This lesson is adapted from the Illinois Science Performance Assessments before the adoption of the NGSS. It is the third lesson in our constructing a mental model of the structure of the atom as prior knowledge for HS PS1-1: Use the periodic table to predict properties of elements based on patterns of valence electrons.
One of the most difficult parts of Science and Engineering Practice 2: Develop and use models is letting go as a teacher and allowing students to make errors in the process of developing their mental models. At this point, students may have conceived the "plum pudding" model of the atom with negative and positive charges throughout the atom, or another incomplete or inaccurate atomic model. This lesson develops, through inquiry in the lab, the concept that the atom is mostly empty space with a dense positively charged nucleus, thus helping students to refine and improve their models. This process of developing models is slower than simply telling students about Rutherford's nuclear model and students may make mistakes along the way. But the inquiry process is worthwhile in developing student understanding that it is OK not to know the answer -- in fact, that is the whole reason to do a scientific experiment in the first place!
In the previous lesson students discovered the electron. Today's lesson revolves around Rutherford's discovery of the nucleus. For the hands on activity, you will need an atomic target practice apparatus per table. They can be made from wood and cardboard, or purchased via the link in the experiment section.
Before beginning new content, I want to cement that the focus of the previous day on Thomson's experiments with the Cathode Ray leading to discoveries about the atom and the electron.
I ask students what they learned about the atom from Thomson's Cathode Ray experiments and the behavior of the beams.
Confirmation that they understood the lesson are shown in responses of:
I explain to students we are going to continue to develop our model of the atom. I begin to pass out the worksheet for the activity and instruct them to not look underneath the lab materials. The target practice apparatus is a piece of cardboard with wooden feet on the corners, and a shaped piece of wood underneath. Students will roll the marble under the cardboard and record its path to deduce the shape of the wood.
I pass out an atomic target practice apparatus and marble to each table. I remind students of the obscertainer activity(Exploration) from a couple of days before. Their job is to determine the shape underneath the cardboard without looking. I tell them they have 6 minutes to experiment and record their results before we come back together. I will then put on a 6 minute song, such as this example. While students are working, I circulate the room to observe how groups are testing out the shapes, answer questions, and provide advice if I feel a group is struggling.
When the song ends, I ask the groups to turn over their worksheet and swap apparatus with another table. I give them 5 minutes to run their second sample with another song as their timer.
At this point, I allow them to turn over the apparatus and check their results. I ask what features were easier to determine. Many students will cite angled faces because the ball turns in response to the angle. Other students will cite flat sides because sending the marble from a certain direction causes it to bounce straight back. I have them pass back the original apparatus to check their success on the first one.
I then ask "How is this similar to our struggle with the atom?" Due to our focus, one of the first three student responses will be "We had to observe how the ball moved and figure out the arrangement without seeing, just like scientists couldn't see atoms."
I ask each table to return the apparatus to the front desk and go back to the page in their binder from the bell ringer.
I turn on the projector, and ask students to write a summary of what they are seeing in this short video clip. I usually show clips that are this short twice, to give students time to see what they might have missed due to writing.
Now I ask volunteers to share their observations of what they are seeing. Students will include the following:
Finally, I play the video a third time, explaining the parts of the system, that the dots are alpha particles with a +2 charge, the yellow square is super thin gold foil (if I can, I get some gold leaf from a craft store to show them) and the outside ring glows whenever the alpha particles hit it.
Now I pass out the Rutherford Experimental Data sheet to each table. I remind them of the video and the set up and we look at the data table.
I ask if the number of deflections they see in the data table roughly matches what they observed in the video. Most students will respond yes. I then ask them to independently come up with a conclusion and record it in the left hand box.
After two minutes, I ask them to get with a partner at their table, share their explanations and reasoning, and come up with the best explanation from the two of them. During this time I am circulating the room reading what is on their papers, and listening to their discussions.
Based on what I'm observing and hearing, I may stop the class and ask them to focus on why a POSITIVE alpha particle would deflect away from the straight line, or why 99% of the particles go straight through. If students are already engaging these ideas, I let it lie and wait for the full share to expose all students to the ideas.
After three minutes to partner, I ask them to share as a table and create a table explanation. I give them five minutes to agree upon an explanation and reasoning for the whole table and record it on the class whiteboard.
Now is the crucial part of the lesson, and again I focus the class on the positive alpha particle deflecting, and connect to our experiments with charges the day before. Students will recall that like charges repel, so something in the atom must be positive to push the positive alpha particle off track.
I seize on this idea: "What is off track?" Students will state that most of the particles went straight through. I ask "What does this mean about this solid gold foil?" It is VERY hard for students to realize that the atom is mostly empty space given what they have experienced with solids and liquids at the macro scale.
I can refer back to the lab apparatus from the day and ask "Why would the marble roll straight through?" to elicit "Because it didn't hit anything." I connect this to our explanations by asking "So why would the alpha particle go straight through?" Now students connect and realize "It doesn't hit anything, or get deflected because of the charge"
I finally ask "What is our evidence that the particles hit something in the atom?" If needed, I can replay the video. Students should cite the particles that bounced backwards, or deflected greater than 105 degrees.
So I sum up from the student responses on the board: What did we learn about the atom today? "The atom has a heavy, positive area" "The atom is mostly empty space" If a student knows the term nucleus and introduces it today, we will use it, otherwise we will wait for tomorrow's lesson.
If students are still struggling, I project this video without sound and narrate over it myself. This shows the alpha particles passing through the gold foil and the occasional particle bouncing off the nucleus. The extra visual was needed in three of my four chemistry classes this year.
I wrap up the lesson with projecting this picture of Soldier Field in full-screen mode and put a tiny smiley sticker in the center of the 50 yard line and explain "If that is the center of the atom, the nearest electron is at the back of the end zone. The rest is empty space." This provides a sense of scale which allows them to better visualize and understand the emptiness of the atom.
I ask students to take five minutes tonight to view the diagrams and read about Rutherford's experiment on pages 72-73 of their Modern Chemistry text, paying special attention to figures 3-6 and 3-7. Before students leave I collect the data sheet from each table to review and assess the group discussions.