Radiometric Dating Activity

Transcription

1 Teacher Notes: Radiometric Dating Activity This hands-on activity is a simulation of some of the radiometric dating techniques used by scientists to determine the age of a mineral or fossil. The activity uses the basic principle of radioactive half-life, and is a good follow-up lesson after the students have learned about half-life properties. See the background information on radioactive half-life and carbon dating for more details on these subjects Objective: Students will use half-life properties of isotopes to determine the age of different "rocks" and "fossils" made out of bags of beads. Through this simulation, they will gain an understanding of how scientists are able to use isotopes such as U-235 and Pb-207 to determine the age of ancient minerals. National Science Education Standards: Grades 5-8: CONTENT STANDARD A (Science as Inquiry): Mathematics is important in all aspects of scientific inquiry. CONTENT STANDARD E (Science and Technology): Science and technology are reciprocal. Science helps drive technology, as it addresses questions that demand more sophisticated instruments and provides principles for better instrumentation and technique. Technology is essential to science, because it provides instruments and techniques that enable observations of objects and phenomena that are otherwise unobservable due to factors such as quantity, distance, location, size, and speed. Technology also provides tools for investigations, inquiry, and analysis. Grades 9-12: Instructions: Mathematics is essential in scientific inquiry. Mathematical tools and models guide and improve the posing of questions, gathering data, constructing explanations and communicating results. Before class begins, prepare five bags filled with about 100 beads each. For each bag, count a specific number of "parent isotope" beads of one color and "daughter isotope" beads of another color. Once you have a set of parent and daughter isotope beads in the bag, fill up the bag with a mixture of all the other colors. Next, label each bag with a number (1-5), put it at a separate station around the room, and make a sign that identifies the parent isotope type and color, daughter isotope type and color, and half-life. For instance, your five bags might be set-up something like:

2 Colors vary in the bags, but are marked on the baggies. The bags should be checked before use. 1. Blue = Parent isotope Uranium 235 (15 beads), Red = Daughter isotope Lead 207 (45 beads), U-235 has a half-life of 704 million years 2. Blue = Parent isotope Uranium 235 (5 beads), Red = Daughter isotope Lead 207 (35 beads), U-235 has a halflife of 704 million years 3. Green = Parent isotope Uranium 238 (30 beads), Orange = Daughter isotope Lead 206 (10 beads), U-238 has a half-life of 4.5 billion years 4. Green = Parent isotope Uranium 238 (25 beads), Orange = Daughter isotope Lead 206 (25 beads), U-238 has a half-life of 4.5 billion years 5. Yellow = Parent isotope Thorium 232 (45 beads), Purple = Daughter isotope Lead 208 (5 beads), Th-232 has a half-life of 14 billion years When class begins, tell the students that in this activity they will use their knowledge of radioactive decay and half-life properties to figure out the age of five different "fossils" at different stations around the room. The bag itself represents the fossil and the beads inside represent some of the millions of atoms that make it up. As scientists, their job is to count the number of parent and daughter isotope atoms in each bag, and from this data to determine how many half-lives the isotope has gone through and therefore the age of the rock. Have the students rotate in groups from station to station until they have figured out the age of all five fossils. Students will calculate the percent of remaining parent isotopes using the following equation. # of parent isotopes / (# of parents isotopes + # of daughter isotopes) this is the initial # of parent isotopes X 100= % This ratio gives you the percentage of parent isotope atoms left after radioactive decay. Instead of using exponents and natural logs, the students can just use a graph of predicted decay rates to determine the number of half-lives the isotope has gone through based on this percentage (see graph). For instance, in fossil one, the students will take 15 divided by 60 and come up with the percentage.25 or 25%. Next, they will look at the graph of decay and see that when 25% of the parent isotope atoms are left, the isotope has gone through two half-lives. In this way, they get practice reading graphs and using them to understand and interpret data. A good idea is to have the graph printed on the worksheet with the data table so that the students can have it right in front of them. Finally, to figure out the age of the fossil, they will take the number of half-lives, two in this case, and multiply it by the length of the half-life (704 million years for fossil one): # of half-lives x length of half-life = age of sample If you are using the five example bags, the correct answers that the students should come up with are: Answer Key 1. 2 half-lives: 1,408,000,000 years old (1.408 billion years) 2. 3 half-lives: 2,112,000,000 years old (2.112 billion years) 3. 1/2 half-life: 2.25 billion years old 4. 1 half-life: 4.5 billion years old 5. 1/5 half-life: 1.4 billion years old

