Modeling the Concept of Ionic Bonding

When describing abstract concepts like chemical bonding, it always seems to feel far too easy for both teachers and students to resort to the “wants” and “needs” of atoms. After all, we understand what it means to want, need, or like something, so it often feels appropriate (and easier) to use a relatable metaphor or subtly anthropomorphize these atoms to accommodate our students’ current reasoning abilities. While predicting the types of bonds that will form and the general idea behind how atoms bond can be answered correctly using such relatable phrases or ideas, the elephant in the room still remains—do our students really understand why these atoms bond?

The following explanation should feel rather familiar.

“Since chlorine wants 1 electron and sodium wants to lose 1 electron, chlorine will steal sodium’s 1 valence electron so that they both have a full outer shell and are stable.”

I don’t know about you but that feels awfully familiar to me and it’s not too difficult to realize why. I was taught that way, I taught it that way, my students learned it that way from me, and my students explained it that way to me for years. It wasn’t until a colleague of mine shared a great JChemEd article,¹ which focused on placing a greater emphasis on the electrostatic interactions in chemical bonding, that I started to really question the depth of understanding on this topic that I was expecting from my students.

So for the first time ever in my career, I decided to pump the brakes on the wants and needs of atoms and instead focus more on the electrostatic interactions between atoms to account for chemical bonding. Much of this new approach was centered on a topic we had recently learned—electronegativity. However, I started to notice this whole idea of electronegativity difference wasn’t sitting so well with a significant portion of my students. It became obvious to me that many of them had just simply memorized the consequence of what would happen if one atom had a significantly larger electronegativity than another. For example, they might say that because fluorine’s electronegativity is so much larger than sodium’s, fluorine will end up taking sodium’s valence electron. Statements like this bothered me because they didn’t convince me that the student actually knew what was taking place.

To satisfy this uncomfortable feeling, I proposed to them the following two scenarios that I knew couldn’t be correctly answered unless they understood what was going on at a conceptual level.

Scenario 1:  If atom X has an electronegativity of 3.6 and atom Z has an electronegativity of 1.2, draw vectors to represent the attractive and repulsive forces present between the atoms.

Scenario 2: Using your understanding of attraction, provide an explanation for why phosphorus (P) and chlorine (Cl) form a covalent bond instead of an ionic bond.

In short, the majority of my students couldn’t accurately represent the attractive/repulsive forces present in the diagram. They would do things like draw a vector displaying that atom X was being pulled on by atom Z with more force or that the repulsive vectors were actually larger than the attractive vectors, which wasn’t accurate for the situation provided. In addition, for scenario 2, the vast majority of them resorted to explanations like “phosphorus is a nonmetal and chlorine is a nonmetal, so they form a covalent bond” or “the difference in electronegativity between phosphorus and chlorine is 0.9, which falls within the window of being polar covalent, so that’s why they don’t form an ionic bond.”

In order to deepen their understanding, I needed to take a different approach. That’s when a colleague of mine, Erica Posthuma (@eposthuma), told me about Seth Furlow’s (@Furlow_teach) video that modeled the idea of ionic bonding using film canisters, magnets, and washers in a way that I had never seen before.

It was then that I started to use phrases like “pull strength” or “strong/weak positive core” to account for electron transfer. In addition, I had shared the magnets and washers idea with a colleague in my department and that same day, he quickly assembled some old prescription pill bottles, magnets, and washers so that I could have my own items to model the concept with my students whenever I wanted.

Having these pill bottles and magnets at my disposal completely changed everything for many of my students. There were numerous times when students would be struggling to understand something about this topic and I would sit right next to them and model it in front of their eyes. A typical conversation would look something like this.

Me: Ok, check this out. I’ve got these two pill bottles and you see the one washer that appears to be stuck to one of the bottles?

Student: Yea

Me: Ok, well watch what happens when I slowly bring them together.

Once the pill bottles are close enough, the washer all of a sudden leaps from one bottle to the other. Since I didn’t tell the students about the magnets ahead of time and they can’t see them, they are usually really surprised and it’s clear that they didn’t expect that to occur.

Me: So what happened?

Student: That pill bottle took the washer away from the other bottle.

Me: Why do you think that happened?

Student: Do you have magnets under there or something?

Me: Yea, I do. That explains why the washer was stuck to the original bottle and why it’s stuck to the new bottle. But why did the washer switch bottles?

Student: The magnet in that bottle (new bottle) must be stronger than the other magnet in the original bottle.

Me: True, I have a much stronger magnet in here. But why does having a stronger magnet matter?

Student: Because the stronger magnet will pull on it with a greater force than the other magnet.

Me: Exactly. So why do you think sodium’s 1 valence electron transfers to chlorine without using the term electronegativity?

Student: Chlorine must pull on sodium’s valence electron with a greater force than sodium does.

Me: Awesome. So would you say that it appears as though sodium wants to give its electron up or that fluorine really wants an electron?

Student: Well….no, not really wants. I guess it’s more about whatever happens to pull with a greater force.

I must have had 15-20 conversations like this, which would not have taken place if I didn’t have the models with me. Sometimes I would even tell them to do it and allow them to literally feel the attractive force building until the washer transfer was made. From then on, I saw many more explanations centered on “pull strength” and forces to account for electron transfer or lack thereof.

Not only did this open a new pathway for me to communicate an idea but it effectively allowed my students to visualize an abstract concept in a way that was far less likely to lead to a misconception or weak understanding due to oversimplification.

I highly encourage you to make your own models! All you will need are the following materials:

1. Film canisters (or pill bottles)
2. 2 magnets of different strength. I attached 2 small neodynium magnets* together for the stronger magnet.
3. Glue or tape to keep the magnets attached to the inside of the canister lid.
4. 1 or 2 small metal washers.

Thank you to Erica Posthuma (@eposthuma...You can also follow her ChemEd X blog) for giving me the idea in the first place and to Seth Furlow (@Furlow_teach) for making the video and allowing the idea to spread!

*I purchased my neodynium magnets from Amazon. The link provides several options.

¹Venkataraman, B. Emphasizing the Significance of Electrostatic Interactions in Chemical Bonding. J. Chem. Educ. 2017, 94, 296−303.