Radioactive Dating

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Radioactive Dating

• Calculating the age of a sample based
• on the remaining amount of a radioactive isotope
  the sample contains.

By calculating the amount of uranium-238 and
thorium-232, this meteorite was dated to be 14.5
billion years old, give or take 2 billion.
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How is Relative Dating Different?

• Date fossils relative to their position in rock
• No info on age in years

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Half-life

• The amount of time it takes for half of
  radioactive atoms in a sample to decay.
• Different elements have different half lives and
  are used to date samples of various time periods

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Radiocarbon Dating

• Used to date fossils and artifacts such as the
  Ice Man
• found to be 5500 years old

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Radiocarbon dating

• Method developed by William Libby at U. of
  Chicago
• Won Nobel Prize in 1960
• Also worked on Manhattan Project in 1940s
• Peaceful application of radioactive elements!

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How does it work?

• Carbon-14 is taken in by living things
• After organism dies, C-14 decays to form N-14
  (escapes into air)
• C-12 is common carbon isotope found in all living
  things and does NOT decay because it is stable!

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How does it work? continued

• Half life of C-14 is 5,730 years.
• Ratio of C-14 to C-12 in living things is
• 1 to 1 trillion.
• Materials are tested to see ratio of
• C-14 to C-12 and date can be determined

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Radiocarbon Dating
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Sample problems

• What is the half life of a substance if 1/64th of
  the original amount remains after 1,500 years?

• An isotope of cesium (cesium-137) has a half
  -life of 30 years.  If 1.0 mg of cesium-137
  disintegrates over a period of 90 years, how many
  mg of cesium-137 would remain?