This lesson addresses NGSS HS-PS1-1, Use the periodic table as a model to predict the relative properties of elements based on the patterns of electrons in the outermost energy level of atoms. The goal of the lesson is to provide evidence that electrons exist and they are located in different energy levels, in varying quantities. This is aligned with the NGSS Disciplinary Core Idea (DCI) PS1.A (Structure and Properties of Matter): “each atom has a charged substructure consisting of a nucleus, which is made of protons and neutrons, surrounded by electrons”.
In this lesson students continue to explore the structure of the atom using the NGSS Practice: Developing and using models. In day 2 of the investigation my students use the same POGIL inquiry activity to investigate Coulombic attraction. This process illustrates evidence that subatomic particles are present within an atom and further builds a foundation for exploration of HS-PS1-1. This lesson also lays the ground work for HS-PS1-2 and HS-PS1-3, the next two units I will teach.
The Cross Cutting Concept (CCC) that is addressed in this lesson is Patterns. Patterns are illustrated as students continue to see that as elements atomic numbers change so do the number of protons and electrons. However, this concept is expanded when students see that electrostatic forces are present between subatomic particles.
I start class by showing my student the same picture (POGIL Model 1 & 2) that was shown on the previous day which is a diagram of attractive force between a proton and electron. I instruct them to write in their science journals a one sentence summary of the picture and what they learned the previous day.
A good answer should state that as the distance between a proton (nucleus) and electron increases, the attractive force decreases.
This is followed by a quick class discussion to summarize the previous day’s lesson and leads quickly into model 3 and 4 of the POGIL activity. The discussion should focus on what causes attractive forces (protons & electrons) and how force can change as distance between particles increases or decreases. This may lead to the idea of inverse relationships; however, it's not important that they know the relationship is inverse, per-say, but that as distance increase, force decreases.
On day two I begin this part of the lesson with the students observing the diagram and answering the questions in Model 3. As result of yesterday's POGIL, my students have re-learned the idea behind independent (IV) and dependent (DV) variables and have virtually no problem answering questions 9 a & b. They also start to realize that scientists are not moving electrons to determine distances in the atom, but instead measuring the distance between different electrons and the nucleus in an atom. As I walk around, I emphasize to my students the importance of modeling and how we as "scientists" can gain a great deal of understanding.
Model 3 is different from Model 1 & 2 in that distance stays constant with the IV being the quantity of protons and the DV being attractive force. Some students need help with questions 9-10, but the majority of students can make decent observations and realize that as the number of protons increase, the size of the attractive force increases.
To make sure all my students understand model 3 we go over it as a class.
During the process of giving them the answers I use 8 magnets to illustrate how adding protons can have a greater pull on an electron. I illustrate this on the ELMO (overhead projector) by put 2 magnets 3 cm apart and add 1 magnet at a time. Each time I ask them if the force is getting stronger…most agree yes. By the time I add the 3rd magnet all students agree that the force is getting stronger because the magnet get pulled together (see magnet video). My students really get the concept after this.
After Model 3 I instruct them to go through the same process as they did with Model 3 for Model 4. This one take a little longer, so I give them 10 minutes to work on this. I find it important to walk around during this whole lesson, especially now to keep their focus. It is a long process for them to work through the four models, but it is extremely effective in helping students understand attractive forces.
After 10 minutes we go over the questions. By this time almost all students are saying the answers as a class because POGIL does an excellent job of build knowledge and understanding progressively. Every question builds from the previous one and gets students to scaffold information. From Model 4 students learn that as elements change across a row on the periodic table the number of protons and attractive force increases, but the size of the atom do not increase significantly.
The importance of attractive forces ties into many of the NGSS physical science DCIs and is a foundational concept for understanding periodic trends, bonding, IMF’s and bond energy, just to name a few. It is important that students understand this and are given ample time to build understanding. It will help them tremendously in the next unit on the periodic table and bonding.
As a final assessment I skip question 17 and the extension questions, and give them with a take-home quiz (here is the answer key) that summarizes all four models. The following day we grade it as a class so they can they receive immediate feedback. After students have viewed their grade and assessed their strengths and weaknesses on the quiz, I collect it and enter it as a grade.
Overall they do very well on the quiz because we work on it for two days and it is essentially an open note quiz. Another option that I have done in the past is give it to them in class and grade that same day. If this time this is a great way to ensure that the student work is authentic.