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7.2 Half-Lives

7.2 Half-Lives. By: HULK And Bruce Banner. 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

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7.2 Half-Lives

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  1. 7.2 Half-Lives By: HULK And Bruce Banner

  2. 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

  3. 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

  4. Decay Curve • Graph of decay of radioisotope • After each half-life, amount of radioisotope drops by half

  5. Decay Curve • For example: • Iodine-131 is used for treating thyroid cancer • It has a half-life of 8 days • Suppose you have 20g of iodine-131 • You can find out how much will remain after 16 days: • 16 days = 2 half-lives

  6. 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

  7. Common Isotope Pairs

  8. 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

  9. 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

  10. Video http://www.youtube.com/watch?v=KWAsz59F8gA

  11. Quiz • Which two isotopes get compared in radiocarbon dating? • What is used to measure radioactive decay rate? • Does a half-life change for a given radioisotope? • What is a decay curve? • What is the daughter isotope of uranium-235? • What happens to the amount of argon-40 as the amount of potassium-40 decreases?

  12. Answers • Carbon-12 and carbon-14 • Half-life • No • A graph of the decay of a radioisotope • Lead-207 • The amount of argon-40 increases

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