I’ve been struggling with creating a T-GEM lesson because I’ve been hung up on the Generate phase. I can’t get over the need for a large data set to observe and generate a relationship! I’ve settled for periodic trends, but have some rough drafts for:
- intermolecular forces and changes of state
- real gas behaviour
- different ways to fill/inflate a balloon
- solubility (compound in a solvent vs. temperature)
Background: Periodic trends are a challenge for students.
When I marked my undergraduate chemistry students’ quizzes on periodic trends (e.g., defining the trend, explaining it, applying it) there were often challenges with explaining why the trend was as is and how to apply them in novel concepts. In particular, students memorize the trends to apply them to questions. When they are given elements to compare, they can easily use the trend without really understanding what’s happening. However, this misconception becomes clear when students are asked to rank information about isoelectronic species. What I’ve realized from teaching periodic trends to students is that they do not understand what these atoms look like nor do they recognize the importance of Coulombic attractions (re: charge and distance between charged particles).
Periodic Trends T-GEM Lesson
Generate
Using atomic radius data, the teacher will model how to explain the trend for atomic radius going down a periodic table:
Screen shot from periodic trends simulator (American Association of Chemistry Teachers)
Students will identify that the trend going down the periodic table is that the atomic size increases. The Bohr Rutherford diagrams from H, Li, Na, and K will be drawn and connections to Coulomb’s law will be explained. It’s important for students to recognize the importance of:
- protons attracting electrons
- electrons repelling electrons
- impact of distance between charged particles
With the Bohr Rutherford diagrams, the teacher must be very careful to explain that the circles where electrons are drawn represent energy levels, not orbits. The energy levels represent the distance from the nucleus to where the electrons can most likely be found; electrons in higher energy levels are further away from the nucleus.
Visually, students will see that the elements going down this group are increasing in size. This helps them follow along when the teacher models an explanation using Coulomb’s law.
Evaluate
To put their explanation skills to the test, students will predict the trends for atomic radius going left to right and ionic size as compared to the atom an ion was formed from. They are expected to explain this using Coulomb’s law (PhET simulation on Coulomb’s law) and can compare their responses to this periodic trends simulator. The prediction and explanation for the atomic radius should be checked before students can move on to make predictions about ions.
Outside of the classic size ranking questions, they will arrange the following isoelectronic species in order of decreasing size: Na+, Mg2+, Ne, F–, O2-, N3-
Using Coulomb’s law, students should examine the number of protons and electrons to make their prediction.
Modify
As an extension and application of their understanding of atomic size, students will predict and explain the trends for ionization energy (going down the periodic table, going left to right). They will be provided with the definition of ionization energy (the energy required to remove 1 mol of the most loosely bound electron from 1 mol of an isolated neutral gaseous atom).
Students will examine the discrepancy in ionization energy in the oxygen family and explain why it exists. In this case, students will use their trend to make a prediction, and then compare their prediction to data (periodic trends simulator). Students will need to use the quantum mechanic model with an electron configuration diagram to figure this out. This will highlight that the Bohr model is useful in explaining the general trends, but another model is required for explaining discrepancies and developing a more nuanced understanding of the trends.
Another discrepancy to explain is the ionization energy of boron as compared to beryllium. Again, this requires a quantum mechanic model.
A note on technology and simulations
Although the periodic trends simulator I’ve selected is useful, I don’t like some of the language it uses with respect to only being able to remove valence electrons to make an ion stable. Naked ions are NOT stable. Naked ions are not found in nature.
The simulator, along with instructional methods that talk about ionic stability, are missing the point that ions do not readily form in nature. We need to have a conversation about lattice stabilization energy. I remember this being a really confusing point in 2nd year inorganic chemistry. My professor commented that everyone thinks that sodium readily donates its valence electron to chlorine and chlorine willingly accepts to form chloride. This isn’t true as is. My professor specifically showed us the ionization energy of sodium and the electron affinity of chlorine and showed that it would be an endothermic reaction. This contradicts what we had all learned in high school! Then he commented that the missing picture was the lattice stabilization energy. Essentially, elements do not readily become ions!