Class as an Artifact:

A Radioisotope Dating Activity

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Half-life is a mathematical concept that can be difficult for students to visualize and interact with. This activity seeks to illustrate the concept of half-life in a way that students can understand, while providing an opportunity for students to practice their graphing and computational skills.

Overview:

Objective: Students will learn the concept of half-life, be able to identify the half-life of an isotope from a plot of its decay, and understand the potential application of the concept of half-life in the dating of biological and geological artifacts.

Materials:

• 30 Pennies
• Graph paper for each student

Time: 20-30 minutes in class

National Science Education Standards Addressed:

• Content Standard A - Science as Inquiry
  • Mathematics is important to all aspects of scientific inquiry. This activity not only illustrates the importance of math in graphing and understanding half-life, but also allows students to practice their own math and graphing skills.
  • Technology used to gather data enhances accuracy and allows scientists to analyze and quantify results. By discussing accuracy in this activity and repeating the in-class study to improve results, students can see one way scientists test and reinforce the accuracy of their results.
  • Science advances through legitimate skepticism. This activity allows students to gather data, question their results and gather new data to either confirm earlier results or lead to new understanding.
• Content Standard E - Science and Technology
• Technological designs have constraints. These include the constraints that limit technological design illustrated here through the limitations of carbon-14 dating to relatively recent fossils.
• Perfectly designed solutionss do not exist. The activity also shows that perfectly designed solutions do not exist by emphasizing error in the data collected in class and discussing the significance of this error and ways to minimize it.
• Science and technology are reciprocal.This activity illustrates the ways in which technology has evolved to further science and the ways in which scientific inquiry has led to the growth in technological design using the advancement of radiometric dating as one example of this theme.

 

Additional Background for Teachers:

Teachers may wish to seek additional background on carbon-14 or half-life before beginning this activity. This additional information may also help teachers to generate additional follow-up questions and extensions of this project.

The Activity:

1. Tell the class that today they will become an artifact. You may wish to bring in an artifact to show or a picture of the artifact you want your class to image to be. Your students may be familiar with the Dead Sea Scrolls, a famous artifact dated using carbon-14 dating. Remember carbon-14 dating cannot be used to date most fossils. For more information on the restrictions on carbon-14 use in fossil dating see the carbon-14 background information page.

2. Explain that each student will represent one carbon-14 atom in the artifact. Make sure to be clear that the carbon-14 atoms in this artifact represent only a small percentage of all the atoms that compose it. Remind students that the majority of the artifact will be made up of carbon-12 or other atoms. However, students will play the part of carbon-14 because those are the atoms you wish to observe when measuring radioactive decay.

3. Give each student a penny. Have all the students stand beside thier desks. Have each student flip their penny. If the penny is heads the student decays to carbon-12 and must sit down. If the penny comes up tails the student remains undecayed and may remain standing. Each round of flipping the coin will be called a trial. As the students flip their coins record the number of decayed and undecayed atoms on the board along with the trial number. Continue flipping he coins until all students have "decayed."

4. When all students have decayed have all students record the class data on the board. Have students work in pairs to plot the data on a graph. Their plots should show the number of undecayed carbon-14 atoms on the y-axis and the trial number on the x-axis. Student plots should show a half-life decay pattern as in the plots below:

In the plot above, the number of remaining undecayed atoms has been converted to a mass, and the trial number has been converted to years. The same pattern of decay should be visible in your class data.

5. When students have created their plots discuss the results as a class. The half-life of the carbon-14 in your class should be 1 trial time (half of the students "atoms" should have decayed in each trial). You may wish to have your students convert theri trial numbers into years. You could have 1 trial = 10,000 years, for example. Then students can determine the half-life of the class carbon-14 in years.

6. Have students consider some of the following questions:

• How could you change the half-life of the element in the class trial?
• Was the class curve perfectly smooth like the one shown above?
• What are some possible sources of error in the class sampling? (Note that for most small classes a small sample size is probably the greatest source of error. This can lead students to consider the limitations of carbon-14 dating on older fossils that have very little carbon-14 remaining.)

7. You may wish to repeat the class sampling again near the end of the hour to see what differences exist between the two smaplings. With small classes there will likely be differences between the two. Have students plot all the class samplings and average them to see if they gain a smoother curve. Discuss the method of averaging many trials with your class as a way of obtaining a more accurate result.

Assessment:

This activity seeks to teach students the concept of half-life and radioisotope decay. To test student understanding teachers may wish to use a short quiz. A quiz is provided here along with an answer key. Teachers may also wish to use a more interactive assessment model.

A more interactive assessment could include having each student invent an isotope. Students should name thier isotope, decide its half-life and plot its decay. This part of the assessment could be completed as homework and give students practice both using the idea of half-life and graphing. The teacher would then collect slips from each student with the decay plot of thier isotope. The teacher would redistribute the plots to other students and create a key that listed the name of the element along with its half-life. Students would then be assigned to determine which element they had based on the decay plot they recieved. Note that the teacher should check student plots before redistributing them to assure that they are readable and reflect the half-life the student originally assigned.

Finally, teachers may wish to do an in-class assessment by putting out a series of artifacts. The teacher should attach a radioisotope name, half-life and decayed:undecayed ratio to each artifact. Students would then circulate among the artifacts determining the aget of each base on the half-life of the radioisotope used to date it and the decayed:undecayed ratio.

Another great Half-life activity:

Another great half-life activity is available through Resources for Educators. This activity invovles having students act out radioactive decay using pennies in a box. It offers a greater sample size with similar opportunities for graphing and assessment.

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