Chem 100 Section _______
Experiment 12
Name ____________________________
Partner’s Name ___________________________
Radioactivity
Introduction
This experiment is designed to enhance your understanding of the process known as radioactivity. In
this exercise you will measure the radioactive emissions of a particular nuclide under different
conditions and will eventually compute a value for its half-life.
Background Information
Near the end of the 19th century, it was found that the nuclei of the isotopes of some elements undergo
a spontaneous decay to different nuclei through the release of small particles and the release of
electromagnetic radiation. These isotopes are termed radioisotopes. This emission process is termed
radioactivity. For example, iodine-131 undergoes a radioactive decay to yield xenon-131 and an
electron.
131
53
I
131
54
Xe
+
0
-1
e
In this nuclear reaction, the symbol for each particle is written to show its mass number (A) as a
superscript and its atomic number (Z) or nuclear charge as a subscript. Notice that the Law of
Conservation of Mass applies to nuclear reactions, as does the conservation of nuclear charge. The
sum of nuclear charges on the right equals the charge on the left, i.e., the sum of atomic numbers is
equal on left and right sides. By 1903, three types of radioactive emission had been identified.
Alpha particle emission results in the release of a helium ion (an α particle) from a radioactive
nucleus. For example, radium-222 undergoes α emission to produce a radon atom
222
88
Ra
218
86
Rn
+
4
2
He
Likewise, polonium-210 decays by the release of an α particle to produce a lead atom
210
84
Po
206
82
Pb
+
4
2
He
Notice that in both of these nuclear reactions, mass is conserved, i.e., the superscript on the left side of
the equation equals the sum of the superscripts on the right side. Also the subscripts equal each other.
Beta particle emission results in the release of a high energy electron (a β particle) from the nucleus.
This can be viewed as a neutron being converted to a proton and an electron, and the latter then is
ejected from the nucleus. For example, radium-228 undergoes spontaneous β particle emission to form
an actinium atom.
228
88
Ra
228
89
Ac
+
0
-1
e
Since a neutron is converted to an electron and a proton, the atomic number of the product has
increased, while the mass number remains unchanged.
12 - 1
Gamma ray emission results in the release of a form of electromagnetic radiation (γ-rays) that are
similar to X-rays, although higher in energy. Since only energy is released in this decay, the isotope
retains its original identity; it simply changes from a higher energy nucleus to one possessing a lower
energy.
The emission of these various types of radiation can be measured by the use of a Geiger counter. The
general scheme of a Geiger counter is shown in Figure 12.1.
Figure 12.1.
The radiation passes through the window of the probe and causes argon gas atoms to be ionized. This
results in the flow of current to the positively charged electrode, causing a click to be heard from the
speaker and an increase in the value shown in the LED digital display.
In this experiment, you will use a Geiger counter to measure the radioactive emissions from potassium
compounds. Potassium consists of three naturally occurring isotopes (39K, 40K and 41K). Only the
isotope with mass number 40 is radioactive and emits β particles. By measuring potassium chloride at
room temperature and at an elevated temperature, you will be able to determine the effect of
temperature on the rate of radioactive decay. You will also measure the radioactive emissions of
potassium sulfate and potassium phosphate to determine the effect of chemical combination on the rate
of decay of potassium-40.
The rate of decay of radioisotopes varies considerably. Some radioisotopes decay rapidly in a few
seconds, while others decay slowly over billions of years. In addition, not all of the atoms of a
radioisotope decay at the same time. The time required for half the atoms of a radioisotope to decay is
termed the half-life and is symbolized as t1/2. For example, the half-life of iodine-131 is eight days. If
you had a 100 g sample of pure I-131 today, the amount remaining after different periods of time is
shown by the data in Table 12.1.
Table 12.1
Day
0
8
16
24
32
Amount of I-131 Remaining, g
100.0
50.0
25.0
12.5
6.2
After 10 half-lives, about 0.1 g of I-131 would remain. The rest would have decayed to Xe-131.
