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Announcements. These stone tools from the archaeological site in Cactus Hill, Virginia, are at least 15,000 years old. Scientists estimated the age of the site based on rates of nuclear decay. Half-life. How do nuclear decay rates differ from chemical reaction rates?.

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Announcements

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  1. Announcements

  2. These stone tools from the archaeological site in Cactus Hill, Virginia, are at least 15,000 years old. Scientists estimated the age of the site based on rates of nuclear decay.

  3. Half-life How do nuclear decay rates differ from chemical reaction rates? Unlike chemical reaction rates, which vary with the conditions of a reaction, nuclear decay rates are constant.

  4. Half-life A half-life is the time required for one half of a sample of a radioisotope to decay. • After one half-life, half of the atoms in a radioactive sample have decayed, while the other half remains unchanged. • After two half-lives, half of the remaining radioisotope decays. • After three half-lives, the remaining fraction is one eighth.

  5. Half-life The half-life for the beta decay of iodine-131 is 8.07 days.

  6. Half-life The half-life for the beta decay of iodine-131 is 8.07 days.

  7. Half-life The half-life for the beta decay of iodine-131 is 8.07 days.

  8. Half-life The half-life for the beta decay of iodine-131 is 8.07 days.

  9. Half-life Every radioisotope decays at a specific rate. Half-lives can vary from fractions of a second to billions of years.

  10. Half-life Iridium-182 undergoes beta decay to form osmium-182. The half-life of iridium-182 is 15 minutes. After 45 minutes, how much iridium-182 will remain of an original 1-gram sample? • Calculate how many half-lives will elapse during the total time of decay.

  11. Half-life • After three half-lives, the amount of iridium-182 has been reduced by half three times. • After 45 minutes, 0.125 gram of iridium-182 remains. • 0.875 gram of the sample has decayed into osmium-182.

  12. Radioactive Dating How do scientists determine the age of an object that contains carbon-14? In radiocarbon dating, the age of an object is determined by comparing the object’s carbon-14 levels with carbon-14 levels in the atmosphere.

  13. Radioactive Dating The artifacts from Cactus Hill were dated by measuring levels of carbon-14, which has a half-life of 5730 years. Carbon-14 is formed in the upper atmosphere when neutrons produced by cosmic rays collide with nitrogen-14 atoms. Carbon-14 undergoes beta decay to form nitrogen-14.

  14. Radioactive Dating Plants absorbing carbon dioxide during photosynthesis maintain the same ratio of carbon-14 to carbon-12 as in the atmosphere. • Animals have the same ratio of carbon isotopes as the plants they eat. • When a plant or animal dies, it can no longer absorb carbon. • After death, the organism’s carbon-14 levels decrease as the radioactive carbon decays.

  15. Radioactive Dating If the ratio of carbon-14 to carbon-12 in a fossil is half the atmospheric ratio, the organism lived about 5730 years ago. Because atmospheric carbon-14 levels can change over time, the calculated age of the fossil is not totally accurate. To get a more accurate radiocarbon date, scientists compare the carbon-14 levels in a sample to carbon-14 levels in objects of known age.

  16. Radioactive Dating Radiocarbon dating can be used to date any carbon-containing object less than 50,000 years old. Objects older than 50,000 years contain too little carbon-14 to be measurable, so scientists measure the amounts of radioisotopes with longer half-lives than carbon-14.

  17. Radioactive Dating Radiocarbon dating helps archaeologists learn more about ancient civilizations. This Egyptian mummy case, containing the remains of a cat, is 1900 years old.

  18. Assessment Questions • Cesium-137 has a half-life of 30 years. You find a sample with 3 g of cesium-137. How much cesium-137 existed in the sample 90 years ago? • 9 g • 27 g • 24 g • 18 g

  19. Assessment Questions • Cesium-137 has a half-life of 30 years. You find a sample with 3 g of cesium-137. How much cesium-137 existed in the sample 90 years ago? • 9 g • 27 g • 24 g • 18 gANS: C

  20. Assessment Questions • What factors influence nuclear decay rates? • pressure • temperature • concentration • number of neutrons in nucleus

  21. Assessment Questions • What factors influence nuclear decay rates? • pressure • temperature • concentration • number of neutrons in nucleusANS: D

  22. Assessment Questions • What radioisotope is most commonly used to determine the age of archaeological artifacts made of wood? • lithium-7 • carbon-14 • potassium-40 • uranium-235

  23. Assessment Questions • What radioisotope is most commonly used to determine the age of archaeological artifacts made of wood? • lithium-7 • carbon-14 • potassium-40 • uranium-235ANS: B

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