Chemistry Solutions

As a chemistry teacher, I’ve found that one of the most rewarding experiences is observing my students explain and critically evaluate the physical properties of the everyday objects they interact with, especially regarding their structure and bonding qualities.

Figure 1. An example of a traditional electronegativity scale used by students when learning about ionic and covalent bonding.

But making these connections does not always come easy. One of the pitfalls of the traditional bonding pedagogy is that it tends to focus only on salts for ionic compounds and small molecules for covalent compounds. This focus can lead students to associate covalent compounds with low melting points, causing them to believe that all solids they interact with must either be salts or metals. More significantly, it fails to leave space to explore more exciting and technologically relevant materials such as polymers, ceramics/glasses, and composites.

Figure 2. An alternative approach is represented by the “bonding triangle,” which helps students consider differences in electronegativity and average electronegativity.

In my experience, chemical bonding is an excellent topic through which to introduce materials science in my chemistry classroom. For example, replacing the one-dimensional ionic/covalent bonding scale¹ with the van Arkel-Ketelaar “bonding triangle”² is an easy and effective way to introduce more exotic materials (see Figures 1 and 2). By utilizing the bonding triangle, students can use both the difference of electronegativity and the average electronegativity to predict the type of bonding as ionic, covalent, polar covalent, and metallic.

One advantage of the bonding triangle approach is that it allows the inclusion of metals, along with observations about network covalent materials that are commonly utilized in ceramics and glasses, such as SiO₂, and that fall near the ionic and polar covalent boundary. The bonding unit can also offer many practical lab-based activities to explore materials science. Described below is a series of hands-on activities and demonstrations that I use in my classroom to more thoroughly integrate chemical bonding, and to introduce my students to the exciting field of Materials Chemistry. My teaching approach leans heavily on the tools and knowledge I gained from attending the ASM Materials Science Camp for teachers.

Materials ID activity

Figure 3. Sample items that students categorized in the Materials ID lab.

After spending a few classes introducing the bonding triangle and discussing ionic, covalent, and metallic bonding, I like to introduce materials science via the Materials Identification lab, a classic activity from ASM (shared with permission, copyrighted by ASM Materials Education Foundation). In this lab, students are asked to classify a variety of objects as either metal, polymer, ceramic, or composite — without being provided any other information about the objects. This allows them to connect their existing understanding of the world around them with the basic concepts of bonding. Even hesitant students who think they don’t know anything about polymers or ceramics, are usually able to credibly identify most materials based on their properties. 

In my experience, composites are often the category of materials that students struggle with the most, perhaps because it can seem the most disconnected from traditional bonding concepts. However, I find that composites can serve as a great example of heterogeneous mixtures. Because they’re made up from a matrix phase and reinforcing phase, students can readily draw on previous course content.

After completing the Materials ID lab, I lead a discussion to help students talk about what they learned by making reference to the structure and bonding of elements that make up each material, and why they lead to the observed physical properties. A summary chart relating these concepts is a useful and tangible take-away for the students (see Table 1).

Moving forward I plan to replace the post-lab discussion of the Materials ID lab into a CER (Claim, Evidence, Reasoning) activity which can be downloaded by readers. For this version of the activity, I use questions like, “What properties are characteristic of metals, polymers, ceramics, and composites?” or “How do the characteristic properties of metals, polymers, and ceramics relate to the structure and bonding of the materials?”

Demonstrations

I like to describe the following activities as “show-and-tell” demonstrations that are less formal for students, and generally used to introduce concepts. Depending on the needs of the class, some of these are done with samples for students in small groups to test and collect observations on a whiteboard. Another set of demos includes short formative CER activities, while others are just used to generate class discussions. They are excellent tools for drawing connections between the various types of bonding and everyday objects. I have found these help students understand how materials science is present in their day-to-day lives. Be sure to use a fume hood or safety shield when performing demonstrations and make sure to wear goggles and a lab apron—or coat—and always require students to take the same precautions while observing demonstrations.

Ceramics

Figure 4. Two strips of iron metal. The one shown at the top of the image is essentially new, while the other had sat outdoors for a few years and rusted.

