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3-1 Half-Life Decay

Simulating Half-Life Decay with Pennies


In order to better understand how the idea of half-life is used in dating rocks, you can experiment with a simulated radioactive substance that we will call “pennyonium.” A sample of pennyonium consists of 100 pennies. Atoms of pennyonium decay to lead by emitting high energy particles. In this activity you will need to work as a member of a 2- or 3-person team.


100 pennies
1 small box with a lid

  1. Create a chart and graph for collecting data as shown below.
Number of Years Since Sample Formed Atoms of Pennyonium Remaining
0 100


  1. Put all the pennies into the container. Imagine that the pennies represent atoms of pennyonium, which has a half-life of 1,000 years.

  2. Put on the lid, shake the container, and drop the coins onto a table. Remove the coins that are tails up. These coins represent the atoms that have decayed to lead during the first 1,000 years.

  3. Count the number of heads-up coins remaining. Enter this number in the data table to show how many atoms of pennyonium remain.

  4. Repeat steps 2 and 3 until there is just one coin or none remaining. Each shaking represents another 1,000 years.

  5. Graph your results. On the vertical axis plot the number of coins remaining after each period of 1,000 years. On the horizontal axis plot the number of years that took place in your experiment.
Questions For Thought
  1. Compare your graph with graphs of other teams. Try to explain similarities and differences that you notice.

  2. How many years did it take for the sample to decay from pure pennyonium to lead?

  3. How many years passed before approximately half the sample decayed?

  4. Imagine that you find a sample of pennyonium in a layer of rock and analyze it. You find that out of 100 atoms, 13 are pennyonium and 87 are lead. How long do you think it has been since that layer of rock formed? Explain how you arrived at your answer.