Radioactivity and Stability of Isotopes
14 Grade 10 Physics questions on Radioactivity and Stability of Isotopes, with worked marking schemes. 14 questions are available on this topic in full.
Question 1
1 markGamma radiation given out by a nucleus is
A. a helium nucleus
B. a fast-moving electron
C. a stream of protons
D. an electromagnetic wave of very high frequency
(1 mark)
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4 marksExplain why the activity of a radioactive source falls as time goes by, and why radioactive decay is described as a random process.
(4 marks)
Marking scheme
As time passes, nuclei decay, so the number of undecayed nuclei left in the sample becomes smaller (1 mark). The activity is proportional to the number of undecayed nuclei present, so the activity falls as that number falls (1 mark). It is impossible to tell which particular nucleus will decay next, or at what instant it will decay (1 mark). Only the average behaviour of a very large number of nuclei can be predicted; counts taken over equal intervals of time fluctuate about an average value (1 mark).
Question 3
1 markA radioactive nuclide has a half-life of 4 days. The fraction of the original undecayed nuclei still remaining after 12 days is
A. one quarter
B. one sixth
C. one third
D. one eighth
(1 mark)
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1 markA researcher at an agricultural research station in Kitale wishes to use a radioactive tracer to follow the uptake of phosphate fertiliser by young maize plants. The most suitable tracer is one that emits
A. alpha particles and has a half-life of several years
B. alpha particles and has a half-life of a few seconds
C. gamma rays and has a half-life of thousands of years
D. beta particles and has a half-life of a few days
(1 mark)
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1 markA sample of a radioactive isotope is cooled to a very low temperature, then squeezed under very high pressure, and finally combined chemically with another element. Throughout these treatments the rate at which the sample decays will
A. stay the same, because the decay rate depends only on the nuclide itself
B. decrease, because cooling slows down the nuclei
C. increase, because squeezing brings the nuclei closer together
D. change, because the isotope is now part of a different compound
(1 mark)
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3 marksA Grade 10 class is to use a sealed radioactive source during a physics lesson. State three precautions the learners should take while handling the source.
(3 marks)
Marking scheme
Any three of the following, 1 mark each: hold the source with long forceps or tongs, never with bare hands; keep the open end of the source pointed away from the body and away from other people; keep the source as far from the body as is practicable; keep the time of exposure as short as possible and return the source to its container immediately after use; store the source in a lead-lined container when it is not in use; do not eat or drink in the laboratory, and wash hands after the lesson.
Question 7
1 markState what is meant by the half-life of a radioactive nuclide.
(1 mark)
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1 markWhen a nucleus emits a beta particle,
A. a neutron in the nucleus changes into a proton and an electron, and the electron is ejected
B. the nucleus loses energy only, with no change to the particles in it
C. two protons and two neutrons leave the nucleus together
D. an electron is knocked out of the outermost shell of the atom
(1 mark)
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1 markA Geiger-Muller tube connected to a counter is switched on in an empty physics laboratory in Kisii with no radioactive source anywhere near it. The counter still records a small count rate. The best explanation is that
A. background radiation from rocks, soil, the air and outer space is reaching the tube
B. the counter is responding to sound and vibration in the room
C. charge left in the tube from an earlier experiment is leaking away
D. the tube is faulty and should be replaced
(1 mark)
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5 marksWater from a treatment works in Eldoret is suspected to be leaking from an underground pipe. Suggest how a radioactive source could be used to locate the leak, and give reasons for the type of source you would choose.
(5 marks)
Marking scheme
Introduce a small quantity of a radioactive isotope into the water flowing along the pipe (1 mark). Walk along the line of the pipe at ground level with a Geiger-Muller tube and ratemeter, recording the count rate at intervals (1 mark). The count rate rises sharply above one point, because the tracer escapes with the leaking water and collects in the soil there; this locates the leak (1 mark). A gamma emitter is chosen because only gamma radiation is penetrating enough to pass through the pipe wall and the soil to reach the detector (1 mark). The isotope should have a short half-life so that the water supply and the ground do not remain radioactive for long (1 mark).
Question 11
4 marksA nuclide W has proton number 92 and nucleon number 238. It emits an alpha particle to form nuclide X. Nuclide X then emits a beta particle to form nuclide Y. Determine the proton number and the nucleon number of X and of Y.
(4 marks)
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4 marksExplain why alpha particles have a much greater ionising power but a much smaller penetrating power than gamma rays.
(4 marks)
Marking scheme
An alpha particle is relatively massive and carries a double positive charge (1 mark). It therefore interacts strongly with the electrons of the atoms it passes, knocking many electrons off atoms along a short length of its path, so it ionises heavily (1 mark). Because it gives up its energy over so short a distance, it is stopped quickly, being absorbed by a few centimetres of air or a sheet of paper (1 mark). Gamma radiation is an uncharged electromagnetic wave, so it interacts only weakly with atoms, produces few ions and therefore travels a long way through matter before being absorbed (1 mark).
Question 13
4 marksCarbon-12 and carbon-14 are isotopes of carbon. Carbon-12 is stable while carbon-14 is radioactive. Explain, in terms of the structure of their atoms, why the two have identical chemical properties but differ in stability.
(4 marks)
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1 markTwo atoms are isotopes of the same element if they have
A. the same proton number but different numbers of neutrons
B. the same number of neutrons but different numbers of protons
C. the same nucleon number but different proton numbers
D. the same nucleus but different numbers of electrons
(1 mark)
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