Simulation of Radioactive Decay

Certain elements are made up of atoms whose nuclei are naturally unstable. The atoms of these elements are said to be radioactive. The nucleus of a radioactive atom will decay into the nucleus of another element by emitting particles and energy. It is impossible to predict when the nucleus of an individual radioactive atom will decay. However, if a large number of nuclei are present in a sample, it is possible to determine the time period in which half the nuclei in the sample will decay. This time period is called the half-life of the element.

Radioactive materials are harmful to living tissues. Their half-lives are difficult to measure without taking safety precautions. To eliminate these problems, you will simulate the decay of unstable nuclei by using harmless materials that are easy to observe. In this experiment, you will use pennies to represent nuclei. Each observation you make will be one half-life for the "radioactive" pennies.


  1. Count out 100 pennies and place them in the cup. Each penny represents a radioactive atom.
  2. This starting count of 100 radioactive pennies is observation 1.
  3. Carefully dump the pennies from the cup on to your desk top and arrange them into a single layer.
  4. Remove all the pennies that have landed as heads and count them. These pennies represent decayed nuclei. They are now stable atoms. Set them aside on your desk.
  5. Record the number of decayed atoms (heads) in the data table under observation 1
  6. Record the number of remaining radioactive atoms (tails) in the data table under observation 2.
  7. Scoop up the radioactive atoms (tails) and put them in the cup.
  8. Repeat steps 3-7 until all of the pennies have decayed (you will have between 5-10 observations).
  9. Calculate the percent of the original material that decayed according to the following formula:
    % decayed =    # of pennies decayed          x 100
                # of pennies before shaking
  10. Use a full page to make a line graph of your results, with the number of radioactive atoms (pennies before dumping) on the y-axis and the observation numbers (half-lives) on the x-axis.


Answer in complete sentences.

  1. How does your average % decay compare with the amount of decay for a real radioactive element, such as uranium-238?
  2. Is the half-life of your pennies or of real radioactive elements affected by the number of pennies or atoms you start with? Explain.
  3. Compare your graph with the graphs of at least two other groups in the class. Are they similar or different? Explain.
  4. If each of your observations represents 1000 years, what is the maximum age you could measure using your method? Explain.
  5. Use what you learned from this activity to explain how scientists can use radioactivity to determine the age of a rock or fossil.


Observation Remaining number of radioactive pennies (tails) Number of pennies decayed (heads) % decayed

100 (starting number)






































Average % decay