University of Virginia Physics Department Candy Half Life A Physical Science Activity

Adapted from an activity called "The Radioactive Decay of Candium" from North Carolina State University's Science House learning outreach program called Countertop Chemistry.

2003 Virginia SOLs

• PS.1
• PS.4

Objectives

Students will

• define the terms isotope and radioactive isotope;
• understand the concept of half-life;
• use the following skills: observing and recording data (observations), inferring from observations, graphing.

Motivation for Learning

How fast does a radioactive isotope decay? How long will it take for a sample to completely transmutate (change) into a new element?

Background Information

Most elements have atoms that come in two or more forms called isotopes. Isotopes are atoms of the same element, but with different atomic masses. This occurs because different isotopes have different numbers of neutrons. For example, hydrogen has three isotopes that are listed in the table below.

Some isotopes are unstable or radioactive. For instance, in the example above, tritium is an unstable isotope of hydrogen. Radioactive isotopes slowly decompose by discarding part of the nucleus. This nuclear decomposing process is called nuclear decay. The length of time required for half of the isotope to decay is the substance's half-life. Each radioactive isotope takes its own particular amount of time to decay. However, when the amount of remaining isotope is plotted against time, the resulting curve for every radioisotope has the same general appearance.

Student Activity

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Teacher Preparation

1. Purchase a box of ziplock bags.
2. Purchase large bags of candy that have one side labeled (Plain M&Ms and Skittles work well).
3. M&Ms and Skittles will work as is, but you could also buy those M&Ms that are sold around a certain major winter holiday - the one closest to New Year's. These M&Ms come in only two colors (red and green) which could signify atoms of different elements. The idea here is to get more than one variety of flat candy that is labeled on one side.
4. This lab requires that the students have some knowledge of atomic structure.

Materials

• 100 candy pieces (The amount of pieces of candy can be adjusted based on the size of the lab group. Give each lab group a distinct kind of candy.)
• Resealable bag
• Stop watch or visible clock that displays seconds
• Graph paper

Procedure

1. Place atoms (candy pieces) in the bag.
2. Seal the bag and gently shake for the specific amount of time that corresponds to the half-life of your candy.

   Half-life of M&Mium (M&Ms) is 1 minute.

   Half-life of Skittlium (Skittles) is 2 minutes.

3. Gently pour out candy.
4. Count the number of pieces with the print side up. These atoms have "decayed."
5. Return only the pieces with the print side down to the bag. Reseal the bag.
6. Record the time. (For M&Ms it would be 10 seconds on the first trial. On the second trial it would be 20 seconds (10 + 10). On the third trial it would be 30 seconds (10 + 10 + 10) and so on).
7. Consume the "decayed" atoms.
8. Gently shake the sealed bag again for the prescribed amount of time.
9. Continue shaking, counting, and consuming until all the atoms have decayed.
10. Graph the number of undecayed atoms vs. time.

Data Sheet

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Half-Life Total Time # of Undecayed Atoms # of Decayed Atoms 0 0       0   1         2             3             4             5             6             7             8             9             10               Using a whole sheet of graph paper and the data above, make a graph with time on the x-axis and number of atoms on the y-axis.

Extensions

Repeat the experiment starting with 50 atoms and 75 pieces of candy. Compare the resulting graphs. (The graphs can be plotted on the same paper used for the first graph.)

Students with special needs

Some students may have difficulty manipulating small objects.

Click here for further information on laboratories with students with special needs.

Assessment

To print out the Assessment Sheet only, click here.

(Please write complete sentences):

1. Define half-life in your own words.

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2. In the experiment, what was the half-life of the isotope M&Mium? Skittlium?

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3. At the end of 2 half-lives, what fraction of the atoms had not decayed?

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4. Describe the shape of the curve from the graph of your data?

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5. As a class, compare and contrast the graphs made by the different lab groups.

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Answers to Assessment

1. Half-life is the length of time required for one half of the isotope to decay.

2. The half-life of M&Mium in this activity was 10 seconds. The half-life of Skittlium in this activity was 20 seconds.

3. At the end of two half-lives 1/4 of the original sample remained; 3/4 of the sample had decayed into a new element.

4. The graph is a decreasing logarithmic curve.

5. The graphs will be almost the same.