Name: Period: Half-life and Radiometric Dating

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Melinda Wade
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1 Name: Period: Half-life and Radiometric Dating Purpose: The purpose of this lab is to understand half-life and how it is used in determining the age of materials. Students will also understand the limitations or challenges of radiometric dating. Background: Radioactivity was discovered in This discovery lead to further investigations which, in turn, lead to the discovery of several atoms that have unstable radioactive isotopes that, over time, will spontaneously decay into stable isotopes and give off radiation. The new stable isotope is referred to as the daughter product. The rate at which the nuclei of these unstable isotopes decay is constant. While decay is constant, the actual amount that decays is not. Half-life describes the interval of time during which half of the unstable isotope will decay. After one half-life, half of the original unstable atoms will have decayed to daughter product. After a second half-life, half of the unstable atoms left over after the first half-life will have decayed, leaving one quarter of the original unstable atoms still in their unstable state. Each radioactive isotope has a different half-life. If we know the half-life of an isotope and if we have the ability to measure the ratio between parent product and daughter product, then we should be able to figure out how long the isotope has been present in a material. This should give us a very good idea of how old that material is. This process is known as radiometric dating. Radiometric dating is only possible if we can make accurate measurements of the parent isotope and the daughter product. Very small amounts of either can make measurement difficult. Vocabulary Atoms- the smallest particle of an element that has all the element s chemical properties Consists of protons and neutrons in a nucleus surrounded by electrons. Daughter product the resulting atom after undergoing radioactive decay. This may be a final stable atom or may be an intermediate, unstable atom that will, in turn, have its own daughter product. Half-life the interval of time during which half of the unstable atoms undergo radioactive decay. Isotopes atoms that have the same number of protons but different numbers of neutrons. One element can have several isotopes, and some isotopes are more stable than others. Nucleus - center of an atoms that contains protons and neutrons. Parent atom the original unstable isotope that undergoes radioactive decay. Radioactivity process of a nucleus emitting energetic particles. Radiometric dating using information such as the half-life of an unstable isotope and the parent to daughter product ratio found in an object to determine the age of the object. Materials: 1 shoe box with lid 100 pennies 100 paperclips 100 wood cubes Procedure: 1. Place all of the pennies in the box with heads up. The box represents a rock with a given isotope. The pennies are the radioactive isotope. Heads up represents the parent isotope. Tails up represents the daughter product. At time 0 all of the isotopes are in their unstable form. 2. Cover the box and shake by turning over twice. 3. Remove all atoms that have decayed into the daughter product. In this case, all pennies that are tails up. 4. Record the number of pennies that remain (still heads up) in the Results section. 5. Replace the removed pennies with paperclips to represent the stable daughter products. 6. Repeat steps 2-5 until most or all of the isotopes have decayed to daughter product.

2 7. Using the table from the results section make a graph of the decay rate for your penny. Time here will be represented by the number of trials and should be on the x-axis. The number of parent atoms should be on the y-axis. 8. Now combine the results of all the groups into a new chart in the Results section and graph the decay rate in a new graph using the total information. 9. Using the class data, determine the half-life of the isotope. Look at the graph and determine at which time interval half of the original parent atoms had decayed. To double check your results, determine at which time interval ¾ of the parent atoms had decayed. This should measure two half-lives and therefore, should be twice as much time from time = and the first half-life. 10. Repeat this experiment with a new isotope. Take the 100 wood cubes that have two sides marked and put them in the box. Blank sides up will represent the original radioactive isotope, while marked (black) sides up will represent daughter product. Replace the decayed blocks with paperclips. Determine the half-life of the new isotope. Observations/ Results Group Data From Penny Experiment Time Interval Number of Unstable parent Atoms Remaining Time Interval Class Data of Unstable Parent Atoms Remaining (Pennies) Group 1 Group 2 Group 3 Group 4 Group 5 Group 6 Group 7 Group 8 Total

3 Number of parent Atoms Remaining Over Time in Group Number of Parent Atoms Remaining Over Time in Class 1. One half of atoms of the isotope have decayed after shakes of the box 2. Three quarters of the atoms of the isotope have decayed after shakes of the box. 3. Seven eighths of the atoms of the isotope have decayed after shakes of the box 4. The half-life of the isotope is shakes of the box

4 Group Data From Wood Cube Half-life Experiment Time Interval Number of Unstable parent Atoms Remaining Time = 7 Time = 8 Time Interval Class Data of Unstable Parent Atoms Remaining (wood cubes) Group 1 Group 2 Group 3 Group 4 Group 5 Group 6 Group 7 Group 8 Total Time = 7 Time = 8

5 Number of parent Atoms Remaining Over Time in Group Number of Parent Atoms Remaining Over Time in Class 5. One half of atoms of the isotope have decayed after shakes of the box 6. Three quarters of the atoms of the isotope have decayed after shakes of the box. 7. Seven eighths of the atoms of the isotope have decayed after shakes of the box 8. The half-life of the isotope is shakes of the box

6 Discussion: 1. If one shake of the box is equal to 1,000 years, then what is the half-life of the penny isotope? What is the half-life of the wood cube isotope? Show any math work. Which of the two isotopes has a longer half-life? 2. Define, in your own words, the concept of half-life. 3. Are the half-lives determined from the class data the same as the half-lives determined from the group data where there were less parent atoms to begin with? 4. If the half-lives are different when you compare the different data sets, which do you think is more reliable data and why? 5. Mystery Isotope X has a half-live of 100 years. If you start with 500 grams of Mystery Isotope X, how much will be left after 600 years? Show your math work. 6. Look at the graphs that you made for the class data of the pennies isotope. What would happen to the graph line after 9 or 10 shakes of the boxes? 7. If you wanted to date an object that you hypothesized was pretty old, would you feel more confident using an isotope like the penny isotope or like the wood cube isotope? Why? 8. Carbon 14 has a half-life of 5730 years. Carbon is found in all living, or organic, matter but only a very small fraction of all carbon is carbon 14. Think about how long ago dinosaurs lived (over 65 million years ago). Would carbon 14 be useful in dating dinosaur fossils? Why? 9. Uranium 238 will radioactively decay to lead 206 with a half-life of 4.5 billion years. This make it an excellent isotope for determining the ages of very old rocks that contain uranium such as zircon crystals that form in volcanic explosion. However, uranium-lead dating is practically useless for rocks that have formed in the past 100,000 years. Why do you think that might be?

Page 17a. Objective: We will identify different types of radioactive decay. Warm-up:

Page 17a. Objective: We will identify different types of radioactive decay. Warm-up: Page 17a Objective: We will identify different types of radioactive decay. Warm-up: What are the three subatomic particles? Where is each particle located in the atom? What is an isotope? Page 17a (again)