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Determining the Age of Fossils

Grades: 9-12
Author: Vicki Peplin
Source: Ideas gathered from: Radiometric Dating Activity and Determining the Age of Fossils


Abstract

This lesson is designed to help students understand the concepts of radioactive dating to help determine the approximate age of fossils and rocks. After this lesson students will have an understanding of how materials undergo radioactive decay and that the rate of decay allows scientists to predict the approximate age of the specimen. This lesson is geared to help take the "mystery" out of scientific dating of rocks and fossils.


Objectives

What should students know as a result of this lesson?

  • Students will explain the concept of half-life in radioactive decay.

What should the students be able to do as a result of this lesson?

  • Students will use half-life properties of isotopes to infer the age of different rocks and fossils.
  • Students will compare and contrast individual statistical data results to the class average of various statistical data results to determine reliability and predictability of the two groups.

Materials

Each student group will need the following:

  • 5 small plastic bags (to be filled by teacher with beads according to the procedure)
  • Approximately 500 plastic beads of different colors
  • A calculator
  • 100 M&Ms (or other two-sided materials such as pennies, bi-colored tokens, etc.)
  • Activity worksheets (teacher-downloaded from websites, see procedure section)

Procedures

Engagement

Have students work in groups of 2-3 per group. The teacher may assign groups or allow students to pick with whom they would like to work. These groups will be used throughout the remainder of the lesson. Go over the "proper use of materials for their intended purpose" policy to remind students of the difference between acceptable and unacceptable behaviours in class. Follow the directions from the Determining the Age of Fossils website for activity 2a. Before the lesson, the teacher should copy the data tables found in Figures 2 and 3 (on the website) and paste the tables to a student worksheet so that the students have a place to record their data. On the back of the worksheet the teacher should copy and paste the three after-activity questions found next to Figure 3 on the website. Once all individual data are recorded have participants share their data in a way that all participants can observe (on a large white board) so that a class average can be calculated. At the end of the activity the groups may eat their M&Ms.

Assessment: Monitor students' work to check that they are carrying out procedures carefully, making observations, and recording data accurately. Redirect their attention to the task, as needed.

Exploration

Follow the directions on the Radiometric Dating website. Before the lesson the teacher will need to make a student activity worksheet with the data table, graph, and follow-up questions found on the website. Also, the teacher will need to assemble the five different bead bags and place them in various locations around the classroom. The amount of each type of bead for each bag is found in the directions on the website.

Assessment: Monitor students' work to check that they are carrying out procedures carefully, making observations, and recording data accurately. Redirect their attention to the task, as needed.

Explanation

Students report their findings. Each team shares their data from the activity worksheets. Discuss with the class as a whole the student responses to the end of the activity questions from the "Engagement" and "Exploration" sections of the learning cycle. During the discussion explain to students how this decay is at a set rate for a given element and that by measuring the percent rate of decay scientists can accurately predict the approximate age of a fossil or rock. This is not to be confused with the statistical probability of the change in the population over time as demonstrated with an individual group's data during the M&Ms activity. Note that the average of all of the groups' data more closely resembles the statistical probability of the change over time not the individual group's data.

Assessment: Have students explain in their own words the concept of half-life in radioactive decay, demonstrate how the rate of radioactive decay and the buildup of the resulting decay product are used in radiometric dating of rocks, and compare and contrast individual statistical data results to the class average of various statistical data results to determine reliability and predictability of the two groups.

Elaboration

Have students research a radioactive isotope of their choice and find the following information:

  • Parent isotope
  • Daughter isotope
  • Half-life
  • Fossils and/or rocks dated with this isotope
  • Where in the world the fossils and/or rocks were located
  • How old the fossils and/or rocks are according to the isotope

Types of radioactive isotopes:

  • Uranium-238
  • Uranium-235
  • Thorium-232
  • Rubidium-87
  • Potassium-40
  • Samarium-147
  • Carbon-14

Assessment: Teacher should develop a rubric to assess students' research papers.


