Alpha Decay

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SWBAT explain how alpha decay occurs and model it using nuclear equations.

Big Idea

Alpha decay is the process by which atoms with too many protons can become stable.


This was the first lesson coming out of the extended Thanksgiving weekend.  Therefore we are starting slow, with a "What do you remember?" style bell ringer, a brief review of alpha decay, and introducing nuclear equations via alpha decay.

Prior to the extended weekend, students had learned about nuclear fission and the basics of radioactivity and spontaneous alpha decay.

 This lesson is aligned to the NGSS via HS-PS1-8Develop 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 nuclear equations as models to understand how alpha 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.

Radioactivity Bellringer

15 minutes

When the bell rings, I welcome students back and inquire if anyone wants to share about their Thanksgiving break.  This provides a chance for students to get some stories out of their system that they will talk about on the side with friends.  It also provides an opportunity to discuss different family traditions within the diverse population of my school.

Some students in each period talk about either going to see family, or hosting family.  I talk briefly about hosting for my family and doing all the cooking.  The Wednesday prior to Thanksgiving I shared some chemistry-based tips for making the meal, and students ask if I used any of them.  Sadly, this year I did not, but I do take a couple of minutes to talk about the chemistry we have already done and how it applied to the meal, melting butter, bonding in jello, or evaporating too much water from a dry turkey.  These real world connections are often lost on students, and the kitchen is the most likely place for them to experience chemistry at home.

I tell the class that we are going right back into our nuclear chemistry unit, because we only have 8 class days to finish the unit.  I then pass out the Nuclear Chemistry Review (cut in half to save paper) to each student.  I ask them to try their memory for 3 minutes, then circle what they were missing and use their notes.

While students are brainstorming, I circulate the room and chat with the students, both about break (how they did in athletics and activities) and the chemistry, reminding them of what we did on the computer and in class before the break.

When I see most students are done, I ask them to pay attention to the screen, and I use my document camera to project a copy of the review.  We then go over each question, and I encourage students to fill in what they are missing.  I also include citations with each answer, such as "We took notes on radioactivity" or "You saw the alpha decay on the Gizmo with the computers."  I find placing the little bit of context helps students access and re-connect ideas in their heads.  A completed student example can be seen here.

When we are finished, I have a student pass out the nuclear equations sheet, and swap it under the document camera.

Nuclear Equations -- Alpha Decay

35 minutes

Once everyone has the Nuclear Equations paper, I remind them to take a second and put their name on it.  Then I ask what the symbol for the alpha particle is.  I write the symbol on the top next to the title of the page where they can see it on the screen.

I ask the students what an equation needs in the middle of it.  A student responds "An equal sign" and I follow up with "What do chemists use instead?"  There is a pause, and in two classes, students looked at their Gizmo packet and replied "An arrow?"  In the other two classes, I had to tell the class.  It is always my preference to see if someone can help teach the class, rather than just tell them.

The video below shows how I would walk them through the first problem.

Now that they have done the first problem, I ask them to do the second on their own, using the first one as a model.  About 2/3 of each class flies through this practice, and 1/3 struggles, either uncertain why we used the alpha symbol or how we subtracted to get the daughter atom.  I make a quick lap around the room to check and see who is struggling before bringing students back together. 

I ask a student who is confident in what they did to tell me what to write, and let them walk the class through the example.  When they do, I probe them asking "Why are we using this for the alpha particle?" or "How did you get 60 for the atomic number?" to ensure that their classmates can hear their thought process. 

Now I give students time to finish the page, they can check with me if they are confused, or check with a table mate.  I tell them that when two or more people at their table are ready to do the back, to let me know and I'll explain it to them.

While students are working, I circulate the room and provide reinforcement and clarity for students who are still a little unsure.  After about 5 minutes I point out that there is a trick to both #7 (three alpha particles) and #9 (asking for the parent atom), but that I want them to try each one before calling me over.

When students are ready to label the decay series on the back, I have them only find the alpha decays.  If they are uncertain which ones they are, I ask them to compare the parent and daughter atoms on the front of the page to determine how much they changed in each problem and use that information to find the alpha changes.  When students realize their looking for changes of 4 to the mass and 2 to the atomic number, they fly through the decay series.

Since I walked the room so much, I have the students keep their sheets, and we will review them the start of the following day.  I am very confident in how the students performed, and know that reviewing these tomorrow will build confidence going into the beta decay, which can be more difficult.