Radioactive Decay and Half-lives

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Students will be able to model how unstable nuclei decay based on half-lives through taking notes, performing practice questions, and doing a Marie Curie Activity.

Big Idea

Radioactive nuclei decay in different amounts of time based on their half-lives.


In this lesson students learn about how unstable elements will decay based on half-lives through performing notes, practice questions, and a Marie Curie activity.

  • This lesson covers the Next Generation Science and Engineering Performance Expectation 1-8: 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. It does so because students are learning about nuclear half-lives and Marie Curie's work with radioactivity.
  • This lesson aligns with the Next Generation Science and Engineering Practice 2: Developing and Using Models: It does so because students are using models to understand what happens during radioactive decay
  • There are no additional resources needed for this lesson.


20 minutes

To begin this lesson I review the idea of unstable nuclei and then teach students about how atoms decay based on their half-lives.  I do this by presenting the unit10 lecture 2 PowerPoint as students take notes on their unit10 lecture 2 student notes graphic organizer.

  • I begin by reviewing the idea of strong nuclear force and electromagnetic repulsions on slide 1.
  • I then show students a cartoon on slide 2.  Most students don't really get the joke of duct tape holding the nucleus together, so I usually have to explain it to students.
  • I then go into the idea of radioactive nuclei and unstable nuclei on slide 3.  As I go over the example of sodium and positron emission I draw out the nuclear equation for the positron emission of sodium-22 on the board to help tie together what we had learned in the previous lesson.
  • I then go into half-lives on slide 4 and give some examples on slide 5.  As I go over the examples on slide 5 I ask students if they have heard of these before, and I usually get some responses of having heard of carbon-14 dating.
  • Finally, I go through the example using barium-122 on slide 6.  As I go through this example I explain to students how using a table is the easiest way to solve these tricky types of questions.   This is a movie of me doing this in my classroom. 
  • This is a copy of one student's filled in notes.

Elaborate Part I

20 minutes

For the next section of the lesson I have students do example problems 2-5 related to half-lives.  

These problems include several types of examples including:

  • Students determining the amount of substance remaining after a certain amount of half-lives have passed.
  • Students figuring out the amount of half-lives that have occurred given a starting and ending amount.
  • Students determining the amount of time for a half-life given the starting amount, ending amount, and elapsed time.
  • I do not get into complicated problems such as those dealing with partial numbers of half-lives.

The example problems are on the second page of their notes graphic organizer and on slides 6-10 of the PowerPoint.

The way I do this is show the example problem and have students start to try on their own.  After a minute or two I start to do the problem on the board for those students who are stuck.  I make sure to help them with setting up a table, filling in the information that they have and use that to find what they want.  

This is a copy of one student's notes with the answers to the problems. 

Elaborate Part II

40 minutes

After students have gotten a general idea about half-lives I have them read about Marie Curie and do an activity related to half-lives.  

  • I start by passing out the Marie Curie Activity Paper and the Marie curie reading to each student.  I reuse the reading in each class so I make that a different color and tell students that I will be collecting the reading when they are done so to please not write on it. 
  • I then lead students in the reading using Popcorn Reading.  To learn more about this strategy check out my reflection.
  • I then have students do the activity on their activity paper.   Depending on the class, I will sometimes lead students through each step of the activity on the document camera.  The goal of the activity is to help visualize how uranium decays to lead over time.  I will usually use different colors as I grid for each half-life so students can see how much decays each time.  This is a copy of an answer key which shows how I do this.
  • After the activity portion students answer questions in the analyze/conclude/critical thinking sections of the paper.  For the most part students are able to answer these questions using their graphs and then going back to the readings.  Some students get confused with explaining how the uranium changes in analyze/conclude #1.  This is an example of one student's filled in activity.
  • As students complete the questions I have them start to work on the homework.


For this section of the lesson students are working on their homework.  This homework is a review of what they have learned in both this lesson and in the previous lesson, Types of Nuclear Decay.

Students get a copy of the unit10 homework as they are complete the Marie Curie activity.

Most students do not complete the paper in class so they take home and complete for homework.  I then review the answers with students in the next class using the answer key.

The biggest struggle that students have with this paper are question #3 with changes in atomic number and mass number (I remind them to look at the numbers at the top and bottom of the symbol) and questions #6 with half-llives.  I go over how to answer the half-life questions with students on the board.  These are videos showing me explaining the answers to the questions for example b and example c