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Half-life and Radioactive Dating

Radioactive decay can be used to measure the age of rocks and fossils.

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The Everlasting Cookie

How is it possible to have an everlasting cookie?

Credit: Helena Steffens
License: CC BY-NC 3.0


Half-life is the amount of time it takes a radioactive substance to reach one half of its original mass.  Let’s use a cookie as an example. On the first night of the week, you are hungry and decide to split your 16 g cookie in half and eat one half of it. The next night you aren't as hungry as the first night and decide to split the remaining half of your cookie in half. You continue to split the cookie in half every night. The diagram above shows what the cookie looks like every night before you have your desert. 

Creative Applications

  1. How much will the cookie weigh on the second night after you've eaten dessert? (Hint- you have cut it in half twice)
  2. How much will the cookie weigh on the fourth night after you've eaten dessert?
  3. Predict how long it will take for you to eat the entire cookie. Will it last the week?
  4. How does this relate to how long a radioactive isotope takes to break down?

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Image Attributions

  1. [1]^ Credit: Helena Steffens; License: CC BY-NC 3.0

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