Name: _____________ANSWER KEY________________ Date: ______________________________ Period: _______
Radioactivity and Half-Lives Review Worksheet
1. What is a half-life?
The time it takes for half of the material you started with to decay.
2. If we start with 400 atoms of a radioactive substance, how many would remain after one
half-
life? 200 After 2 half-lives? 100 After 3 half-lives? 50 After 4 half-lives? 25
3. If we start with 16 grams of a substance, how much will remain after 3 half-lives?
16 8 4 2 2 grams after 3 half-lives.
4. A sample of carbon-14 has been decaying for 22,920 years and is now 35.0 grams. What
was the size of the original sample? (The half-life of carbon-14 is 5,730 years)
22920/5730 = 4 half lives You will need to work backwards
35 70 140 280 560. grams
7. How old is a skeleton sample if the current amount of carbon in the bones is 3.125%?
(Assume you started with 100%)
100% 50 25 12.5 6.25 3.125 5 half lives x 5,730 = 28,650 years
8. If you started with 120 grams of a radioactive substance, and now you have 15 grams,
how many half-lives have passed?
120 60 30 15 3 half-lives
9. The half-life of isotope X is 2.0 years. How many years would it take for a 4.0 mg sample
of isotope X to decay and have only 0.50 mg of it remain?
4.0 2.0 1.0 0.50 3 half lives x 2 years = 6.0 years
10.The half-life of Po-218 is three minutes. How much of a 200 atom sample will remain after
15 minutes?
15 minutes/3 = 5 half lives 200 100 50 25 12.5 6.25 atoms
11.The half-life of Au-198 is 2.69 days. How long does it take a 180g sample to decay to 1/8
its original mass?
1 ½ ¼ 1/8 3 half lives x 2.69 days = 8.07 days
12.What is the half-life of a radioactive sample if 100.0 grams of it decays to 12.5 grams in
24.3 hours?
100 50 25 12.5 24.3/3 half lives = 8.10 hours
13.How long will it take a sample of 131I to decay to 12.5% assuming its half-life is 8.07 days?