From your measurements of the radioactivity of potassium chloride, you will be able to estimate the
half-life of potassium-40.
12 - 2
Materials Needed
Chemicals
• Sodium chloride, NaCl
• Potassium chloride, KCl
• Potassium sulfate, K2SO4
• Potassium phosphate, K3PO4
Equipment
• Geiger counter
• Hot plate
• Thermometer
• 3 mini-petri dishes (glass)
Experimental Procedure
A. The Determination of Background Radiation:
1. Obtain a Geiger counter, turn it on, and let it warm. Your instructor will demonstrate its operation
and how to determine the reading of the radioactive emission from a sample.
2. Label three mini-petri dishes as #1, #2, and #3. Tare the mini-petri dishes and record their tare
weights in the space provided on the data sheet.
2. Determine the level of background radiation in counts per minute by taking six one-minute
measurements with only air and again with one of the empty mini-petri dishes. Record these data
in the appropriate places of Table 12.2.
B. Determination of Radiation Emitted by NaCl and KCl at Room Temperature:
1. Add enough solid sodium chloride (KCl) to mini-petri dish #1 to give a depth of solid of about 1
cm. Reweigh the mini-petri dish with its contents and record its mass. In mini-petri dishes #2 and
#3, prepare a second sample of potassium chloride (KCl) and a sample of sodium chloride (NaCl),
respectively. As before, reweigh the dishes with their contents and record their masses.
2. By difference, determine and record the masses of the potassium chloride and sodium chloride in
the three mini-petri dishes.
3. Obtain a hot plate, turn it on to the lowest setting, and place the beaker containing KCl Sample 2
on it.
4. Determine the radiation count from the KCl Sample 1. Tap the beaker so that the surface of the
KCl sample is level. Make sure that the Geiger counter probe is as close to the sample as possible,
without actually touching the solid. As before, take six one-minute measurements and record them
in Table12.2. Also, determine the radiation count – six trials - of the NaCl sample; record them in
Table 12.2.
.
C. The Determination of Radiation Emitted by Potassium Sulfate at Room Temperature:
1. Clean out mini-petri dish #1 and add enough solid potassium sulfate (K2SO4) to give a depth of
solid of 1 cm. Weigh and record the mass of this beaker with potassium sulfate.
2. By difference, determine and record the mass of the potassium sulfate.
3. As before, make six one-minute measurements of the radiation emissions from the solid and record
your data in Table 12.2.
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D. The Determination of Radiation Emitted by Potassium Phosphate at Room Temperature:
1. Clean out mini-petri dish #1 again, and now add enough solid potassium phosphate (K3PO4) to
give a depth of solid of 1 cm. Weigh and record the mass of this beaker with its contents.
2. By difference, determine and record the mass of the potassium phosphate.
3. Once again, make six one-minute measurements of the radiation emitted by this sample and record
your data in Table 12.2.
E. The Determination of Radiation Emitted by Potassium Chloride at Elevated Temperature:
1. By now the KCl Sample 2 should have warmed up. Place a thermometer into the granular solid
and determine its approximate temperature.
2. Remove the sample from the hot plate. Again, take six one-minute measurements of radiation
being emitted from the hot potassium chloride and record the data in Table 12.2.
F. The Determination of the Net Activity of the Samples:
For each of the five (5) sets of measurements, determine the average value and record these in
Table 12. 2. Subtract the average background count from each of the sample count averages to get
a corrected sample count, i.e., the Net Activity of the samples. Record these results in Table l2.2.
