Day 2: Calculating Radioactive Decay

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Objective

SWBAT Compare and contrast nuclear processes (fission, fusion and radioactive decay) in terms of changes in subatomic particles and mass and type of energy emitted.

Big Idea

After learning about the types of decay, students will practice determining daughter isotopes that are formed during alpha, beta and gamma decay.

Introduction

In this lesson students continue to explore the NGSS Performance Expectation 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 This lesson is the second of two (Day 1: Radioactive Decay Using A Gizmo) where they learn various isotopes emit radiation in the form of alpha particle, beta particle and/or a gamma ray.  Students will elaborate on this information by learning how to determine serial decay products in a decay series.

Students will apply Science and Engineering Practice 4, Analyzing and interpreting data, by graphing a series of nuclear decays and determining how atomic number and mass change.   As a result of being able to model and evaluate different types of radioactive decay, students will begin to see that the Cross Cutting Concept Energy and Matter, is an underlying theme in this unit. Evidence is obtained when students see that energy in the form of gamma radiation can be emitted during decay.

 

Homework Check-in

10 minutes

As students walk in to class I have them pick up a guided notes sheet, take out the previous day’s assignment and instruct them to answer the bell ringer in their journal.  The questions are below in italics, answers are in bold.

  • Write a nuclear equation for the beta decay  of  16561Pm:  16561Pm--> 16562Sm + 0 -1e
  • Write a nuclear equation for the alpha decay of  16562Sm:  16562Sm--> 16160Nd + 42He

After students answer these questions I ask for two volunteers to put these on the board, so that students can check their answers.

As they are answering these problems I walk around with a roster and check in their homework (student work).  They either get a + (10 pts) for good effort, √ (5 pts) for partial effort, or – (0 pts) for no effort.  While am doing this I have the first page of the answer key on the doc cam so they check their answers after the bell ringer.  After checking in the homework I will put the second page up and answer any questions.  I try not to spend too much time on this because they will have more opportunity to practice after they receive notes.

Some students struggle with the back side where they have to identify what type of decay is occurring in a decay series. This is the first time they have seen a decay series, so they are a little uncertain what was expected of them. Upon seeing the answer key it made sense what they need to do.  After teaching the lesson a simple explanation would alleviate any uncertainty. 

Explain

25 minutes

An important part of the 5E model is getting a formal explanation that helps clarify any difficulties they are having.  To do this I show a Powerpoint that summarizes decay symbols, types of decay and the dangers of decay and their penetrating ability.   For these notes I provide students with a Notes Sheet to help organize the information. I also use the note sheet because it provides them with a picture that illustrate how radiation penetrates. 

Powerpoint slides

  • As I show each slide I read the slide and explain what they are seeing.  
  • I tell them to fill out the graphic organizer

Slide 1:  Title

Slide 2: Is a summary of chemical reactions compared to a nuclear reaction, definition of radioactivity and a list of the 3 types of radiation.  

Slide 3: Summarizes stability and how as elements get heavier the neutron begin to out number protons causing the nucleus to become unstable (radioactive)

Slides 5-6:  Summarize each type of radiation with a description of the particle, what it symbol is, a picture to illustrate the particle being emitted from the nucleus and how far it can travel.  

Slide 7:  Illustrates what a nuclear equation looks like.  Students can use this as reference.

Slide 8: This slide is a review of the nuclear symbol that reinforces the proper isotope notation using uranium-238

Slide 9:   This shows students how to write alpha, beta and gamma decay of uranium-238 and thorium-234.  This is important because it provides them with a visual of each decay and how the daughter product of one decay undergoes another decay.

Slide 10:  Is a small decay series that illustrates how the daughter product of one decay can decay into another isotope and so on till a stable atom is formed.  I chose a simple decay series because they will have a chance to create one in the homework.

Slide 11: Is a picture showing the three types of radiation (alpha, beta and gamma) being penetrated and blocked by paper, aluminum foil and lead.

Slide 12:  Summarizes each particle emission in a table format.  The table goes one step further than what they have learned from the previous lesson which provides new information about penetration, protection needed to block radiation and the relative danger of each particle.  This information will be explored during the next lesson when students elaborate on nuclear decay using a Geiger counter and various radiation sources to determine penetration and protection. 

After completing the Powerpoint students can use the notes to complete the homework. 

 

Practice

20 minutes

As the class is completing the table I hand out the decay series practice worksheet.  The remainder of the period is used to practice writing nuclear decay equations and reinforce the associated concepts presented during the Powerpoint.  

All of the questions on the worksheet (decay series key), (student work) can be answered using the notes.  The first portion of the assignment has students answer 10 questions about alpha, beta and gamma radiation and their penetrating ability. Questions 11-16  asks students to identify daughter products and particle emissions.  This is a review of day 2 and should be relatively easy to complete for students.

Questions 17-19 build on the previous questions by having students take the daughter products from one decay and write several decays based on daughter products.  This will help develop an understanding that isotopes can go through a series of decays before coming stable.

The final part of the worksheet has students graph a series of decays based on atomic number and atomic mass of the daughter isotopes produced at each step of the decay.  Before graphing students must determine the daughter products resulting from alpha and beta decay in the same way that they did in Questions 11-6.  Once the daughter isotopes are determined, students create a graph showing all of the daughter isotope symbols' atomic masses and atomic numbers. 

Since students work at different paces and I want to provide specific directions about the graphing portion I stop them after about 10 minutes and give them a brief explanation about what the graph should look like.  

  • Using the graph from question 17 as an example, I let them know atomic number should be on the x-axis and mass number on the y-axis. 
  • I explain the numbers will be different because their decay series is slightly different but still needs to spread out according to their numbers so that most of the graph is used.
  • Instead of dots make a circle and write the symbol for the element inside the circle.
  •  Connect the points as you make the graph, writing α, β or γ on the line to indicate the mode of decay.  Make sure your graph has an appropriate title and covers at least half of the page.

I find it important to walk around and help with the graph so the axis are numbered and labeled correctly.  Some students struggle with this so a little explanation can help student complete the graph correctly.

This graph illustrates how beta decays only changes atomic number as a beta decay step appears as a horizontal line, while alpha decay changes the element and the mass and appears as a diagonal line.