Demonstration of radioactive decay using pennies

Jennifer M. Wenner, University of Wisconsin-Oshkosh,, based on material from "West's Great Ideas for Teaching Geology".
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A demonstration (with full class participation) to illustrate radioactive decay by flipping coins. Shows students visually the concepts of exponential decay, half-life and randomness. Works best in large classes -- the more people, the better.

Used this activity? Share your experiences and modifications

Learning Goals

  • to illustrate the exponential nature of radioactive decay
  • to demonstrate the concept of half-life
  • to establish the randomness of radioactive decay
  • to illustrate the concept that quantity determines the number of isotopes that decay

Context for Use

This demonstration is suitable for use in a large lecture class (>75 students). It is a great way to introduce or reinforce the concepts involved in radioactive decay. You will need enough coins (I use pennies) for each person in the class, some sort of graph paper printed on an overhead, an overhead projector and a pen for the overhead. The demonstration usually takes approximately 5-10 minutes depending on how long it takes to count the students.

Description and Teaching Materials

In a large class, give each student a penny. Have the class stand up. Tell them that they will be flipping the penny (you will tell them when); each time they flip one half life will have passed. If their penny lands on heads, they are radioactive and have decayed and they should sit; if their penny lands on tails, they have not decayed and may remain standing. After each "half life", count the people remaining standing and plot it on a piece of graph paper (Acrobat (PDF) 42kB Jun21 04) on the overhead. After about 3 or 4 "half-lives" ask students to predict what's going to happen to the numbers of remaining parent isotopes. Continue the experiment until only one or 2 people are left (usually 6-7 "half-lives").

This can set up a good discussion of what is happening to the number of students still standing (i.e., that the number of isotopes decayed is determined by the initial quantity) and can lead into a discussion of exponential decay, half-life and various other concepts involved in radioactive decay.

Some questions to get the students started thinking about these concepts:

  • (After two or three "half-lives") What is happening to the number of students standing? Do the same number of students sit down each time we flip the coins?
  • About how many students would have had to sit down if we started with twice as many students? What about if we only had half as many in this class? What does that tell you about how the quantity of "radioactive isotopes" affects the number that decay?
  • Can you predict which of you is going to be the first to sit down? Why or why not?
  • How is radioactive decay sort of like gambling or playing the lottery?

Teaching Notes and Tips

If your class is particularly large, you might want to have a few students help you count the "undecayed" isotopes. This demonstration is a great way to break up the lecture with a short activity -- students get to stretch their legs. I often give students the penny at the beginning of class with no explanation -- or say something like, "Today, you're getting paid for coming to class!" After introducing some of the concepts involved in radioactive decay, I do the demonstration. I usually let them keep the penny at the end of the class.


It is relatively easy to get a quick check on whether students have grasped several concepts: half-life, probability, dependence on quantity and exponential functions. There are several ways that this can be done:
  • If you have a student response system, a quick quiz with questions that cover these four concepts is an easy way to determine the students' understanding.
  • Having students work through a short problem (in groups or on their own) that applies these concepts in a geologic context -- a problem where they have to read a graph or calculate how many isotopes are left after x half-lives -- can also provide a quick check.
  • A short written quiz might also be a way to assess comprehension.

References and Resources

Here are links to several variations on this theme. Some of them include handouts to use for homework in smaller classes.