Radioactive Dating: Looking at Half-Lives Using M&Ms

Kathleen Babich, South St. Paul Junior High School, South St. Paul, MN, based on an original activity retrieved from also with the help of Jenni Johansen (other 8th grade science teacher at So. St. Paul Junior High School


In this activity, students gain a better understanding of radioactive dating and half-lives. Students use M&Ms to demonstrate the idea of radioactive decay. Each group begins with 100 M&Ms in a container. Parent isotopes are represented by the M side up (radioactive). Daughter isotopes are represented by the M side down (stable). Students begin by pouring the 100 M&Ms on the table, and set aside the "stable" isotopes (M side down). They then gather the radioactive, or M side up M&Ms, put them back in the container, and then pour them out again. They then set aside stable isotopes... and continue this process until all M&Ms are stable, or M side down.

During each trial, students record the number of radioactive parent isotopes and record this in a data table. Once all groups finish, each group records their info on the class decay table (on the board) and we calculate the averages of the class. Once this info is calculated, students create a graph comparing the class average of parent isotopes to the number of half-lives.

Used this activity? Share your experiences and modifications

Learning Goals

1. Students will be able to explain what a half-life of a rock is.
2. Students will have a more in-depth understanding of what radioactive decay is.
3. Students will understand how scientists use half-lives to date the age of rocks.

-critical thinking
-data analysis
-graphing and data collecting

Vocab Words:
1. Relative Dating
2. Radioactive Dating
3. Half-Lives
4. Isotope

Students should begin to see the pattern that each time they dump out their M&Ms, about half become stable. Students then should be able to see the connection of the M&Ms and radioactive elements in rocks, and how scientists can determine the age of rocks by looking at the amount of radioactive material in the rock.

Context for Use

This activity can be adapted for older students, but is used in an 8th grade earth science classroom. Class size can vary, but activity should be done in groups of 2-3. Equipment that is necessary is M&Ms-- a lot because each group needs to begin with 100, and a container with a cover for each group. Students should have the skill to set up a data table and a graph, however, if you want to use this activity with students that have not, you can provide them a template with that information. As far as mastery of content, this activity is done in our rocks and minerals unit. Students should have some prior knowledge of rocks and how they are dated. This activity would also be easy to adapt when talking about half-lives within a chemistry course.

Subject: Physics:Modern Physics:Nuclear Physics, Chemistry, Geoscience
Resource Type: Activities:Classroom Activity
Grade Level: Middle (6-8)
Theme: Teach the Earth:Teaching Environments:K12

Description and Teaching Materials

Time Needed: Appx. 60 minutes.
Materials Needed:
-100 M&Ms (per group)
-Piece of Paper
-Plastic Container with a Lid

Lesson should be introduced by reviewing the 2 broad ways scientists age rocks (relative dating and radioactive dating). Also, review what a half-life is (info given the day prior during lecture/ notes/ reading). New information needed to be introduced with parent and daughter isotopes. Once students are in their groups, with supplies, and general directions are given, they are on their own for doing their runs. Students will record the number of M&Ms that are still "radioactive" (M side up) in their data table after each run, and set aside the "stable" (M side down) M&Ms. They will only re shake the radioactive M&Ms each time. They will do this 8 times. Once they are finished with their 8 runs, they will record their data on the class data table (which can be on the board). Once all groups data is on the table, you can calculate the average for each run (1-8) and determine a class average. Students should recognize each time the number should go down by appx half. Then students take the class data and create a graph comparing the number of parent isotopes to the number of half-lives. Once this is done, students have some post questions they are given that they should record in their science notebook.

Adapted from: ( This site may be offline. ) PowerPoint for Half-Life Lab (PowerPoint 2007 (.pptx) 121kB Aug3 09) Student Lab Sheet (Microsoft Word 2007 (.docx) 75kB Aug3 09) M&M Half Life Lab Rubric (Microsoft Word 2007 (.docx) 13kB Aug3 09)

Teaching Notes and Tips

For Special Ed and ELL kids, you could give them a template of the data table/graph.

The first post question caused some confusion: Why didn't each group get the same results? A lot of the students said because they shook the containers differently... this may be one you want to discuss as a class.
I also have students wash their hands before the activity, because of course after, the students eat the M&Ms.
This was a new activity we implemented last tear. Radioactive decay and half-lives can be a very difficult concept for our 8th graders to grasp. They not only enjoyed this activity, but they really gained a better understanding of it as well.


I have attached my rubric, this is what I use for assessment. Students graph, post questions, and data table will be looked at.


8th Grade Earth Science: Interpret successive layers of sedimentary rocks and their fossils to infer relative ages of rock sequences, past geologic events, changes in environmental conditions, and the appearance and extinction of life forms

References and Resources