Half-Life Data-Teacher Answer Key
1.	Hypothesize what half-life is:
Half-life is the amount of time it takes for approximately half of the radioactive atoms in a
sample to decay into a more stable form. Every radioactive element has a different half- life.
2.	Calculate the number of radioactive atoms remaining after each half-life. Write the number of
atoms in the "Number of Radioactive Atoms" column. Plot the number of radioactive atoms on the
graph provided. Note that the number of unstable (radioactive) atoms decreases as they are being
transformed into stable atoms.
Half-Life
Number
Number of
Radioactive
Atoms
0
1024
1
512
2
256
3
128
4
64
5
32
6
16
Half-Life Number
Observations: Answers will vary, but students should recognize that the number of radioactive atoms
decreases by half after each half-life.
Questions:
3. If you had a sample of 4,000 radioactive atoms, how many atoms would remain after 5 half- lives?
125 radioactive atoms.
Half-Life
Number of
Number
Radioactive

Atoms
0
4000
1
2000
2
1000
3
500
4
250
5
125
oEPA
United States	RadTown Radioactive Atom Activity Set	EPA 402-B-19-026
Environmental Protection
Agency	https://www.EPA.gov/RadTown/Activity-5-Half-Life

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4. If you had a sample of 210 atoms, and you started with a sample of 3,360 atoms, how many half-
lives have elapsed? 4 half-lives.
Half-Life
Number of
Number
Radioactive

Atoms
0
3360
1
1680
2
840
3
420
4
210
5. If the half-life of the sample from question 2 is 30 minutes, how many hours did it take to decay
from 3,360 atoms to 210 atoms? 2 hours.
Half-Life
Number of
rime that has
Number
Radioactive
passed

Atoms
(minutes)
0
3360
0
1
1680
30
2
840
60
3
420
90
4
210
120
6.	Can you determine the age of something (like a fossil) by examining its half-life? Explain. Yes, this
process is called carbon dating. Basically, all living things are made of carbon that continuously
cycles through the environment. A small portion of this carbon is in the form of carbon-14, an
unstable (radioactive) element. Once an organism dies, the carbon-14 begins to disintegrate.
Because it disintegrates at a steady known rate, scientists can measure the amount of carbon-14
remaining and use a scientific formula to determine the age of the sample.
7.	In what other ways might it be useful to know a sample's half-life? It helps in determining how
long radioactive material must be safely stored, when radioactive material will be safe to handle,
or how long a source will remain radioactive. For example, radioactive dye (called a tracer) and
radioactive seeds are used in medical imaging and cancer treatment. Knowing the half-life helps
doctors and patients know how long the radioactive material will be effective and when it will
stop producing radiation.
^PP^V Environmental Protection RadTown Radioactive Atom Activity Set	EPA 402-B-19-026
* * Agency	https://www.EPA.gov/RadTown/Activity-5-Half-Life

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