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Radioisotopes, and their use in “dating” rocks

Radioisotopes, and their use in “dating” rocks. Radioactive Decay. Certain isotopes of some elements are not stable. They naturally change (decay) over time into other elements. For example: uranium-238 (U-238) decays into lead-206 (Pb-206)

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Radioisotopes, and their use in “dating” rocks

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  1. Radioisotopes, and their use in “dating” rocks

  2. Radioactive Decay Certain isotopes of some elements are not stable. They naturally change (decay) over time into other elements. For example: uranium-238 (U-238) decays into lead-206 (Pb-206) potassium-40 (K-40) decays into argon-40 (Ar-40) carbon-14 (C-14) decays into nitrogen-14 (N-14)

  3. Example – decay of uranium-235 into lead-207 Half-life = 700 million years (one half-life later) 50% remaining (two half-lives later) 25% remaining

  4. As the parent isotope decreases, the daughter isotope increases Daughter isotope percentage Parent isotope percentage

  5. Many steps in the decay of Uranium-238 into lead-206

  6. Many steps in the decay of Uranium-238 into lead-206 (another way of looking at it)

  7. Many steps in the decay of Uranium-238 into lead-206 BUT – the first one determines the half-life (in this case) Symbol Element Radiation Half-Life Decay Product U-238 Uranium-238 alpha 4.46 billion years Th-234 Th-234 Thorium-234 beta 24.1 days Pa-234 Pa-234 Protactinium-234 beta 1.17 minutes U-234 U-234 Uranium-234 alpha 247,000 years Th-230 Th-230 Thorium-230 alpha 80,000 years Ra-226 Ra-226 Radium-226 alpha 1,602 years Rn-222 Rn-222 Radon-222 alpha 3.82 days Po-218 Po-218 Polonium-218 alpha 3.05 minutes Pb-214 Pb-214 Lead-214 beta 27 minutes Bi-214 Bi-214 Bismuth-214 beta 19.7 minutes Po-214 Po-214 Polonium-214 alpha 1 microsecond Pb-210 Pb-210 Lead-210 beta 22.3 years Bi-210 Bi-210 Bismuth-210 beta 5.01 days Po-210 Po-210 Polonium-210 alpha 138.4 days Pb-206 Pb-206 Lead-206 none stable (none)

  8. SO: U-238 has a half-life of 704 million years. Say a sample of a rock contains 800 atoms of the “parent” isotope (U-238), and only 200 atoms of the “daughter” isotope (Pb-206). How old is the rock?

  9. 800 atoms of the parent isotope, 200 atoms of the “daughter” isotope This means that 80% of the parent isotope remains, so about 0.4 half-lives have passed since the rock was formed (see the red line). 0.4 x 704 million = 281 million years.

  10. Say a sample of a rock contains 250 atoms of the “parent” isotope (U-238), and 750 atoms of the “daughter” isotope (Pb-206). How old is the rock? (U-238 half-life = 704 million years)

  11. This means that 25% of the parent isotope remains.so 2 half-lives have passed since the rock was formed. 2 x 704 million = 1408 million years (1.4 billion yrs)

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