M & M Decay

Rebecca Teed & Carrie Wright
Earth & Environmental Sciences, Wright State University; Dept. of Geology & Physics, University of Southern Indiana
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Initial Publication Date: February 22, 2012 | Reviewed: November 3, 2013


This is a simulation of radioactive decay which illustrates what a half-life is and explains some of the challenges involved with radiometric dating. Pennies or other cheap coins can be substituted for M&Ms if needed.

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Introductory Earth science lab

Skills and concepts that students must have mastered

Must have been introduced to the concepts of radioactive decay and radiometric dating. Students need to have a rudimentary understanding of probability.

How the activity is situated in the course

After doing reading and hearing a lecture on radioactive decay and radiometric dating


Content/concepts goals for this activity

Probabilistic nature of radioactive decay
Importance of sample size
Time limits to effectiveness of radiometric dating

Higher order thinking skills goals for this activity

Observation and explanation of the differences between real events and probabilistic models
Extending understanding of this simulation to radiometric dating

Other skills goals for this activity

Graphing and probability

Description of the activity/assignment

This simulation is intended to help students use what they already know (elementary probability) to the concept of radioactive decay and how it can be used to determine the absolute age of an igneous rock. The students will be simulating atoms of a radioactive isotope such as radiocarbon or uranium with M & M's or some other flattish object with differently-marked sides.

They simulate a half-life by shaking the M & M's and dropping them onto a sheet of paper. Roughly half of them will fall with the blank side up, just as half of the radioactive atoms decay into another element during a half-life. The students will remove the blank M & M's, graph the number of undecayed ones, and shake those, simulating another half-life. The students will pool their data, examine the effect of sample size, and assess how many half-lives it takes to run out of countable undecayed atoms. This model explains why real radiometric dating labs require minimal starting sample sizes and cannot give dates beyond a certain range (>40,000 yrs) for radiocarbon.

Determining whether students have met the goals

The class has a debriefing after finishing the worksheet. Each group of students presents its answer to one or two of the discussion questions.

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