# Demonstration of radioactive decay using pennies

#### Summary

## 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

## 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

## Assessment

- 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

- Science Net Link's Isotopes of Pennies (more info)