1. Radiometric Dating Continuation of Journal Entry #5

2. Radiometric Dating A technique used to date materials such as rocks based upon a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products using known rates of decay. Some elements have isotopes that predictably decay over time to more stable forms

3. Types of radioactive decay Alpha decay – Isotope emits 2 n 0 and 2 p + Beta decay – Isotope emits 1 e - Neutron decay – Isotope emits 1 n 0

4. Common radioactive isotopes and their products C-14  N-14 (beta decay) K-40  Ar-40 (beta decay) U-235  Pb-207 (all three forms of decay)

5. Radioactive Half-life The rate at which an isotope decays can be precisely measured in the lab and have been found to occur at a constant, predicable rate. An isotopes half-life is the amount of time necessary for ½ of the original amount of the radioactive isotope to decay to the more stable form.

6. Commonly used half-lives C-14 = 5,730 years U-235 = 704 million years K-40 = 1.25 billion years

7. Question: If I found a bone with 12.5% of the expected amount of C-14, approximately how old is the bone?

8. Answer: 5,730 years x 3 half-lives = 17,190 yrs

9. U-235 decay The geologic processes that form uranium are very different than those that form lead. Therefore, any lead found in a rock containing uranium must have been formed by decay. A measurement of the ratio of the two will reveal how many half-lives have passed – and thus will determine the age of the fossil.

10. Say hello to your lab partner!

11. Analysis Question If I found a fossil sample with 12 m&m’s remaining from the original sample, how old would it be if m&mdium has a half-life of 4.7 million years?

12. #2 – Explain how an evolutionary biologist would use radiometric dating, relative dating, and fossils to determine something about how species have changed over time. #3 – Personal thoughts and reflections