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ABSOLUT DATING Explanations collected from three online presentations.

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1 ABSOLUT DATING Explanations collected from three online presentations

2 For example: – If a rock has a parent/daughter ratio of 1:3, the remaining parent proportion is 25% – 25% = 2 half lives Determining Age –If half life is 57 milliion years then the rock is 57 million years x 2 = 114 million years old

3 Half-Life Element Uranium-238 4.5 x 10 years 9 9 Carbon-14 5730 years Bismuth-210 5.0 days Polonium-214 1.6 x 10 sec - 4

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9 Radioactive Half-Life The time it takes for one- half of a radioactive sample to decay Look at factors of 2 One half-life (1/2) Two half-lives (1/4) Three half-lives (1/8) Look at factors of 2 One half-life (1/2) Two half-lives (1/4) Three half-lives (1/8) For Example: A material has decreased by ¼ of its original amount it has gone through two half-lives

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11 N-14

12 77

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15 C-14

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17 86

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19 CO 2 14

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21 Carbon-14 is a radioactive isotope that is naturally incorporated from carbon dioxide into living organisms, the amount remains relatively constant during the life of the organism When the living organisms dies the carbon 14 is no longer being replaced in the organism and will start to decay. The amount of loss from the that compared to living organisms can be used to determine when the organism died. Carbon-14 is a radioactive isotope that is naturally incorporated from carbon dioxide into living organisms, the amount remains relatively constant during the life of the organism When the living organisms dies the carbon 14 is no longer being replaced in the organism and will start to decay. The amount of loss from the that compared to living organisms can be used to determine when the organism died.

22 22,920 years ago

23 17,190 years ago

24 11,460 years ago

25 5730 years ago

26 Present

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28 Calculate Age Problem: The carbon-14 radioactivity in the bones of a body was measured to be 1/8 of that compared to a living person How long ago did the person live? Problem: The carbon-14 radioactivity in the bones of a body was measured to be 1/8 of that compared to a living person How long ago did the person live?

29 Calculate Age Calculation of Age: The carbon-14 has decreased by 1/8 which is three half lives (1/2 times 1/2 times 1/2 = 1/8) Carbon-14 half life = 5730 years 3 times 5730 = 17,190 years Calculation of Age: The carbon-14 has decreased by 1/8 which is three half lives (1/2 times 1/2 times 1/2 = 1/8) Carbon-14 half life = 5730 years 3 times 5730 = 17,190 years

30 Present

31 One Half-Life 5730 years ago One Half-Life 5730 years ago

32 Two Half-Lives 11,460 years ago Two Half-Lives 11,460 years ago

33 Three Half-Lives 17,190 years ago Three Half-Lives 17,190 years ago

34 Radioactive Decay Radio Isotope is an isotope that undergoes radioactive decay. It naturally breaks down into a different element called the decay product. Half –life: the time it takes for ½ of the original amount of atoms to decay to the decay product. – Note: Each element decays to a different decay product

35 Each radioactive isotope has a specific decay product and rate of decay (half-life). See page one of the reference table

36 The half-life of a radioactive nuclide (atom) is the amount of time it takes for half of that nuclide to decay into the decay product. The half-life of Carbon-14 is 5730 years After 5730 years, ½ the mass of an original sample of Carbon-14 remains unchanged. After another 5730 years, ¼ (half of the half) of an original sample of Carbon-14 remains unchanged. The half-life of a radioactive nuclide cannot be changed. Half- Life

37 Determining how much of a radioactive isotopes remains unchanged after a period of time. Determine how many half-lives have gone by (Time/half-life) Halve the mass of the starting material for each half-life period that goes by. – How much of a 20.g sample of 14 C remains unchanged after 17,100 years? – The half-life period is 5,700 yrs. So 17,100 years is 3 half-lives (17,100 / 5,700). Half the mass three times. 5,700 yrs 20 g10 g 5 g 2.5 g Each arrow represents one half-life

38 Absolute Age The ratio between the radioactive element and the decay product is the decay-product ratio. Using the decay product ratio, a scientist can determine the products absolute age by calculating the number of half lives that have past.

39 Regents Question #23 August 2008 1 (the whole) 1/21/4 Each arrow represents one half-life With each half-life ½ of the previous amount decays, so that after two half-lives ¼ of the original amount remains

40 Regents January 2010

41 Selecting the best Radioactive Element The scientists must choose the best element to use for dating: – Carbon-14 is common in living organisms but has a short half live and is not useful for samples older than 50,000 years. Few atoms will be left after 10 half lives – U-238 has a half life of 4.5 billion years. Useful for very old samples. But samples too young may not have enough Lead-206 to measure


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