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7.2 Half-Lives. By: HULK And Bruce Banner. Radiocarbon Dating. Used to calculate the age of plant and animal remains While organism alive carbon-14 to carbon-12 ratio stays same When organism dies, carbon-14 decays without being replenished

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7.2 Half-Lives

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7 2 half lives

7.2 Half-Lives


And Bruce Banner

radiocarbon dating
Radiocarbon Dating
  • Used to calculate the age of plant and animal remains
  • While organism alive carbon-14 to carbon-12 ratio stays same
  • When organism dies, carbon-14 decays without being replenished
  • Puny scientists like Bruce Banner measure ratio of carbon-14 to carbon-12 to find how old organism is
rate of radioactive decay
Rate of Radioactive Decay
  • Radioactive decay is measured using half-lives
  • Half-life is constant for any radioisotope
  • Half-life is the time it takes for half a sample of a radioisotope to decay
  • Ex: Strontium-90 has a half-life of 29 years
  • If you had 100g of strontium, after 29 years there would be 50g left
decay curve
Decay Curve
  • Graph of decay of radioisotope
  • After each half-life, amount of radioisotope drops by half
decay curve1
Decay Curve
  • For example:
  • Iodine-131 is used for treating thyroid cancer
  • It has a half-life of 8 days
  • Suppose you have 20g of iodine-131
  • You can find out how much will remain after 16 days:
    • 16 days = 2 half-lives
common isotope pairs
Common Isotope Pairs
  • Isotope that decays called parent isotope
  • Stable product(s) of parent isotope's decay called daughter isotope(s)
  • On next slide, some common isotope pairs that can be used for radioisotope dating
potassium 40 clock
Potassium-40 Clock
  • Potassium-40 has a half-life of 1.3 billion years
  • It’s daughter isotope is argon-40
  • When rock is created from lava, the argon-40 is forced out leaving only potassium-40
  • After time the potassium-40 decays and creates argon-40 which gets trapped in the rock
  • When scientists examine the rock they can use the amount of argon-40 to find the age of the rock
  • Since these rocks were probably formed at the beginning of the earth’s life, the age of the rock is close to the age of the earth
potassium 40 clock1
Potassium-40 Clock
  • As the amount of potassium-40 decreases, the amount of argon-40 increases
  • Using the graph and the ratio of potassium-40 to argon-40, scientists can find how old the rock is


  • Which two isotopes get compared in radiocarbon dating?
  • What is used to measure radioactive decay rate?
  • Does a half-life change for a given radioisotope?
  • What is a decay curve?
  • What is the daughter isotope of uranium-235?
  • What happens to the amount of argon-40 as the amount of potassium-40 decreases?
  • Carbon-12 and carbon-14
  • Half-life
  • No
  • A graph of the decay of a radioisotope
  • Lead-207
  • The amount of argon-40 increases