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Science 10: 7.2 Half-Life

Text: Chapter 7, pages 302-311

Part A: Geologic Dating Overview

-Examine the diagrams below. What are some ways you know to discover how old each depicted organism is?

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Part B: Radioactive Decay

-Remember, radioactive isotopes decay into a stable atom over time. This decay rate is not affected by environmental factors and is always consistent.

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Gorgosaurus ~75 million years old.Methuselah: World’s Oldest Tree ~4780 years old.

-For example, all animals obtain equal proportions of carbon-14 and carbon-12 by eating plants.

-However, when the organism dies carbon-14 radioactively decays into nitrogen-14 at a fixed rate without being replaced (no more plants are ingested).

-This means that the ratio of carbon-14 to carbon-12 changes. By measuring this ratio, we can determine the age of the organic remains.

-Here’s how it works:

a) Radioactive isotope decay is measured as a half-life.

-One half-life is equal to the amount of time required for half of the parent isotope present in a sample to decay into the daughter isotope. This length of time varies depending on the parent isotope being analyzed (see Table 7.6)

b) By multiplying the half-life time for the parent isotope by the number of half-lives elapsed, you can determine how old a sample is.

-For example, an archaeologist finds a Woolly Rhinoceros and through testing discovers that of the carbon present in the skeleton, 25% is carbon-14.

-How many half-lives have elapsed since the Rhino died?

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-What is the time required for one half-life to elapse for carbon-14?

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-How long ago did the Rhino die?

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-Let’s look at the idea of half-lives in a slightly different way. If a Neanderthal skeleton starts with 1000 grams of carbon-14, how much would remain 17,190 years after the organism died?

a) How many half-lives have elapsed?

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b) What percentage of the parent isotope remains?

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c) Initial Grams of Parent Isotope x Percentage of Parent Isotope Remaining = Grams of Parent Isotope Remaining

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d) How many grams of Nitrogen-14 have been produced over 17,190 years of Carbon-14’s radioactive decay?

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-Carbon-14 is very useful for dating the remains of living things, but is limited to materials under 50,000 years old. Why is this?

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-For materials that are not organic or are older than 50,000 years old we have to use other types of radioactive isotope dating.

*Complete Practice Problems on page 306

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Part C: Radioactive Decay of Potassium-40

-The Potassium-40 Clock is the use of this radioactive isotope’s half-life to determine the age of very old rocks.

a) Using the Potassium-40 decay curve, determine the age of a rock with 25% Potassium-40 and 75% Argon-40.

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b) What is ratio of argon-40 to potassium-40 remains 3.9 billion years after the rock has formed?

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*Complete Check Your Understanding Questions 6, 7, 9-12 on page 311.

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