I’ve been overcomplicating the way I teach periodic trends. Have you?
The goal of teaching periodic trends on the table is to allow the students to predict outcomes. Which is all of chemistry, really. “What happens when I …?” That’s it. We want them to wonder, and we want them to predict. I think many of us have fallen victim to the mentality, if I just get the kids to memorize these trends with big arrows drawn on laminated periodic tables, then they will know it. But really, what we are creating are non-thinking parrots. The goal of this short article is for you to analyze how you are teaching periodic trends and then gift your students with the ability to critically think about the reasons behind the trends.
This year, I will be teaching two trends before we even look at the periodic table: Coulombic interaction and stability.
Ultimately, these two basic trends drive all the others. If a student has a clearer understanding of these two factors, their ability to predict the other trends (like electronegativity or ionization energy) will increase. Please know as I break these two topics down, I was a biology major. My understanding of physical chemistry and the depths of quantum theory is low. I hope that your ability to satisfy that deeply curious student is more than my ability!
Coulombic Interaction
This equation is known as Coulomb’s Law. It describes the forces of attraction or repulsion between charged bodies. Even in my lower levels of chemistry, they will learn Coulomb’s Law, and even more importantly, the way that it impacts the behavior of charged species in an atom. Functionally, and at a basic level, the kids need to recognize that increased charges would have increased forces due to the direct correlation. After they get the basic idea, it is important to ask the students where in an atom they might experience charges impacting each other. They could hopefully point out proton-electron, electron-electron, nucleus-electron cloud, and proton-proton. All of these except proton-proton can be simply explained within the atom. If the students have a working understanding of the attraction or repulsion of particles based on their charge, they will be more likely to predict other patterns as well.
The other part of Coulomb’s Law is the r squared. The “distance” between the charged particles squared is inversely proportional to the forces. Most students will recognize, at a surface level, that magnets behave this way as well. The closer the magnets, the more likely they are to jump toward one another. Again, understanding distance within an atom will improve the way that the student thinks about the other trends.
Stability
Charge and distance being understood, students can add stability to the mix.
Stability, or a state of lowest energy, is both easier and harder to teach. It is easier because many of us just anthropomorphize the atoms. We say things like, “All atoms want to be stable.” Even though we know that atoms don’t want anything. They don’t have wants, desires, dreams, or aspirations. Rather, systems tend to move to orientations of minimal energy. Which means stability. The hard part about teaching stability within the atom is that the nitty gritty of why full and half full subshells are stable is both quite nitty and quite gritty. Some of you who are much more gifted in understanding this area may disagree with me here. But I teach that full and half full subshells minimize energy and are therefore stable. That’s it. I know it is not deeply satisfying to some. But minimizing repulsions and maximizing attractions lands in two primary electron arrangements; full and half full subshells. That’s it. I don’t go any further.
Once your students are armed with Coulombic interaction and stability, adding on the remainder of the trends becomes easier. You now have an anchor or understanding against which the students can leverage all sorts of trends.
Hopefully, by starting with these two primary trends, you will become more efficient at teaching the trends. Even more importantly, your students will walk away from your class with a deeper understanding of why the trends behave the way they do.
Our newest game, Trendy is now on pre-order sale! It is 20% off if you order before manufacturing is complete. Trendy is a fast, simple game that your students will rapidly be able to play. It allows them to identify the trends of effective nuclear charge, atomic radius, electronegativity, first ionization energy and electron affinity. As students are playing the game you will walk around the class hearing insightful conversations about Coulombic interaction and stability in relation to five trends. That’s pretty cool.
Check out our video that explains the game.
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