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Comparing Half-Life Activity Changes
Table 2 shows the half-lives of radon-222 and vanadium-52.
| Isotope | Half-life |
|---|---|
| Radon-222 | 3.8 days |
| Vanadium-52 | 3.7 minutes |
The scientist measured the radiation emitted by a sample of radon-222 and the radiation emitted by a sample of vanadium-52. The scientist repeated the measurements 7.4 minutes later.
Explain how the activity of the radon-222 and vanadium-52 had changed after 7.4 minutes.
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Marking points
- 1 States that radon-222 activity remained almost the same or decreased only slightly.
- 2 Explains that 7.4 minutes is very small compared with radon-222’s 3.8-day half-life.
- 3 States that vanadium-52 activity decreased by a factor of 4.
- 4 Explains that 7.4 minutes is two vanadium-52 half-lives.
Full-mark answer
Radon-222: its activity stayed almost the same because 7.4 minutes is only a very small fraction of its 3.8-day half-life. Vanadium-52: 7.4 minutes is two half-lives of 3.7 minutes, so its activity fell to one quarter of its original value—a decrease by a factor of 4.
Why this answer loses marks
Says the short-lived sample halves only once.
The interval spans more than one full half-life, so repeat the halving for every completed interval.