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  • 1. Radiocarbon Dating • Used to calculate the age of plant and animal remains • While organism alive carbon-14 to carbon- 12 ratio stays same • When organism dies, carbon-14 decays without being replenished • Puny scientists like Bruce Banner measure ratio of carbon-14 to carbon-12 to find how old organism is
  • 2. Rate of Radioactive Decay • Radioactive decay is measured using half-lives • Half-life is constant for any radioisotope • Half-life is the time it takes for half a sample of a radioisotope to decay • Ex: Strontium-90 has a half-life of 29 years • If you had 100g of strontium, after 29 years there would be 50g left
  • 3. Decay Curve • Graph of decay of radioisotope • After each half-life, amount of radioisotope drops by half
  • 4. Decay Curve
  • 5. Common Isotope Pairs • Isotope that decays called parent isotope • Stable product(s) of parent isotope's decay called daughter isotope(s) • On next slide, some common isotope pairs that can be used for radioisotope dating
  • 6. Common Isotope Pairs Isotope Half-Life of Parent (y) Effective Dating Range (y)Parent Daughter Carbon-14 Nitrogen-14 5730 Up to 50 000 Uranium-235 Lead-207 710 million > 10 million Potassium-40 Argon-40 1.3 billion 10 000 to 3 billion Uranium-238 Lead-206 4.5 billion > 10 million Thorium-235 Lead-208 14 billion > 10 million Rubidium-87 Strontium-87 47 billion > 10 million
  • 7. Potassium-40 Clock • Potassium-40 has a half-life of 1.3 billion years • It’s daughter isotope is argon-40 • When rock is created from lava, the argon-40 is forced out leaving only potassium-40 • After time the potassium-40 decays and creates argon-40 which gets trapped in the rock • When scientists examine the rock they can use the amount of argon-40 to find the age of the rock • Since these rocks were probably formed at the beginning of the earth’s life, the age of the rock is close to the age of the earth
  • 8. Potassium-40 Clock • As the amount of potassium-40 decreases, the amount of argon-40 increases • Using the graph and the ratio of potassium-40 to argon-40, scientists can find how old the rock is
  • 9. Video http://www.youtube.com/watch?v=KWAsz59F8gA
  • 10. Quiz 1. Which two isotopes get compared in radiocarbon dating? 2. What is used to measure radioactive decay rate? 3. Does a half-life change for a given radioisotope? 4. What is a decay curve? 5. What is the daughter isotope of uranium-235? 6. What happens to the amount of argon-40 as the amount of potassium-40 decreases?
  • 11. Answers 1. Carbon-12 and carbon-14 2. Half-life 3. No 4. A graph of the decay of a radioisotope 5. Lead-207 6. The amount of argon-40 increases