Half Lives for Radioactive Elements Radioactive Parent Stable Daughter Half life Potassium 40 Argon 40 Rubidium 87 Strontium 87 Thorium 232 Uranium 235 Uranium 238 Carbon 14 1. Half life a) How can we use radioactive decay to figure out the age of a sample ? b) If you have 50% of the daughter isotope and 50% of the parent isotope what does this tell you about the number of half lives that have passed? 2. Clair Patterson a) What question was Clair Patterson trying to solve? What was his answer? b) What did Patterson find out about environmental hazards while searching for this answer? 3. Radiometric Dating Percentages Fill out the following table in percentages: Number of half-lives % Parent Material Lead 208 Lead 207 Lead 206 Nitrogen 14 % Daughter Material 0 100 0 1 1.25 billion years 48.8 billion years 14 billion years 704 million years 4.47 billion years 5730 years 2 3 4 5 6 7 4. Daughter/Parent Plot the ratio versus half life after completing this table: # Half lives Parent 0 1 2 3 4 5 6 7 8 9 10 Atoms 1000 Daughter Atoms 0 01 2 Pnow/Porig as fraction 1 Plot Pnow/Porig ratio And Dnow/Porig versus half life for the Parent and Daughter atoms. Label both axes with titles and numbers. Dnow/Porig as fraction 0 3 4 5 6 7 8 9 5. Radiometric Dating 40. How old is a rock if the mineral biotite from the rock has 100,000 K atoms and 40 Ar atoms? 100,000 40. 40 What if it had instead 125 K atoms and 875* Ar atoms? 10 6. Isotopic analysis a) Isotopic analysis of a granitic intrusion indicates that 25% of the original 40K is present. What is the absolute age of the granitic intrusion (in years)? b) Analysis of a nearby basaltic intrusion indicates that 50% of the original amount of 40K is present. What is the absolute age of the basaltic intrusion (in years)? 7. Radiocarbon dating A bone was recently recovered from a site in eastern Africa. Radiocarbon dating suggests that the bone contains 12.5% of the original amount of C. How old is the bone? (Half life of 4C - 5730 years) 8. Radiocarbon dating Would it be possible to use 4C age dating to estimate the age of a rock containing carbon from the time of the dinosaurs? Explain.