12 - 4
Experiment 12
Name ____________________________
Partner’s Name_____________________________
Date ____________Chem 100 Section _______
Radioactivity – Data Sheet
Table 12.2
Radiation Emission, counts/minute
Trial
1
2
3
4
5
6
Average
Net
Activity
Background
( in air)
Background
(Dish #____)
NaCl
KCl (Dish #1)
Sample 1
KCl (Dish #2)
Sample 2
K2SO4
K3PO4
B.1 Tare Weight Petri Dish #1:
__________ g
Tare Weight Petri Dish #2:
__________ g
C.1
Mass of Dish 1 with K2SO4:
__________ g
Tare Weight Petri Dish #3:
__________ g
C.2
Mass of K2SO4:
__________ g
Mass of Dish 1 with KCl:
__________ g
Mass of Dish 2 with KCl:
__________ g
D.1
Mass of Dish 1 with K3PO4:
__________ g
Mass of Dish 3 with NaCl:
__________ g
D.2
Mass of K3PO4:
__________ g
Mass of KCl:– Sample 1
__________ g
Mass of KCl – Sample 2:
__________ g
E.1
Temperature of KCl – Sample 2: ________ °C
Mass of NaCl:
__________ g
B.2
B.3
12 – 5
Questions To Be Answered After Completing This Experiment
Write out answers to the following questions in the space provided and turn them in along with
the entire experiment (procedures and data sheets). BE NEAT AND WELL ORGANIZED!
1. Compare and contrast the average background count of air versus a mini-petri dish. What is the
effect of having sodium chloride a mini-petric dish?
2. Convert the number of grams of the solids that you weighed out to moles.
a. KCl Sample 1:
b.
KCl Sample 2:
c. K2SO4:
d. K3PO4:
3. Calculate the number of potassium atoms in each of the above samples. Remember that the gram
formula weight of a compound contains Avogadro's number of formula units of that compound.
a. KCl Sample 1:
b. KCl Sample 2:
c. K2SO4:
d. K3PO4:
12 – 6
4. Since potassium consists of only 0.0118 % K-40, not all of the atoms in your samples were
radioactive. Calculate the number of K-40 atoms in each of the above samples.
a. KCl Sample 1:
b. KCl Sample 2:
c. K2SO4:
d. K3PO4:
5. In Table 12.2 you determined the Net Activity for the four samples. In Question 3 you determined
the number of K-40 atoms that resulted in this Net Activity. Determine the Net Activity that would
have been expected for Avogadro's number of K-40 atoms for each of the samples.
a. KCl Sample 1:
b. KCl Sample 2:
c. K2SO4:
d. K3PO4:
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6. What is the effect of temperature on the emission of radiation from K-40?
7. What effect does the chemical combination of a radioactive element have on its rate of radioactive
emission?
8. Use the data for KCl Sample 1 to determine the half-life of K-40 (express you result in years). The
half-life, t1/2, can be determined from the following equation:
t1/2 = 0.693 N
Rate
where N is the number of K-40 atoms in the KCl Sample 1 (determined in Question 4) and Rate is
the Net Activity of the sample multiplied by 50 (the factor 50 is used since the Geiger counter will
only detect about 2 % of the radiation emitted).
How does your calculated value compare with a published value? Published value: __________
List source of published value:______________________________________________________
9. Radiocarbon dating was used to suggest that the Shroud of Turin was no older than 1200 AD.
This evidence was based on measuring the ratio of C-14 to C-12 and C-13. The latter two isotopes
are stable, while C-14 undergoes a natural radioactive decay. Write the nuclear equation for the
emission of a beta particle from carbon-14.
10. If radium-226 undergoes alpha emission, what isotope is produced?
12 - 8
11. A radioisotope produces xenon-128 and a beta particle. What is this radioisotope? Write the
complete nuclear reaction.
12. Complete the following nuclear reactions:
a.
213
83
Bi
213
84
Po + __________
234
91
Pa
234
92
At + __________
221
87
Fr
217
85
At + __________
b.
c.
13. a. Technetium-99 has a half-life of 6.0 hours. How many grams of a 5.00 g sample of Tc-99
remain after one day?
b. How long would it take this sample to disintegrate to 0.04 g?
12 - 9