Nothing motivates me to create new teacher materials more than a failed lesson. After this year’s first lesson about periodic trends and Coulomb’s Law ended in blank stares when asked to explain why atomic radius decreases across a period, I knew I had to do better for my AP Chemistry students.
Here is what I came up with. Feel free to use any of these resources in your classes!
The rest of this post explores the process of creating the materials and how I used them in class.
Creating the Bohr Model Cards
In the initial lesson, I had students draw their own Bohr models to explain the trends, but their understanding of atomic structure wasn’t at a point where they were ready to make strong connections with these. They could create basic Bohr models, but the process was slow.
Instead of students drawing the models, I wanted them to be able to quickly reference and compare the models, looking for patterns and trends. I wanted something they could manipulate and rearrange. There are other printable Bohr model cards out there, but nothing quite fit what I wanted.
So I created these cards in Google Slides, being careful to make sure the atomic radii (sizes) of each atom is correct relative to all the other atoms (though not truly to scale). With some helpful input from chemistry teacher Melissa Hemling, I edited it to make sure the inner electron shells always get smaller as the number of protons increases. I also color-coded the electrons to show orbitals, and made sure the cards went up to scandium to show one d-orbital electron.
I printed the cards on the “4 slides per page” setting, which worked well. Feel free to make a copy of my Bohr model cards to print for your own classes if you’d like!
With the cards made, next I needed some structure for my students to follow in order to make the connections I wanted them to make.
Creating the Model Exploration Activity (fake POGIL®)
So, POGIL® already has an activity for periodic trends and Coulomb’s Law, but I specifically wanted something to get them using the Bohr model cards. I also wanted a focus on how to explain the trends using Coulomb’s Law on AP free response questions. So I made my own!
Side note: If you don’t know what POGIL® is, it stands for “process-oriented, guided-inquiry learning.” This method of instruction starts with visual models, then has a list of questions that guides a group of students through the process of understanding the model.
Here’s the general structure:
- Model 1 – Coulomb’s Law – For any of this to make sense, students need a good foundation of Coulomb’s Law, which basically states that a greater charge and a shorter distance between particles results in a stronger attractive force between those particles. I made a diagram of a nucleus and an electron to show this, with an equation included to help students make connections with the mathematical relationship.
- Model 2 – The Shielding Effect and Effective Nuclear Charge – This basic idea here is intuitive: protons attract the valence (outer) electrons, but inner electrons repel those outer electrons. But the application of this in calculating effective nuclear charge is tricky, so this model guides them through the reasoning.
- Using the Bohr model cards – Students set out all the Bohr model cards for one period, so they can analyze the trends across a period. They then repeat this for the models down a group.
- Model 3 – Explaining the Trend – This part explicitly guides them through how to formulate a free response answer on the AP exam. By this point, they have a good understanding of the concepts, but haven’t had to put it all into sentences yet.
- Model 4 – Ionic Radius – This adds a layer of difficulty because students now have to picture what happens when electrons are added to or removed from a neutral atom. The model here allows them to analyze this easier, and introduces the idea of repulsion within an energy level.
- Model 5 – Successive Ionization Energies – This is almost its own lesson, but it’s an often-tested concept on the AP exam. Students find connections between boron’s ionization energies and its electron structure, then apply these concepts to another table of ionization energies.
Here is the full Model Exploration activity:
Insights from the Activity
- Orbitals on the Bohr models – One group had a debate about the colors on the Bohr model atoms, with one student sure that the colors meant something, but no one knew what. I had the group get out the scandium card, note the blue electron, and look at scandium on the periodic table. The lightbulb clicked for them shortly after, with students realizing the color corresponded to the s-, p-, and d-orbitals. These connections between Bohr models and electron configurations wouldn’t have happened without the cards.
- Shielding Effect and a Misleading Aspect of the Bohr Model – One group had trouble calculating the effective nuclear charge of the atoms because of where the core electrons are drawn on the models. Students asked me, “Do core electrons drawn below the nucleus repel a valence electron drawn above the nucleus?” It was a great question. I told them the Bohr model is just a model, and it’s far from accurate. I asked them whether electrons actually existed where they are in the Bohr model, and they responded by talking about the orbitals, how an electron is constantly moving and can be anywhere within its orbital. This quickly got them back on track with the understanding that all core electrons repel the valence electrons.
- These can be used for a lot more than periodic trends – In future years, I think I’ll start with these models when we first do electron configurations. No, they don’t show orbitals, but the Bohr model is so simple and helpful, them making connections between the quantum mechanical model and the Bohr model is huge. I’ll use them some for photoelectron spectroscopy (PES) because it shows why the binding energies all increase as protons are added. I think they might also be helpful for basic Lewis structures and helping students see why certain elements tend to form a certain number of bonds. Just look at the number of lone valence electrons on the Bohr model…the element will tend to form that many bonds!
- Use the POGIL® role cards – I used the official POGIL® roles for the first time, and I think it’s a good structure. It also set some great norms. I got these resources (which are copyrighted, so I don’t share them online) from the book POGIL® Activities for Life Science—Designed to Support the NGSS.
- This took a lot of class time – I mean, I think it was definitely worth it for this lesson because student thinking was great. But we didn’t get done with all of this even within a 90-minute block. It’s the same critique I have about the official POGIL® activities as well. There’s just not enough time to do all of our lessons this way.
I hope you found this post and these resources helpful! Click here for the rest of my AP Chemistry, Anatomy & Physiology, and Physics resources.