Materials Included In Kit

Additional Materials Required

Prelab Preparation

Divide the bingo chips and place them into the 15 Petri dishes according to the first four columns in the following chart. The exact number of green and yellow chips is important for determining the correct ages; the exact number of red chips, however, does not matter. The total number of chips in each dish should be approximately 40. Label each dish according to excavated layer number. Three samples (A, B, C) will be made for each layer.

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. This laboratory activity can reasonably be completed in one 45- to 50-minute class period. The pre-laboratory assignment may be completed before coming to lab, and the Post-Lab Analysis and Questions may be completed after the lab.
• Place the prepared Petri dishes in a central location for students to obtain. The order in which the groups “date” each sample is not important. Groups do not have to date the same letter sample from each layer.
• Remind students to return all the colored chips to each Petri dish before returning the samples. You may want to advise them to count the green and yellow chips as they put them back, so the next group will have the correct number. Placing the chips on white paper as they are removed from the Petri dish may make them easier to locate.
• The most important point students have to remember is that the original number of parent isotopes in the sample is equal to the total number of parent and daughter isotopes at the time of discovery.
• Students may discover a different method for determining the number of half-lives of a sample. The approach given with the worksheet is summarized.
  {12642_Hints_Equation_1}
  This ratio is the percentage of parent isotopes left at the time of discovery. This percentage is then matched to the graph to determine how many half-lives have transpired since the formation of the artifact.
  {12642_Hints_Equation_2}
  Example: Artifact Label 1A

  24/(24 + 8) x 100 = 75%
  75% is the percentage of parent isotopes left after ½ half-life
  ½ x 1500 years = 750 years

Teacher Tips

• This activity is appropriate for a physical science study of radioactivity or an earth science study of Earth’s history.
• Students should have an understanding of atomic structure, atomic number, and mass number.
• Place “real” artifacts by the Petri dishes as an additional visual aid. Explain that the samples in the Petri dishes were “taken” from each artifact. The artifacts could be simple toys, school supplies, wooden spoons, flower pots, etc.
• The ratios and ages used in this activity have been simplified for ease of calculations. Numbers may be modified for the dating of rock, using uranium-238. U-238 undergoes a series of decay steps by both alpha and beta decay to lead-206. U-238 has a half-life of 4.5 billion years!
• Carbon-14 dating does not use the parent-to-daughter ratio. The daughter isotope of carbon-14 is a gas, nitrogen-14, which easily escapes from the artifact. Instead, the ratio of carbon-14 to carbon-12 is employed. The accuracy of carbon-14 dating depends on the ability of a living organism (plant or animal) to absorb C-14 throughout its lifetime. Ideally, no more C-14 is incorporated into a sample or the object after it has been made (and the plant or animal is dead).
• The following kits may be used to further explore radioactive decay—Radioactivity Half-Life Simulation (Flinn Catalog No. AP6721) and Radioactive Decay Cards (Flinn Catalog No. AP4555).

Answers to Prelab Questions

1. The radioisotope chlorine-36 breaks down by alpha decay. (a) Look up the atomic number of chlorine. How many protons and neutrons are present in chlorine-36? (b) How many protons and neutrons does the resulting daughter isotope contain? (c) Identify the element and the mass number of the daughter isotope.
   1. Chlorine-36 has 17 protons and 19 neutrons.
   2. The resulting daughter isotope has 15 protons and 17 neutrons.
   3. Phosphorus-32 is the daughter isotope.
2. The half-life of radium-226 is 1600 years. How old is an artifact in which 25% (¼) of the original radium-226 is present?

   After two half-lives, 25% of the parent isotope would be left. The artifact is 3200 years old.

Sample Data

See the table in the Prelab Preparation section.

Answers to Questions

1. Assuming there were no daughter isotopes in the original artifact sample (when the artifact was made), determine the total number of parent isotopes in the original sample (parent + daughter) for each artifact. Enter the data in the results table.
2. Calculate the percentage of original parent isotopes remaining in each artifact by dividing the number of parent isotopes in the sample by the original number of parent isotopes and then multiplying by 100. Record the percentage in the results table.
3. Using the graph above, determine the number of half-lives for each sample. Record in the results table.
4. Multiply the number of half-lives by the half-life of the parent isotope from the Experiment Overview section to estimate the age of each artifact. Record in the results table.
5. Which excavated layer number contained the oldest artifact? Which contained the most recent?

   The oldest artifact is represented by layer 5 and the most recent is layer 1. Note: Groups that did not date a sample from either layer 1 or layer 5 may have layer 4 as the oldest or layer 2 as the most recent.

6. Based on the results of this activity, estimate the possible age range of the artifact from the layer that was not dated by your group.

   Layer 1: <1500 years 
   Layer 2: >750 to <3000 years 
   Layer 3: >1500 to <4500 years
   Layer 4: >3000 to <6000 years
   Layer 5: >4500 years
   

7. What do the red chips in the artifacts most likely represent?

   The red chips represent atoms of another element that are not relevant to dating the artifact.

