Summary

In this activity, students will apply their understanding of radioactive decay to establish that radiometric dating (specifically C-14) can be used to reliably determine the age of Earth’s materials.

Grade Level

High School

NGSS Alignment

This activity will help prepare your students to meet the performance expectations in the following standards:

• HS-ESS1-5: Evaluate evidence of the past and current movements of continental and oceanic crust and the theory of plate tectonics to explain the ages of crustal rocks.
• HS-ESS2-1: Develop a model to illustrate how Earth’s internal and surface processes operate at different spatial and temporal scales to form continental and ocean-floor features.
• HS-PS1-8: Develop models to illustrate the changes in the composition of the nucleus of the atom and the energy released during the processes of fission, fusion, and radioactive decay.
• Scientific and Engineering Practices:
  • Analyzing and Interpreting Data

Objectives

By the end of this activity, students should be able to:

• Use radioactive isotope data to determine how long a sample has been decaying.
• Explain the scientific principles that allow us to accurately determine the age of decaying organic material using C-14.

Chemistry Topics

This activity supports students’ understanding of:

• Nuclear Chemistry
• Radioactive decay
• Radiometric dating
• Half-life

Time

Teacher Preparation: 30 minutes
Lesson: One 60-minute class period (optional extensions potentially 1-2 class periods more)

Materials

• Copies of the student handout.
• Access to PHeT (optional follow-up, see teacher notes)

Teacher Notes

• This activity is part of a series of activities designed to guide students in arriving at an understanding of HS-ESS1-5 and allow them to develop hypotheses for HS-ESS2-1. Prior to this activity, students have already developed a basic understanding of the types of radioactive decay and the role that time plays in that decay. Students have learned about alpha and beta decay and can write nuclear equations for each. Students have also completed activities like, The Demise of Frosty to understand of a time-based phenomenon can be used to explore past events and Using Dice to Explore Radioactive Decay to understand decay rate and half-life.
• I suggest that teachers review this background information on C-14 dating if needed.
• After completing this activity, students will be prepared to explore the activity, Radioactive Decay and Seafloor Data.
• The data in this activity was retrieved from a research paper (specifically, Table 1 on p.120), published by The Arizona Board of Reagents, on behalf of the University of Arizona.
• The data was made slightly more student-friendly, but remain essentially unchanged. It may be interesting to have students explore more of the raw data. Particularly interesting to students: The research paper focuses specifically on how the relative distribution of radioactive isotopes changed markedly after nuclear testing began in earnest around 1950. All of the data tables in the paper show a distinct change around that time.
• The pre-activity reading and questions should be completed individually. This may be completed as a homework task or at the beginning of class. Have students discuss their answers to the questions in pairs. Then, have students discuss the driving Question in pairs and share out their thinking. Most will suggest a correlation between the decay of C-14 and the age of a sample.
• In pairs or small groups, have students consider questions 1-3 in the Observations section. Most will find their observations of the data table confirm their initial hypothesis for the Question. Then, questions 3-4 familiarize students with the graphs that are provided and allow them to apply their prior knowledge of half-life. Note: This (and subsequent) activity uses a pre-made graph of the nuclear decay of the isotope in question. The activity was originally created for a 10^th-grade, introductory chemistry course. If it were used in a more advanced course, you may choose to remove the graphs and ask students to use a more mathematical pathway to determine the age of a sample. Students could use the equation where t_1/2 is the half-life of the isotope and N is the portion of the original sample remaining, written as a decimal. For example, if a sample of C-14 (t_1/2 = 5,730 Yr) is 75% of its original concentration, then = 2,380 years old.
• Students now understand that there is a correlation between the age of a layer of peat and the amount of decay the C-14 has gone through. But, how closely do these two data sets correlate? How accurately can one be used to predict the other? The activity directions ask students to first calculate the age of a layer based on the Year of Layer column in Table 1. Then, they are asked to use the Percent of C-14 left data and the provided graphs to determine how long the C-14 has been decaying. Students should find that the actual age (based on depth of the layer) and the predicted age (based on C-14 decay) are very close. The correlation is not exact and this leads to discussion about the uncertainty associated with this technique. Note: In doing this activity, I have found students who wish to use the age of the layer to look up the percent decay on the graphs. This method also allows for validation of C-14 dating. You may consider removing Table 2 completely and allowing students to develop their own methods of data analysis for this activity.
• Lastly, return to the original question and have students share out findings. Generally, students will be able to state that C-14 dating can be used to date decaying organic matter with a high degree of certainty. They will also state that some degree of uncertainty is to be expected in the results.
• As an application of their new understanding, use the Radioactive Dating Game from PHeT. Students can use the graphs to make age predictions of the different objects. I do this as a whole-class activity, projecting the simulation on the screen. The simulation brings up some interesting discussions. First, the living objects at the surface have no decay. This allows for discussion on the nature of C-14 in living organism and that an observable change in that amount only occurs after it dies. Second, for non-living objects, there is no usable C-14 data. So, this technique only works for formerly living things. Lastly, for formerly living things that are really old, the amount of C-14 approaches zero. This allows students to see that the range of time we can explore with C-14 is limited by the half-life of the isotope. This often leads to student questions about other isotopes with different half-lives, which we explore in subsequent activities.
• Included as a separate document is the Ice Cores and Kr-81 Activity. This optional resource can be used as practice, enrichment or formative assessment. Students apply the skills and concepts they have gained in the C-14 activity to a new context. This activity also allows students to see that radiometric dating can be used in different materials and on different time scales. The data for this activity is a simplified sample from this research article.