7.2 Half-Lives

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# 7.2 Half-Lives - PowerPoint PPT Presentation

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

By: HULK

And Bruce Banner

• 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
• 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
• Graph of decay of radioisotope
• After each half-life, amount of radioisotope drops by half
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
• 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 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 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
Video

Quiz
• 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?