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

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. 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) Many steps in the decay of Uranium-238 into lead-206 BUT – the first one determines the half-life (in this case)

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)