Worked example
Bismuth forms very colorful geometric crystals. While at a museum, you see a
sample of bismuth-209 on display. As you explored the IUPAC Periodic Table of the Elements and Isotopes Learning Tool, you may have learned that bismuth-209 is radioactive.
Are you concerned about your exposure to this sample?
Click here to show answer
For a long time scientists thought that bismuth-209 was a stable isotope, but we now know that bismuth has no stable isotopes. However, there is no reason to be concerned because bismuth-209 has a very long half-life (of 2.0 x 1019 years).
Your turn
If a radioactive compound has a half-life of 2.5 years, how much of the compound is left after 5 years?
- none of the original compound remains
- a quarter of the original compound remains
- half of the original compound remains
- all of the original compound remains
Click here to show answer
The correct answer is B. After 5 years two half-lives will have occurred. After the first 2.5 years, half the compound will remain, and after the next 2.5 years this amount will halve again. Thus, only a quarter of the original compound will remain.
Which of the following is a stable isotope?
- Aluminium-26 (Aluminum-26), with a half-life of 7.2 x 105 years
- Neon-19 with a half-life less than 1 hour
- Oxygen-17 with no experimentally detected decays
Click here to show answer
The correct answer is C. Though aluminum-26 is more stable than neon-19, neither of these are stable isotopes. Only oxygen-17 has no experimentally detected decays.