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Worked example

bismuth crystal 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?


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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?

  1. none of the original compound remains
  2. a quarter of the original compound remains
  3. half of the original compound remains
  4. all of the original compound remains
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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?

  1. Aluminium-26 (Aluminum-26), with a half-life of 7.2 x 105 years
  2. Neon-19 with a half-life less than 1 hour
  3. Oxygen-17 with no experimentally detected decays
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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.
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