Half-Life: Teacher Answer Key
Each radioactive (unstable) element has a different half-life. Hypothesize what half-life is: The amount
of time it takes for half of the radioactive atoms in a sample to decay into a more stable form.
Half-Life
Number
Number of
Remaining
Radon Atoms
0
400
1
(3.8 days)
200
2
(7.6 days)
100
3
(11.4 days)
50
4
(15.2 days)
25
5
(19 days)
12.5
6
(22.8)
6.25
7
(26.6 days)
3.13
8
(30.4 days)
1.57
9
(34.2 days)
.79
10
(38 days)
.40
Radioactive Decay
400
 350
E
5 300
<
<4-
o
250
g 200
E
=
s
150
100
50
0
























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Half-Life Number
1.	Observations: Students should observe that the more time that passes, the more radioactive
decaying takes place. After each half-life number, the number of radon atoms is decreased by
half.
2.	Conclusions: Students should reach the conclusion that over time, radioactive atoms become more
and more stable as they decay.
3.	Why is it useful to know the half-life of radioactive materials? 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) or radioactive
seeds are used in medical imagery 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
A PQA United States	RadTown Radon Activity Set	EPA 402-B-19-055
Environmental Protection
^^^1 Agency	https://www.EPA.gov/RadTown/Activity-4-Half-Life-Radon

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radiation. Radium is used in industrial radiography devices (a technology similar to x-ray imaging) to
inspect for flaws in metal parts. Radium also has been added to the tips of lightning rods, improving
their effectiveness by ionizing the air around it. We do not generally use radon for any useful
purposes.
4. Radon is the second leading cause of lung cancer. How does the half-life of radon and its decay products
cause damage to lung tissue that can lead to lung cancer over the course of a lifetime? Radon gas
decays into radioactive particles that can get trapped in your lungs when you breathe. As they break
down further, these particles release small bursts of energy. This can damage lung tissue and lead to
lung cancer over the course of your lifetime. The amount of time between exposure and the onset of
the disease may be many years.
A rnA United States	RadTown Radon Activity Set	EPA 402-B-19-055
Mrnl Environmental Protection ,	..				 
Agency	https://www.EPA.gov/RadTown/Activity-4-Half-Life-Radon

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