature in guidelines for drawing good Lewis structures. Valence shell electron pair repulsion (VSEPR) theory can be introduced at this stage. After understanding the structures of small molecules, one can move on to “giant” structures and lattices be they ionic, covalent or metallic. Metallic bonds get introduced at this stage as related to covalent bonds with a swath of delocalized electrons.

Stage 5 is where Properties are discussed – the main idea is to connect the microscopic world with macroscopic observations. This is in contrast to standard approaches that start with classification of substance-type by different properties and then a proceeding atomistic-molecular “explanation” for each type.

What do I think about this? I partially do some of this in my class already, but not systematically. Honestly, I feel constrained by the textbook. The fact that we have small class sizes and therefore have to offer multiple sections of General Chemistry and agree on a list and order of topics (to mesh with the lab) imposes further constraints. As it is, I already jump around somewhat in the textbook. This leads to student confusion in their reading, and may provide a stronger impetus for me to move towards Open Educational Resources (OERs). While there are good resources out there, they don’t quite do things with the depth and approach that I’m looking for. Perhaps I need to write my open OER textbook. But it’s unclear how helpful it will be to others. If teaching is a relationship, then it’s not surprising that each instructor has an idiosyncratic approach that’s unique – playing to the instructor’s strengths and taking into account the background (and numbers) of students in the class. Context matters. At least I think so. Other voices would argue that General Chemistry can be standardized through an online delivery system that could even provide data-driven personalized approaches.

Will I overhaul my class and follow this approach? I don’t know yet. I spent the previous two afternoons writing out a sample syllabus with some re-ordering of topics. I’m not happy with it yet, and it only partially makes use of the approach favored by Nahum and colleagues. I need to chew on this a bit more. Classes don’t start until early September so it’s still early days in the summer. I have some time for further reflection.