~M&M Half-Life~
Background
Most geological processes occur at an irregular and unpredictable pace. In processes such as erosion, deposition, land uplift and volcanic eruption, periods of activity occur in spurts that are separated by long periods of inactivity. Although geological processes often reveal relative time, they do not indicate absolute time. For example, we can look at a rock formation and determine which layer formed earlier and which formed later, but we cannot tell exactly how many years ago a particular layer formed.
Testing radioactive minerals in rocks best determines absolute time. Radioactive decay goes on like clockwork, at an even and continuous pace. The nuclei of radioactive atoms break down, releasing particles and radiation. Finally, the radioactive element changes to a stable new element. The radioactive element is called the parent, and the stable new element is called the daughter. The rate of radioactive decay is measured by half-life - the time it takes for half of the atoms of a parent element to change into atoms of the daughter element. Consider the element radium-226, which has a half-life of 1,622 years. What happens to 10 grams of radium after 1,622 years? Five grams of radium remain, and five grams will have changed into lead.
Activity
In this lab, you will experiment with a half-life model in which M&M candies represent radioactive atoms. The imprinted "M" on each candy represents whether the atom has become stable or not. Students place the candies "M"-side down in a box, shake them, and then count the number of "changed" atoms. The graphs that students produce also make half-life easier to understand.
Materials
Procedure
1. Place the candies "M"-side down in the shoe box.
2. Close the cover and shake.
3. Open the box and remove all the "changed" candies (those turned "M"-side up).
4. Count and record the number of "unchanged" candies remaining in the box. Record this data on a chart.
| Trials | Number of "unchanged Atoms |
| 1 | 49 |
| 2 | 23 |
| 3 | 14 |
| 4 | 7 |
| 5 | 3 |
| 6 | 2 |
| 7 | 1 |
| 8 | 1 |
| 9 | 0 |
| 10 |
5. Repeat steps 2,3, and 4 until all the candies have turned.
6. Have students construct their own graphs using their data from their charts. On the graph, draw a curve in red for the data. In this model of half-life decay, each shake is comparable to the passing of time: the number of "unchanged" candies is comparable to the number of unchanged atoms.
Enrichment Activities
You can assess student's knowledge by having them draw a cartoon, write a poem, make a collage or poster, or even write a story. You might want to consider having students design their own activity for classmates or younger students to demonstrate half-life.
Source: Earth At Hand, NSTA, 1993. Darnell Giron, Langham Creek High School, Houston, Texas. Adapted by: A. Miles and S. Vance, Rice Model Science Laboratory, Houston, TX.
