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Radioactive Isotopes and Half Life 1

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What is a Radioactive Isotope? What is Radioactive Decay? What is Half Life? (Take notes as we discuss) 2

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Radioactive elements are unstable. They decay, and change into different elements over time. Not all elements are radioactive. Those that are listed below are the most useful for geologic dating of fossils are: U-238Half-life = 4.5 Billion Years K-40Half-life = 1.25 Billion Years C-14Half-life = 5, 730 Years Radioactive Isotopes 3

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Radioactive Decay and Half Life Here are some facts to remember: 1.The half-life of an element is the time it takes for half of the material you started with to decay. 2. Each element has its own half-life 4

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3. Each element decays into a new element C 14 decays into N 14 while U 238 decays into Pb 206 (lead), etc. 4. The half-life of each element is constant. Its like a clock keeping perfect time. Now lets see how we can use half-life to determine the age of a rock, fossil or other artifact. Radioactive Decay and Half Life 5

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The blue grid below represents a quantity of C 14. Each time you click, one half-life goes by and turns red. C 14 – blue N 14 - red As we begin notice that no time has gone by and that 100% of the material is C 14 Half lives % C 14 %N 14 Ratio of C 14 to N %0%no ratio 6

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The grid below represents a quantity of C 14. Each time you click, one half-life goes by and you see red. C 14 – blue N 14 - red Half lives % C 14 %N 14 Ratio of C 14 to N %0%no ratio 150% 1:1 After 1 half-life (5730 years), 50% of the C 14 has decayed into N 14. The ratio of C 14 to N 14 is 1:1. There are equal amounts of the 2 elements. 7

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The blue grid below represents a quantity of C 14. Each time you click, one half-life goes by and you see red. C 14 – blue N 14 - red Half lives % C 14 %N 14 Ratio of C 14 to N %0%no ratio 150% 1:1 225%75%1:3 Now 2 half-lives have gone by for a total of 11,460 years. Half of the C 14 that was present at the end of half-life #1 has now decayed to N 14. Notice the C:N ratio. It will be useful later. 8

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The blue grid below represents a quantity of C 14. Each time you click, one half-life goes by and you see red. C 14 – blue N 14 - red Half lives % C 14 %N 14 Ratio of C 14 to N %0%no ratio 150% 1:1 225%75%1: %87.5%1:7 After 3 half-lives (17,190 years) only 12.5% of the original C 14 remains. For each half-life period half of the material present decays. And again, notice the ratio, 1:7 9

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What is the half life represented in this graph? 11

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The End We will complete a Half Life laboratory using Twizzlers!!!!! 12

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