Making Cents of Radiometric Decay


     What is radiometric decay? Evolution is the unifying principle in biology, as it provides an explanation for the overwhelming diversity of forms and function seen in life on Earth.  The distribution, morphology, and genetics of living populations all provide evidence for evolution, but the complete picture cannot be understood by the study of living population alone.  The origin of major new structures and body plans can be studied through the archive of evolution, the fossil record. Fossils, traces of once living organisms, are found most commonly in layers of sedimentary rock.  This rock type forms when water and wind form layers of sand and silt.  Over time, the sedimentary layers become rock.  The order in which fossils are buried and the findings of transitional fossils (changes seen from one species to another) are two major lines of fossil evidence for evolution.

     A variety of methods can be used to determine the age of fossils. “Relative dating” is determined by comparing fossil and rock position in relation to other fossils or rocks. “Absolute dating” can give actual dates of fossils. The most common type of absolute dating is called, “radiometric dating”. Any radioactive substance can be used for dating, but it is important to select a radioactive material with a “rate of decay” or “half-life” that fits the time range for the fossil being studied. When radioactive atoms decay, they release electrons, protons, and neutrons at a constant rate.  The length of time it takes half of the radioactive atoms in a sample to decay is called a “half-life”.  You cannot predict with individual atoms will decay because the process is random and spontaneous, but you can predict how many will decay if you know the half-life.  Examples of radioactive materials and their half-lives include: Potassium 40 à Argon 40 = 1.3 billion years; Uranium 235 à Lead 207 = 713 million years; Carbon 14 à Nitrogen 14 = 5,740 years.  If one were to model radiometric decay, then the understanding of half-life will become evident. 




Materials: Per Group

Plastic Box with Lid

100 Pennies (in Zip-lock bag)

100 Paper Clips (in Zip-lock bag)



Pencil / Pen

Graphing Software (optional)



1.      Make sure you have 100 pennies by counting them out and placing them “Heads” up in your box; do not worry about the paper clips at this time. If you need more pennies let the instructor know.

2.      For this activity, pennies will represent radioactive isotopes (atoms).  “Heads” will represent unchanged atoms, while “Tails” will represent decayed atoms that change into “Paper Clips” since radioactive isotopes do not just disappear but literally change into a different isotope.

3.      Perform the first trial by closing the lid, placing your hand/fingers on the lid, and shaking the container for 30 seconds.

4.      Count and remove the number of “Changed Atoms” (or Tails), place them back into the bag,  and replace them with paper clips in the box. 

5.      Record the number of “Changed Atoms” (or Tails) in the table provided.

6.      Count but do not remove the number of “Unchanged Atoms” (or Heads).

7.      Record the number of “Unchanged Atoms” (or Heads) in the table provided.

8.      Calculate the “Percent of Atoms Changed” per trial by using a calculator. Record the percentage of “Atoms Changed” in the table provided:        

9.      Repeat steps 3-8 until you have 100% of “Changed Atoms”.

10.  Clean and return all materials as directed and complete the Graphing Exercise and the Result Data Sheet questions.  




Graphing Exercise: 10 Points [Work in partners to complete (one with the lab directions, one with the spread sheet (ie Google Sheets) open)] See Directions Below


Create a column using only the “Number of Changed Atoms” data from your table.

Highlight the numbers (do not include 0) and then select the graphing icon (top right side).

Select “Charts” and then “Line Graph” for this lab and click “insert”.

By using the “advanced tab” on the graph (upper right arrow down) label the following: Title: Radiometric Data Results ; Let X “horizontal” axis: Number of Trials ; Let Y “vertical” axis: Number of Changed Isotopes  (Under “Axis” select down arrow for Vertical Title); Under Series: Point Size = 10px



Result Date Sheet for Radiometric Decay Activity








Number of Changed


(Tails Changed Paper Clips)


Number of Unchanged


 (Heads Left)


Percent of Atoms


(Changed # / Last Unchanged #) x 100
















































1.      What is the half-life of these “atoms”? In other words, how long (time) did it take for about half the pennies (atoms/isotopes) to change into paper clips (changed atoms/isotopes)?

2.      Isotopes do not truly decay and go away but can “change” into different atoms. (Yes or No)

3.      If each trial represented a half-life value of 1,500 years, how old would a fictional fossil be by the 3rd trial?

4.      If the half-life rate of Carbon 14 is 5,740 years, how many half-lives would a fossil plant have gone through if the fossil was dated to be 22,960 years old and thus be made up of about 94% of Nitrogen 14 and about 6% or 1/16th of Carbon 14 left?

5.      Was the hypothesis proven or disproven for this activity? (Start by saying, The hypothesis was ….)

6.       According to the background information, what is “absolute dating”?

7.      Does radioactive decay help to determine the age of fossils? (Yes or No)

8.      What defines “half-life” according to the background information?

9.      Fossils can be mostly found in what type of rock according to the background information?

10.  What would be an improvement to this lab activity? (Start by saying, An improvement to this lab activity would be ….)