A radioactive sample has a half life of 8 days. The fraction remaining undecayed after 24 days is
Correct answer: C. one eighth
- A. one half
- B. one quarter
- C. one eighth
- D. one sixteenth
Explanation
Twenty four days is three half lives, so the sample is halved three times, leaving one half multiplied by one half multiplied by one half, which is one eighth. A common error is to divide 24 by 8 and answer one third, which misunderstands the exponential nature of decay. Iodine 131, used in thyroid treatment, has almost exactly this half life.
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About Half-life and Rate of Decay
Half-life is the time required for half the unstable nuclei in a sample to decay, and it remains constant for a particular radioactive nuclide. Questions use the exponential decay law, decay constant, activity, remaining quantity and mean life. Radioactive decay is random for individual nuclei but predictable for large samples.
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