8. The limit to radiometric dating using a particular radioisotope is usually 8 to 10 half-lives. Suggest a possible reason why this might be true.

   After 8 to 10 half-lives, very little, if any, of the parent isotope is left, making it difficult to measure the ratio of parent-to-daughter isotopes.

9. It is possible that some of the daughter isotope may be already present at the formation of a particular artifact. How would this affect the dating of the artifact?

   If some of the daughter isotope were already present, the age would be calculated as older than the actual age of the artifact. Scientists would need to figure in the original amount of daughter isotope present in their calculations to determine the age of the artifact.

References

Special thanks Annis Hapkiewicz, retired, Okemos High School, Okemos, MI, for providing the idea for this activity to Flinn Scientific.

Bogner, Donna, “Teaching Radioactivity in the Secondary Classroom,” Journal of Chemical Education, January, 1988; Vol. 65, No. 1, pp 47–48.

Introduction

An archeological site in Flinnlandia was recently excavated. Five layers were excavated and various handmade objects known as artifacts were uncovered. Your job is to determine the age of four different artifacts, one each from four of the five layers.

Concepts

• Half-life
• Radioactive decay
• Radiometric dating

Background

Many elements have different isotopes. Isotopes are atoms of the same element that have different numbers of neutrons in their nuclei. Some isotopes are radioactive—their nuclei spontaneously break apart because the nuclear force holding the protons and neutrons together is not strong enough. This breaking up of a nucleus is known as radioactive decay. One way radioactive decay occurs is by alpha decay. When alpha decay occurs, alpha particles identical to a helium nucleus (two protons and two neutrons) are emitted from the nucleus. This produces an atom of a different element with an atomic number that is two less than the original element and a mass number that is four atomic mass units less (see Figure 1). A well-known radioactive isotope, uranium-238, changes to thorium-234 by alpha decay.

Beta decay occurs when a neutron decays into a proton and an electron, and the electron (called a beta particle) is emitted at a high rate of speed from the nucleus. The mass number of the atom does not change, but since the nucleus now has one more proton than before, the atomic number increases by one and a different element results (see Figure 2). Perhaps the best known radioactive isotope that undergoes beta decay is carbon-14, which is used to date artifacts composed of organic material, that is, materials that were obtained from once-living organisms (plants or animals). In the process of radioactive decay, the original radioactive isotope (radioisotope) is known as the parent isotope and the element produced after it decays is called the daughter isotope. In the above example of carbon-14, the parent isotope is carbon-14, and the daughter isotope is nitrogen-14. If a radioisotope is part of the composition of an artifact, the age of the artifact can be determined. After the artifact is made, the amount of parent isotope decreases according to the half-life of the isotope. The halflife of a radioisotope is the amount of time needed for half of the atoms of the radioisotope in a sample to decay to the daughter isotope. For example, the half-life of carbon-14 is 5730 years. In other words, after 5730 years, half of the carbon-14 isotopes in a sample would have decayed to nitrogen-14, with half (50%) still remaining as carbon-14. After another 5730 years, or 11,460 years, another half of the remaining carbon-14 atoms would have decayed, leaving one-fourth (25%) of the original amount of carbon-14. The process of radiometric dating (radio—pertaining to radiation and metric—pertaining to measurement) is used by archeologists to determine the age of artifacts. As long as the ratio of parent-to-daughter isotopes can be measured and the half-life of the radioisotope is known, the amount of time that has elapsed since the manufacture of the artifact can be calculated.

Experiment Overview

The age of four “artifacts” will be determined by counting the number of atoms of parent and daughter isotopes (represented by colored chips) in each sample and then estimating how many half-lives have transpired since the artifact was formed. See Table 1 for information regarding the isotopes and half-life.

Materials

Prelab Questions

1. The radioisotope chlorine-36 breaks down by alpha decay. (a) Look up the atomic number of chlorine. How many protons and neutrons are present in chlorine-36? (b) How many protons and neutrons does the resulting daughter isotope contain? (c) Identify the element and the mass number of the daughter isotope.
2. The half-life of radium-226 is 1600 years. How old is an artifact in which 25% (¼) of the original radium-226 is present?

Procedure

1. Complete the graph on the Artifact Ages Worksheet by plotting the percentage of parent isotope remaining for each half-life transpired. The first two data points have been plotted for you. Connect the data points with a smoothly curved line.
2. Obtain a sample of an “artifact” from one of the excavated layers. Each layer is represented by the numbers 1–5. Three artifacts (A, B and C) are available for each layer. Record the label (number and letter) of the chosen artifact in the results table on the Artifact Ages Worksheet.
3. Count the number of parent isotopes (green chips) in the sample and record the amount in the results table.
4. Count the number of daughter isotopes (yellow chips) and record the amount in the results table.
5. Place all the colored chips back in the Petri dish. Return the artifact and obtain another artifact from a different numbered layer.
6. Repeat steps 1–4 three more times to analyze a total of four artifacts. Be sure to place all the colored chips back in each dish before returning the artifact sample.

Student Worksheet PDF

12642_Student1.pdf