NAME:___________________________________ DATE:_______SECTION________LAB_______
Are We There Yet?
The radioactive half-life for a given radioisotope is a measure of the tendency of the nucleus to "decay"
or "disintegrate" and as such is based purely upon that probability. The tiny nuclear size compared to
the atom and the enormity of the forces which act within it make it almost totally impervious to the
outside world. The half-life is independent of the physical state (solid, liquid, gas), temperature,
pressure, the chemical compound in which the nucleus finds itself, and essentially any other outside
influence. It is independent of the chemistry of the atomic surface, and independent of the ordinary
physical factors of the outside world.
A half-life is the length of time required for one half of a sample of a radioactive substance to decay.
The half-life is different for each radioactive isotope.
Questions:
1) Use the beta decay of Neon-19 to answer the following questions.
A) In the boxes below show amounts of Neon-19 and Flourine-19, as Ne-19 decays into F-19 over
3 half-lives. Shade in the amounts of both the parent and daughter isotopes. Use 2 different
colors to represent each substance.
STARTING
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1st HALF-LIFE
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2nd HALF-LIFE
3rd HALF-LIFE
____
____
B) In the space below each box write the fraction remaining for the parent isotope.
C) What fraction of parent would remain after the 4th half-life? ___________
D) In the space below write the correct equation to show the beta decay of Ne-19.
2) Radioactive Iodine I-131 (also called Radioiodine I-131) therapy is as a diagnosis tool and as a
treatment for an overactive thyroid, a condition called hyperthyroidism. Hyperthyroidism can be
caused by Graves' disease, in which the entire thyroid gland is overactive, or by nodules within the
gland which are locally overactive in producing too much thyroid hormone.
A) Based on Table-N, what type of decay does I-131 undergo. __________
B) For our questions use exactly 8 days as I-131’s half-life not the 8.07 days from Table-N.
How many half-life periods will I-131 undergo in 40 days? Show work.
C) What fraction of I-131 will remain after 40 days? Show work.
D) How much I-131 would be left after 40 days, if you started with 200 grams of I-131?
Show work.
3) Use the space below to answer questions about Potassium-42.
A) What is the half-life for K-42? __________
B) How long would it take a sample of K-42 to decay from a mass of 100 grams to a mass of 25
grams? Show work.
C) How long would it take a sample of K-42 to decay until 1/32 is left? Show work.
4) A wooden axe handle is found to have undergone 3 Carbon-14 half-lives, how old is the axe
handle? Show work.
5) Use the following information to answer the following questions. It takes 4,800 years for a radio
isotope to go from 500 grams to 62.5 grams.
A) What is the half-life of the unknown isotope? Show work.
B) Based on Table-N, what is the isotope? _________
6) List the following radioactive isotopes in order of, which sample will decay the least over a period
of 30 days. Au-198, I-131, P-32, Rn-222. Describe
________
________
________
Least Radioactive
________
Most Radioactive
7)
The half life of iodine-125 is 60 days. What fraction of iodine-125 nuclides would be left after
360 days? Show work.
8)
The half-life of protactinium-234 is 6.75 hours. What percentage of a given sample will remain
after 27 hours? Show work.
9) A medical institution requests 1 g of bismuth-214, which has a half life of 20 min. How many
grams of bismuth-214 must be prepared if the shipping time is 120 min.? Show work.
Reflection:
A) Describe your method to determine the amount of starting sample remaining.
B) Describe your method to determine the fraction of parent remaining.
C) Describe your method to calculate the half-life of an unknown isotope.
D) Describe your method to calculate the amount of parent a decay sequence started with.