# Half-life number – number of radioactive atoms remaining

Half-Life Data Worksheet (Adapted from RadTown Radioactive Atom Activity Set, US EPA)
questions to the best of your ability:
The following image demonstrates how uranium‐238 (a radioactive element) decays and
changes to a stable element (lead‐206). The half‐life of each element is shown in years and days. Use the website below for
the image
1. What is a half‐life (Use the mathematical terminology in your definition):
2. Calculate the number of radioactive atoms remaining after each half‐life listed in the following table. Write the number of
atoms in the “Number of Radioactive Atoms” column. Note that the number of unstable (radioactive) atoms decreases as they are being transformed into stable atoms.
Half-Life Number – Number of Radioactive Atoms Remaining
0 – 1024
1 –
2 –
3 –
4 –
5 –
6 –
7 –
8 –
9 –
10 –
3. If you had a sample of 4,000 radioactive atoms, how many atoms would remain after 5 half‐lives?
Half-Life Number
Number of Radioactive Atoms
0 – 4000
1 –
2-
3 –
4 –
5 –
4. If you have a sample of 210 atoms, and you started with a sample of 3,360 atoms, how many half‐lives have elapsed
(remember fractions of a half-life are possible)?
5. If the half‐life of the sample from question 2 is 30 minutes (0.5 hours), how many hours did it take to decay from 3,360
atoms to 210 atoms (hint: multiply the number of half-lives calculated by the length of the half-life, and convert to hours by
dividing by 60)?
6. Can you determine the age of something like a dinosaur fossil by examining the half‐life of isotopes present in the sample? Why would radiocarbon not be appropriate for these types of fossils (hint: article on dinosaur bones) Explain.
7. In what other ways might it be useful to know a sample’s half‐life (Hint: nuclear power plants)?