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Radiometric Dating

Radiometric Dating. Continuation of Journal Entry #5. 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.

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Radiometric Dating

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  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 n0 and 2 p+ • Beta decay – Isotope emits 1 e- • Neutron decay – Isotope emits 1 n0

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

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