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Radioactive Decay Lab


One way of determining the absolute age of a rock is to measure the percentage of radioactive isotopes found in that rock.  Not all atoms are stable—some atoms are unstable and break down or change spontaneously.  In the process of breaking down, they emit radioactive energy and subatomic particles.  These elements change into different isotopes—elements with different numbers of neutrons.  This process is called radioactive decay.  The original atom is called the “parent” and the resulting atom is called the “daughter.”    

Radioactive decay is a natural process and is not affected by temperature, pressure, chemical change or size of the rock.  Radioactive decay is random, but the rate is predictable.  This means you cannot predict exactly when any given atom will decay, but you can determine by percentage how fast billions or trillions of atoms will decay.  This allows scientists to date objects with a high rate of accuracy.  The time it takes for half of the atoms in a sample of radioactive isotope to decay is its half-life period which is well known for common radioactive isotopes (see table above right).  For example, when an organism dies it is made out of carbon-14.  After 5,700 years, half of the carbon-14 atoms have decayed into nitrogen-14.  

Activity procedure

  1. Open your bag which is a model for a rock, bone or other material which will be radioactive decaying and put 25 dominoes pip side up on the table in front of you.  Each domino represents an atom in your sample.  The black side represents the parent atom of Carbon-14 (14C) and the pip side represents the daughter atom of Nitrogen-14 (14N).  At the beginning of the activity (time T0, or 0 half-lives), all atoms are black side up representing 14C.  Now put all the dominoes back in the bag.
  2. Shake the bag for 10 seconds which represents the passage of 5,700 years or one half-life of 14C.  Carefully dump the dominoes out on the table.  Count the number of dominoes that are black side up, representing atoms that have not decayed and are still 14C.  Record this number in the 2nd column (14C – black) of the 2nd row (T1).  Count the number of dominoes that are pip side up, representing atoms that have decayed into 14N.  Record this number in the 3rd column (14N – pips) of the 2nd row (T1).  Now calculate the percentage of 14C and 14N in the 4th and 5th columns.
  3. Now leave the pip side up dominoes on the table because once a radioactive isotope decays to its daughter, it cannot return.  Put all the black side dominoes back in the bag.
  4. Shake the bag for 10 seconds representing another 5,700 years or one half-life of 14C. Carefully dump the dominoes out on the table.  Count the total number of 14C and 14N and record these numbers in the data table on the next line (T2).
  5. Repeat step (4) four more times.
  6. Graph your results in the graph on the back side.


14C (black)

14N (pips)

% 14C

% 14N

Overall elapsed time






0 years


5,700 years






Graph and analysis questions

  1.  If you cut your sample in half, would that change its half-life?  Why or why not?

  1. If we changed the radioisotope                      

to 238U, would that change the             shape of the graph?  Why or                      why not?

  1. Suppose you found a bone of a mastodon which had a 6.25% 14C and 93.75% 14N.  How long ago did this animal die?  Does this agree with how old mastodons can be?

  1. Would 14C be a good choice to determine the age of a trilobite?  If not, give an alternative isotope from the chart on the front page.

  1. If we determine that a rock sample (the oldest we could find on Earth) has about a 50:50 ratio of 238U to 206Pb, how old is this rock?  What does that mean about the age of our planet?