3 Classroom Copy Radiometric Dating Activity Background Information: Determining a fossils age can be done in a couple of ways. The first is relative dating. Scientists use relative dating to determine which fossils are older or younger. To understand how relative dating works imagine that a river has cut down through layers of sedimentary rock to form a canyon. If you look at the canyon walls, you can see the layers of sedimentary rock piled up one on top of another. The layers near the top of the canyon were generally formed most recently and are the youngest rock layers. The lower down the canyon wall you go, the older the rock layers are. Therefore, fossils found in layers near the top of the canyon are younger than fossils found near the bottom of the canyon. This method does not provide an actual age of the fossil and can only be used when rock layers have been preserved in their original sequence. Actual Age or Radioactive Dating: A revolutionary technique called radioactive dating allows scientists to determine the actual age of a fossil. Using the knowledge that not all elements are eternal scientists came up with a method to date fossils and rocks. Radioactive isotopes fire off subatomic particles and energy. The new particle switches from one element to another in the process. For example, Uranium 238 (parent isotope) decays to thorium 234. Each radioactive element decays at a unique rate. The half-life of a radioactive element is the time it takes for half of the atom in a sample to decay. Thus, the rocks fossils are in or the fossils themselves contain radioactive elements allowing scientist to find the absolute or actual age of a fossil or age of the rock layer in which it is found. The graph in Analyzing Data shows how a radioactive isotope breaks down over time. Look at the graph. When 50% of the parent isotope remains the element has gone through one half-life. When 25% of the parent isotope remains the element has gone through two half-lives. A few ways how the fossil record and radiometric dating support the theory of evolution 1) Using the fact that many naturally-occurring elements are radioactive and they break down, or decay, at known predictable rates. 2) Many isotope pairs are useful in dating the Earth such as rubidium/strontium, thorium/lead, potassium/argon, argon/argon, or uranium/lead, all of which have very long half-lives, ranging from 0.7 to 48.6 billion years. 3) Subtle differences in the relative proportions of the two isotopes can give good dates for rocks of any age. 4) Geologists have made many radiometric age determinations, and continue to refine earlier estimates with new data. Dates are often cross-tested using different isotope pairs. 5) Results from different techniques, often measured in rival labs, continually confirm each other. 6) Repeatable results 7) Comparisons of the radiometric dating data to other methods of absolute dating.

4 Procedures 1) Read the information above and answer pre-lab questions. 2) You will use knowledge of radioactive decay and half-life properties to determine the age of five different fossils. The bag itself represents the fossil and the beads inside represent some of the millions of atoms that make it up. As a scientist, it is your job to count the number of parent and daughter isotope atoms in each bag, and from this data to determine how many half-lives the isotope has gone through and thus the age of the rock or fossil. 3) Count the number of parent isotopes and daughter isotopes. Ignore those of various colors they represent other atoms within the sample. 4) Determine the % of the parent isotope remaining. Remember the original is the parent plus the daughter because the parent has decayed into the daughter element. # of parent isotopes / (# of parents isotopes + # of daughter isotopes) this is the initial # of parent isotopes 5) Use the graph to find the number of half-lives the sample has gone through based on the % parent isotope remaining. 6) Use the table to fill in the half-lives and then calculate the age of the fossil by multiplying the number of half-lives by the specific half-life of the isotope listed below. # of half-lives x length of half-life = age of sample Each radioactive isotope has a specific half-life and is listed below 1. Half-life: 704,000,000 million years 2. Half-life: 704,000,000 million years 3. Half-life: 4.5 billion years or 4,500,000, Half-life: 4.5 billion years or 4,500,000, /5 half-life: 17 billion years

5 Data and Analysis for Radioactive Dating Lab Name Period Pre-Lab Questions 1. Describe what relative dating is and how it works. 2. Is a fossil found in a lower layer of sedimentary rock older or younger than a fossil found in an upper layer? Explain. 3. What is half-life? 4. Describe radioactive dating. 5. If 50% of the parent-isotope remains, how many half-lives has the sample gone through? 6. If 25% of the parent-isotope remains, how many half-lives has the sample gone through? Age of fossils determined by radioactive decay "Fossil" Number Number of parent isotope atoms Number of daughter isotope atoms % of daughter isotope remaining Number of halflives Age of "fossil" Use the data and the lab information sheet to answer the follow-up Questions: RSQ the short answers. 1. Rank the fossils from oldest to youngest. 2. Which two were very close in age? 3. In this activity, which "fossil" came from the time just after the formation of the earth? Do you think any real fossils could come from that time? Explain. 4. How does the technology to determine the actual age of a fossil support the theory of evolution? (read) 5. Why do scientists often use more than one radiometric dating pair to determine the age of fossils?

6