Honors Chemistry – Unit 2 Name ____________________________________ Period ______ Date ________________________ Radioactive Dating of Human Bones A Model of Half-Life Background Forensic scientists can use their knowledge of radioactive decay and half-lives to determine the age of many things, including skeletal remains. One method used to determine the age of bones is the Carbon-14 method. Carbon-14 is a radioactive isotope of carbon that is found in all living things. Carbon-14 is absorbed by living things when they are alive and although it decays over time, it is constantly replaced. When an organism dies, the amount of C-14 begins to decay at a fixed rate determined by the half-life of the isotope. The amount of time it takes for half of the C-14 atoms to decay is known as the half-life. Forensic scientists can measure the amount of C-14 remaining in a skeleton to determine how long ago an individual died. For this model, let’s assume that a penny represents another element found in human bones. We will call this fictitious element “boneyum”. If the penny is tails is up, it represents an undecayed atom of a radioactive isotope. If the penny is heads up, it represents a decayed atom (it has become another element). You will start with 100 pennies to represent 100 “boneyum” atoms. Each time you pour out the cup of pennies, one half-life has passed. Purpose Using pennies, you will model the radioactive decay of a fictitious element “boneyum” found in human bones. Prelab Practice How much of a 100.0 g sample of 198Au is left after 8.10 days if it’s half-life is 2.70 days? Materials 100 pennies paper or plastic cup Procedure 1. Working with your lab groups, designate one classmate to be the coordinator, one to be the “counter,” and one to be the recorder. 2. Place 100 pennies into a paper or plastic cup. The pennies will represent “boneyum” nuclei in this experiment. Record the initial number of pennies in the zero half-life row of your data table. 3. Have the coordinator shake the cup of pennies and carefully empty them onto a flat surface. Have the counter separate the pennies that are heads up from those that are tails up. The heads represent “boneyum” nuclei that have decayed and the tails represent unchanged “boneyum” nuclei. This step represents one halflife, which we will say is ten years of time. Have the recorder note the number of “boneyum” nuclei remaining (tails) after one half-life in the data table. 4. Push the decayed “boneyum” nuclei (heads) to the side. Return the unchanged nuclei (tails) to the cup. Repeat step 3 five more times. Data Table Half-Lives 0 (0 years) 1 (10 years) 2 (20 years) 3 (30 years) 4 (40 years) 5 (50 years) 6 (60 years) Number of “boneyum” nuclei remaining GRAPH RUBRIC full page _____/1 title _____/1 x-axis labeled _____/1 y-axis labeled _____/1 axes correct _____/1 points plotted _____/1 curve correct _____/1 ruler/neatness _____/1 ------------------------------------TOTAL _____/8 Results Graph your results from your data table. Label your axis number of remaining “boneyum” nuclei vs half-life. Refer to the graphing guide and rubric in your comp book for proper graphing procedures! Using the graph above and the missing person’s report on the following page to answer the following questions: 1. What is the half-life for “boneyum”? How do you know this? 2. Tests on the skeletal bones show that 10% of the “boneyum” remains. Determine how many years have gone by if 10% of radioactive nuclei remain. Show all work. 3. Knowing that the case took place in 1998, around which year did this person die? 4. What is the name of the probable missing person? Explain how you determined this. 5. Do you think that after more half-lives you would eventually have no “boneyum” nuclei remaining? Missing Person’s Report Data The year was 1998. Some hikers stumbled across a suspicious looking mound of dirt in an area that would be home to a future high school in Las Flores, California. Crime scene investigators were called to evaluate the scene. After determining that this might be a grave, the investigators ask for help from some forensic anthropologists. Working together, the site is carefully excavated. The skeletal remains of a human are found in the grave. The forensic anthropologists determine that the remains belong to a female, about age 16. She had been shot at close range through the back of the head. X-rays are taken. Police records turn up missing person reports of ten girls this age from all around the country. Anthropologists decide to narrow down the range of possibilities by determining how long the young girl has been dead. To do this they must do radioactive dating of the fictitious element “boneyum”.
