Lesson 4 of 7
Objective: SWBAT explain how beta decay occurs in the nucleus and model it with nuclear equations.
In yesterday's lesson, students learned to do nuclear equations for the first time using alpha decay. Today they learn about beta decay, and how to use equations to represent the process of beta decay.
This lesson is aligned to the NGSS via HS-PS1-8: Develop models to illustrate the changes in the composition of the nucleus of the atom and the energy released during the processes of fission, fusion, and radioactive decay. We will also use drawings and nuclear equations as models to understand how beta decay occurs, in alignment with Science and Engineering Practice 2. The Energy and Matter Cross-Cutting Concept: In nuclear processes, atoms are not conserved, but the total number of protons plus neutrons is conserved is present in the nuclear equations.
Alpha Decay Review
When the bell rings, I ask students to get out their alpha decay paper from the day before. From walking the room, I felt confident that the students did well on it, so I turn on my document camera and display the first three problems. We had done the first two together, so the only one they are checking is number three.
I then display questions 4-6 for students to check. I tend to go slow through these so that I can zoom as tightly as possible on my answer key for the inevitable student who forgot their glasses that day. I then show #7 by itself as it had the wrinkle of losing 3 alpha particles. If students made errors, this was the problem. We discussed why it changed, and I encourage them to do as the above student did and show the work of the three times the mass and atomic numbers.
After we check the final two problems on this page, we flip over and check the decay series. At this point, the students would only have the alpha decays labeled.
I then use the decay to transition to talking about beta decay. Since we had properly identified all the alpha decays because the mass and atomic numbers decreased by 4 and 2 respectively, I ask "how are the unlabeled decays changing?" Students identify that they don't change, and go up one. I tell them to label those as beta decays, and pass out our beta decay paper while they label.
Beta Decay Instruction
Once all students have the Beta decay paper, I put a copy on the document camera and we begin to go through the top notes section together.
I refer back to the decay series they just completed, and ask them how much the mass changed, "Zero" and how much the atomic number changed "Plus one" so I tell them to fill in the top line.
Then I ask "In the alpha particle, did we write the mass on top or bottom?" When students reply "Top," I ask them what the top number should be in the beta particle's symbol, "Zero."
Now I pause them, and have them go to the white space next to the problems, and walk them through a drawing like shown in this video.
Now I ask, "Since we lost an electron, what number goes on the bottom of the beta particle symbol?" Students will differ between one and negative one. When a student points out that it lost an electron, so it lost a negative, we place the "-1" on the bottom. We finish the symbol asking what letter to use, and students immediately think of the B for beta, and I ask what else we could use, since it is an electron. A volunteer says "E, for electron" and we put it in.
Next we use our picture to complete the sentence. "Beta decay occurs when a neutron breaks down, changing it into a proton and an electron. The electron is the beta particle, which is given off"
Then students complete the final sentence "Beta decay makes the nucleus more stable by replacing a neutron with a proton."
Now we are ready to practice the beta decay equations.
Beta Decay Equation Practice
I walk students through the first beta decay equation the same as I did the previous day with alpha decay. Students are faster at the setup, recognizing that it looks the same as alpha with the parent atom, the arrow, then the beta particle. They struggle a little with the mathematical concept of the atomic number going up one.
I refer them to their drawing, that they made a new proton, so the atomic number went up one. I also show them the mathematical representation.
87 - -1 = 87 + 1 = 88
I know students have seen the "minus a minus equals a plus" in algebra. It is still counter intuitive to them, but they accept it and try problem #2. They do a good job of showing At-198 decaying into Rn-198, and again I have a student walk us through her setup and thought process for the benefit of the class. I then have the students finish the page, including the beta decay chains on the back of the page.
This shows a completed version of the sheet, both front and back page. You can see the student drawing of our neutron becoming a proton and emitting the beta particle. This student had issues realizing the decay chain was more than one step. About 25% of students struggled with this, and I didn't catch it before they all left.
The other error on this paper is on number 7, the student didn't catch that it was emitting two beta particles, and then did the math wrong anyhow. The product should have been Plutonium-235.
Some classes finished very quickly, so I had them jump on the computers in the lab to work on the beta decay portion of their ExploreLearning paper from before the Thanksgiving break. Some students realize that they know enough to finish the packet without the papers, but others benefit from the extra visuals afforded by the Gizmo.
As students did better than anticipated, I have them keep their papers to review the following day.