Teacher’s Notes Purpose: The purpose of this activity is to help students discover, through a model, how scientists determine the age of a rock sample by way of radioactive decay. It is important to discuss with the students the use of models in science with its pitfalls and the advantages. This activity also incorporates several reading comprehension strategies throughout the lesson. Science Benchmarks: SC.7.N.3.2 Identify the benefits and limitations of the use of scientific models. SC.8.N.3.1 Select models useful in relating the results of their own investigations. SC.6.N.1.4 Discuss, compare, and negotiate methods used, results obtained, and explanations among groups of students conducting the same investigation. SC.7.N.1.4 Identify test variables (independent variables and outcome variables (dependent variables) in an experiment. SC.7.E.6.3 Identify current methods for measuring the age of Earth and its parts, including the law of superposition and radioactive dating. Science Process Skills: Inferring Identifying variables Organizing and analyzing data Observing/measuring/graphing Interpreting and communicating results Using a model to communicate Reading Standards: The student uses multiple strategies to develop grade appropriate vocabulary. Use new vocabulary that is taught directly. Listen to, read and discuss conceptually challenging words. The student uses a variety of strategies to comprehend grade level text. use background knowledge of subject and related content areas, prereading strategies, graphic representations, and knowledge of text structure to make and confirm complex predictions of content, purpose, and organization of a reading selection; use strategies to repair comprehension of grade-appropriate text when selfmonitoring indicates confusion, including but not limited to rereading, checking context clues, predicting, note-making, summarizing, using graphic and semantic organizers, questioning, and clarifying by checking other sources. Anticipation Guide Directions: Go back through your notes and review what we have done on radioactivity. Then, read each statement below. If you believe that a statement is true, place a check in the Agree column. If you believe the statement is false, place a check in the Disagree column. Be ready to explain your choices. You will repeat the activity after we do the lab. Agree Disagree Agree Radiation is the energy or product given off from an unstable nucleus. A half-life is the time it takes for ½ of an unstable substance to break down. In living matter radiation damages cells that can lead to serious burns and death. The length of an element’s halflife depends on how much of it you have. Gamma rays have a short half-life and can pass easily through paper and the human body. If 80 grams of a substance has a half-life of 8 days, after the first half-life there would be 10 grams remaining. Radioactivity does not transform matter. Disagree Grade 8 Chapter 13, Section 1 Pg 380-386 What is radioactivity? What are the kinds of radioactive decay? 1. 2. a. b. 3. What is an isotope? What are the effects of radiation on matter? 1. living 2. non-living What is a half-life? How do we use radiation? Summary: Name:________________________________ Period: ________ Lab- Candium Detected in Sample from Mars Introduction: Candium, a radioactive substance, has been accidentally brought back to Earth by a recent mission to Mars. It was found in a contaminated sample of rock. NASA believes that the only thing that can be done is send a sample of the substance to the most educated experts on radioactive decay and half lives. (Which is YOU). You have been assigned to the following task. You must study Candium decay and report back to headquarters within one day with a report that details the decay of Candium and the possible effects Candium will have on Earth and its inhabitants. What is the question you are trying to answer? _____________________________________________________________________________ _____________________________________________________________________________ Assignment: Complete the following procedure to discover more about the half-life of Candium. As stated before, this is only for the most educated experts. DO NOT EAT THE CANDIUM (Mm) BEFORE YOUR DATA IS REVIEWED AND CHECKED BY THE LABORATORY ADMINISTRATOR! Remember, only a limited amount of Candium atoms available to the planet. Following is a procedure to conduct the experiment for NASA: Materials: 100 Atoms Candium (Skittles) 2 Cups Graph Paper Foil Procedure: 1. Obtain cup with 100 atoms of Candium. 2. Cover the cup and gently shake for 10 seconds (the pre-determined half-life of Candium is 10 seconds). 3. Gently pour out Candium on to the high-tech receptacle. 4. Count the number of atoms with the “S” decay marker side facing up— and record the data. These atoms have “decayed”, however there is no gamma radiation so your skin will not turn green and you will not become angry. 5. Return only the pieces not showing the “S” marker to the cup. 6. Store the “decayed” atoms in decay receptacle. 7. Gently shake the cup for 10 seconds and repeat steps 3-6. 8. Continue shaking, counting, and storing until all the atoms have decayed. 9. Graph the number of undecayed atoms vs. time on a separate graph paper. 10.Repeat this procedure with 80 atoms. Remember a half-life is similar to a trial run in an experiment. To find the decayed atoms, use the half-life number (trial run). 100 Atom Data: Half Life Initial 1 2 3 4 5 6 7 8 9 10 # Undecayed Atoms Total Time (sec) 80 Atom Data: Half Life Initial 1 2 3 4 5 6 7 8 9 10 # Undecayed Atoms Total Time (sec) 1. Graph the half-life verses the number of undecayed atoms. Use different colors for each trial (100atoms and 80 atoms) Do the following on a separate sheet of paper: 2. At the end of 2 half-lives, what fraction of Candium had not decayed? _____________________________________________________________________________ After 5 half-lives and 9 half-lives, what fraction of Candium had not decayed? Show your work. _____________________________________________________________________________ 3. We used a model of radioactive decay. What did the candy represent? _____________________________________________________________________________ What represented the radioactive decay in the model? _____________________________________________________________________________ What was the limitation of this model? _____________________________________________________________________________ 4.List the variables that affected your results. _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ Communicate Your Results Use the following words to complete the sentences below. Use the completed sentences to write your conclusion. In order for your paragraph to make sense, the sentences must be in correct order. radioactive decay half atoms decayed atoms model cup pieces process repeated simulates 1. The results show that approximately ____________ of the total amount of the element decayed within the first half-life. 2. The experiment began by using 100 candy pieces to represent __________. 3. The purpose of this activity was to use a __________ to demonstrate radioactive decay of certain elements. 4. Scientists can use __________ __________ of elements to determine the age of rocks. 5. All the pieces with the “S” facing up represented __________ __________ and were removed. 6. This __________ continued for 10 half-lives. 7. The candy pieces were placed in a __________ and were shaken for 10 seconds. 8. The experiment was __________ using 80 “atoms”. 9. The __________ were dropped into a tray. 10. The __________ __________ were counted and recorded. 11. This __________ radioactive decay of real atoms. Conclusion answer key The purpose of the activity was to use a model to demonstrate radioactive decay of certain elements. Scientists can use radioactive decay of elements to determine the age of rocks. The experiment began by using 100 candy pieces to represent atoms. The candy pieces were placed in a cup and were dropped into a tray. All the pieces with the “S” facing up represented decayed atoms and were removed. The undecayed atoms were counted and recorded. This process continued for 10 half lives. The experiment was repeated using 80 “atoms”. The results show that approximately half of the total amount of the element decayed within the first half-life. This simulates radioactive decay of real atoms. Dear Scientist, I am a teacher at Sligh Middle School in Tampa, Fl. My students have been working with samples of canedeum and I have been very concerned about the safety of the substance. Please send me a letter that outlines your findings complete with pictures and diagrams. We are most interested in the possible effects that canedeum will have on the Earth’s atmosphere and the living creatures here. Thank you so much for all of your help!
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