A Half-Life Model
Nuclear Chemistry Activity
BACKGROUND
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Radioactive isotopes (radioisotopes) are unstable atoms that decompose spontaneously to the
atoms of a different element.
The breakdown of atoms takes place at a set rate, called half-life, which differs for each
radioactive isotope.
Half-life is the amount of time it takes for one half of the atoms in a sample of a
radioactive isotope to decay to the atoms of a different element.
Because it is not practical for you to study the half-life of a real radioactive isotope, you will use
a model of such an element in this activity.
The purpose of this activity is to study the phenomenon of half-life.
Materials
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100 split peas
10 lima beans
250 mL beaker
graph paper
Procedure
PART A
1. Label the beaker Undecayed Atoms.
2. Count out 100 split peas and place them in the beaker. Each pea will represent one atom of the
imaginary unstable element peanium (a radioisotope).
3. Shake the beaker and dump the split peas onto the table.
4. Notice that some peas landed curved side down and others landed curved side up. Separate the
peas into two groups according to the position – up or down of the curved side. The group with the
curved side down will represent the peanium atoms that have decayed to the atoms of a different
atom. The group with the curved side up will represent the undecayed atoms.
5. Count the undecayed atoms and record the number in the data table.
6. Place the decayed peanium ions aside. Return the undecayed atoms to the beaker.
7. Repeat steps 4 through 7 until there are no more atoms in the Undecayed Atoms beaker. Note the
time in your data table.
PART B
8. Repeat PART A, but this time lima beans will represent a STABLE isotope of peanium atoms (aka
– not a radioisotope … will not decay). Add 10 lima beans to the peanium atoms in step 2. Since
the sides of the lima beans do not differ significantly, always sort them into the group of undecayed
peanium ions.
Data and Observations
Number of Undecayed Atoms
RUN #
PART A
1
100
2
PART B
110
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Analysis and Conclusions
1. Make a graph of your results. Place time (the run number) on the x-axis and the number of
remaining peanium ions (undecayed) on the y-axis. Draw the lines for both ‘series’ on the one
piece of graph paper.
2. What is the relationship between time and the number of remaining peanium ions?
3. In the activity, how did the stable peanium isotope (lima beans) affect the rate of decomposition of
the unstable peanium?