When discussing ionic and covalent bonding through a Material Science lens, I think it is critical to discuss ceramics. Typically comprised of metal or non-metal oxides, ceramics tend to form 3-D network structures. The bonding within elements in ceramics varies significantly, and can include both ionic and covalent compounds. Ceramics are a great example of a material that meets the characteristic properties of ionic compounds (high melting point, brittle, insulating, etc.), but isn’t simply another salt. I often use a demonstration where students observe and compare the physical properties (appearance, conductivity, and malleability) of a new iron sample with those of a rusted iron sample (see Figure 4).

Using the CER model in class, I ask students, “What are the types of bonding in iron and rust?” Students typically begin to notice and theorize that the rust coating is like a ceramic coating on the outside as a result of the oxidation of the metallic iron. The connections can start to be made that iron, a metal substance made up of a lattice of positive ions and delocalized electrons, creates a substance with entirely different properties when exposed to oxygen. The metal is originally malleable, conductive, and shiny, but becomes brittle, non-conductive, and dull. As electrons are lost from the iron to the oxygen atoms, an ionic compound is formed, causing the material to be brittle due to the ionic lattice formed. This process fixes the electrons that were once delocalized, and reduces the conductivity of the material. A simple observation from students can inspire a high-level discussion (oral or written) about the connections between properties and bonding. This simple demonstration shows that rust, a ceramic coating on a metal surface, behaves in the same fashion as ionic salts.

Polymers: Properties of HDPE vs LDPE

Polymers are great for discussing and demonstrating covalent bonding and intermolecular forces. Generally, when I introduce polymers, I start by giving students individual strips of a garbage bag and ask them to start pulling it apart (see Figure 5). This allows them to observe that this plastic material is made up of a fiber-like substance that is aligned in one direction. This hints that there is more to the underlying structure than the monolithic plastic bag material that they are accustomed to.

Comparison of high density polyethene (HDPE, or linear polymer) and low density polyethene (LDPE, or branched polymer) is also a great way to enrich students’ interpretation of intermolecular forces. Since these are materials easily found in household waste or recycling (recycling symbols 2 and 4, respectively), they are easy to acquire. I encourage my students to reason how the varying strength of London dispersion forces (LDFs) between polymer chains can lead to different material properties, despite the polymers consisting of the same repeating unit. A class discussion follows of how these properties make certain polymers more or less advantageous for use in specific products or applications.

A fun demonstration is to use a HDPE jug to create a section of a LDPE-like material. Simply heat up a small area of the jug with a heat gun until it is almost transparent, then blow into the jug and watch the bag-like appendage form (see Figure 6)! This is a great opportunity to discuss the similarities and differences between this thin-walled HDPE section and a LDPE material.

Polymers: Hydrophobicity vs hydrophilicity

Figure 7. Hydrophilic and hydrophobic powder samples interacting with water on a tray.

Polymers can also be used to demonstrate hydrophobicity and hydrophilicity. I like to use materials such as diaper gel (sodium polyacrylate), instant snow (cross-linked sodium polyacrylate), and HDPE powder to show how drops of water interact with each material (see Figure 7). This is a great way to show the hydrophobicity or hydrophilicity of the materials, and to have students explain the phenomena based on the intermolecular interactions between the polymers and water droplets.

Reflection

These are some of the ways I have tried to enrich my own teaching of chemical bonding. Incorporating materials science into the foundations of bonding opens up the possibilities for students to make connections between structure/bonding and the material items they interact with on a daily basis. I think we often forget that there is science behind the stuff we own, purchase, and dispose of — and I truly believe that by having a better understanding of these materials, we as a society can be more thoughtful consumers. Enriching my lessons with demonstrations, discussions, and labs in materials science has been made possible by my attendance at ASM’s Materials Science Camp for teachers. They are arguably the best professional development opportunities I have had during my time as a teacher. I strongly encourage all science teachers to reach out and consider attending one of these events.

References

1. Abozenadah, H.; Bishop, A.; Bittner, S; Flatt, P. M. CH150: Preparatory Chemistry [online textbook]; Western Oregon University: Monmouth, OR, 2017; Chapter 4 – Covalent Bonds and Molecular Compounds. https://wou.edu/chemistry/courses/online-chemistry-textbooks/ch150-preparatory-chemistry/ch150-chapter-4-covalent-bonds-molecular-compounds/ (accessed April 7, 2022).
2. Jensen, W. B. A Quantitative van Arkel Diagram. Journal of Chemical Education. 1995, 72 (5), 395-398.

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