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Half-Life Pg. 870-874

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Half-Life Half-life = the time needed for half of the radioisotope sample to decay into products Half-live can have varying lengths May be a fraction of a second Can last billions of years Examples: Carbon-14 = 5730 years Potassium-40 = 1.25x10 9 years Radon-22 = 3.8 days Thorium-230 = 75,400 years Thorium-234 = 24.1 days

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Calculating Half lives There are 2 ways to solve these problems: Continually divide by 2 Use the equation below N = N o (1/2) n

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Calculating Half-Life Nitrogen-13 emits beta radiation and decays to carbon- 13 with a half-life (t 1/2 ) of 10 min. Assume a starting mass of 2.00g of nitrogen-13. How long is three half-lives? How many grams of the isotope will be present at the end of three half-lives?

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Calculating with Half lives If the passing of 5 half lives leaves 25.0mg of strontium-90 sample, how much was present in the beginning?

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Examples of Uses of Half-Life 1.Can be used to determine the age of an artifact (carbon-14 dating) 2.Artificial isotopes tend to have short have lives - Used in medicine - Dont pose long-term hazards to patients asymmetric scan indicates disease normal

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Radiocarbon dating AKA: Carbon-14 dating Carbon-14 is produced in the atmosphere when high-energy neutrons from space (cosmic rays) collide with nitrogen-14 Carbon-14 decays by - particle production

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Carbon Dating Carbon-14 is continuously produced in the atmosphere and it continuously decomposes by β- decay The two opposite processes have come into balance, so the amount of carbon-14 present in the atmosphere is approximately constant.

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Carbon Dating Carbon-14 dating can be used to date wood and cloth artifacts A living plant consumes carbon dioxide in the photosynthesis process Some carbon dioxide contains the C-14 isotope When the plant uses carbon dioxide with C-14, it is incorporated into the molecules in the plant

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Carbon Dating While it is alive, the C- 14 in the plant molecules is equal to that in the atmosphere. When the tree is cut, there is no more incoming source of C- 14 to balance the C-14 that is decaying. The C-14 content decreases.

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Carbon Dating Using the known half-life of 5730 years for C-14, archaeologists compare the amount of C-14 in an artifact to the amount currently in trees and figure out the age. A wooden bowl with half as much C-14 as a current living tree would have been through one half life and would therefore be 5730 years old. Carbon dating made by Thomas Athol Rafter

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Half life Activity You must get the data today Class data will be posted on Moodle tonight Lab due Monday

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