Prerequisites

Students should have a basic knowledge of the parts of an atom especially concerning protons and neutrons.


Best Teaching Practices

  • Hands-on/Minds-on Learning
  • Learning Cycle

Alignment with Standards

NGSS Standards:

  • 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.

Common Core Standards:

  • RST.9-10.3 Follow precisely a complex multistep procedure when carrying out experiments, taking measurements, or performing technical tasks, attending to special cases or exceptions defined in the text.
  • RST.9-10.6 Analyze the author's purpose in providing an explanation, describing a procedure, or discussing an experiment in a text, defining the question the author seeks to address.
  • RST.9-10.7 Translate quantitative or technical information expressed in words in a text into visual form (e.g., a table or chart) and translate information expressed visually or mathematically (e.g, in an equation) into words.
  • WHST.9-10.2 Write informative/expanatory texts, including narration of historical events, scientific procedures/experiments, or technical processes.
  • WHST.9-10.4 Produce clear and coherent writing in which the development, organization, and style are appropriate to the task, purpose, and audience.

National Standards:

  • Content Standard A: 9-12 Science as Inquiry
  • Content Standard D: 9-12 Earth Science

Ohio Standards:

  • Grades 9-10 Scientific Inquiry Benchmark A
  • Grades 9-10 Earth Science Benchmark C

Content Knowledge

Vocabulary:

  • Isotope: An alternate form of an element due to a change in the number of neutrons.
  • Parent Isotope: The unstable radioactive isotope that will decay over time
  • Daughter Isotope: The stable un-radioactive isotope that the parent isotope decayed into.
  • Radioactive decay: Radioactive isotopes that are unstable and change over time to become more stable.
  • Half-life: The probable rate of nuclear decay of a radioactive isotope.
  • Probability: The statistical chance than an event will occur.

Math Connections:

During the engagement activity students observe what occurs in a population of M&Ms over time due to random chance. The statistical probability of the M&M "decay" is 50% per each half-life but the actual data may not reflect the theoretical probability. Students may confuse this random change of events with the non-random decay of radioactive isotopes. Stress the fact radioactive decay is not random and is highly predictable based upon past empirical data.


Safety

The small beads and M&Ms could be choking hazards or a projectile hazard. Therefore, the teacher should state and enforce a no throwing rule of materials. The M&Ms can be eaten by the students at the end of the activity. Make sure to know any food related allergies of the students before handing them the candy. If such allergies occur, use a substitute material such as pennies.


Applications

This content is applicable to the real world due to the fact that it deals with the scientific dating of rocks and fossils found on the planet.


Assessment

At the end of the lesson each student will be presented with his/her own fossil. This fossil will have information containing how much of the parent and daughter isotopes were found as well as the accepted half-life of the radioactive isotope. Using a graph that relates percentages of parent isotope levels to number of half-lives students will determine the approximate age of the fossil.
Example data set:
Potassium-40 = 12 isotopes
Argon-40 = 88 isotopes
Half-life = 1.25 billion years

Answer: Using the graph from the Exploration activity, this radioactive isotope has undergone three half-lives thus making the fossil (1.25 * 3 =) 5.25 billion years old.


Other Considerations

Grouping Suggestions: The teacher should assign small groups (2-3 students) to ensure diversity within the groups. Teacher discretion is advised especially in the areas of mathematical abilities.

Pacing/Suggested Time: Three class periods (approximately 45-50 minutes for each period).


Printable PDF Worksheets

Radioactive Dating Assessment Worksheet

The activity worksheets need to be created by the teacher with information based off of the websites.

Worksheet #1: This worksheet will correlate with the "engage" portion of the module and will need to include Figure 1 from the Determining the Age of Fossils website. The back of the worksheet will correlate with the "explore" portion of the module and will need to include Figure 2 and Figure 3 from the Determining the Age of Fossils website.

Worksheet #2: This worksheet will correlate with the "elaborate" portion of the module and will need to include the data table, graph, and follow-up questions found on the Radiometric Dating website.


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