This set has twelve original questions, ordered from easier to harder, covering all five lessons in atomic and nuclear models. Questions 1 to 4 are warm-ups, 5 to 8 build accuracy, and 9 to 12 mix skills. All data in the questions are invented for practice.
Attempt each question on paper and write your working as you would in an exam. Only then open the answer. Mark the ones you got wrong and use the routing list at the end.
Questions
1. State whether each event is an atomic change or a nuclear change, and whether the element changes: (a) a neutral sodium atom loses an electron; (b) radium-226 emits an alpha particle; (c) an electron falls to a lower energy level in an atom.
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(a) Atomic. Only the electrons change, so it is still sodium, now a positive ion. (b) Nuclear. The proton number falls by 2, so a new element forms. (c) Atomic. The nucleus is unchanged, so the element is the same.
2. Chlorine has proton number 17. Two isotopes have nucleon numbers 35 and 37. How many neutrons does each have, and why are they the same element?
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Neutrons = A − Z. For chlorine-35: 35 − 17 = 18. For chlorine-37: 37 − 17 = 20. Both have 17 protons, and the proton number decides the element, so they are the same element with different numbers of neutrons.
3. Radon-222, 22286Rn, decays by alpha emission. Write the equation.
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Upper: 222 − 4 = 218. Lower: 86 − 2 = 84, which is polonium.
22286Rn → 21884Po + 42He
Check: 218 + 4 = 222 and 84 + 2 = 86.
4. Sodium-24, 2411Na, decays by beta emission. Write the equation.
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Upper stays 24. Lower: 11 = Z + (−1), so Z = 12, which is magnesium.
2411Na → 2412Mg + 0−1e
Check: 24 + 0 = 24 and 12 + (−1) = 11.
5. A nucleus X decays by alpha emission to lead-208, 20882Pb. Find the nucleon number and proton number of X.
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Work backwards. X → 20882Pb + 42He. Upper: A = 208 + 4 = 212. Lower: Z = 82 + 2 = 84.
X is 21284Po, polonium-212.
6. Give two differences between beta and gamma radiation.
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Beta particles are fast electrons with negative charge, whereas gamma rays are uncharged electromagnetic waves. Beta is stopped by a few millimetres of aluminium, whereas gamma is only reduced by thick lead or concrete. Beta is deflected by a magnetic field, whereas gamma is not.
7. The background count rate is 30 counts/min. A sample gives these measured count rates.
| Time (min) | 0 | 6 | 12 | 18 |
|---|---|---|---|---|
| Measured (counts/min) | 510 | 270 | 150 | 90 |
Find the half-life. Predict the corrected and the measured count rate at 30 min.
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Subtract 30 from each: 480, 240, 120, 60. Each step of 6 min halves the value, so the half-life is 6 minutes.
At 30 min there are 30 ÷ 6 = 5 half-lives. Corrected rate = 480 ÷ 25 = 480 ÷ 32 = 15 counts/min. A detector would measure 15 + 30 = 45 counts/min.
8. A source is counted for six separate intervals of 10 s. The counts are 12, 9, 14, 11, 9 and 13. The background is 8 counts/min. Find the count rate from the source alone, and explain why the counts differ.
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Total = 12 + 9 + 14 + 11 + 9 + 13 = 68 counts in 6 × 10 s = 60 s. So the measured rate is 68 counts/min. Corrected rate = 68 − 8 = 60 counts/min.
The counts differ because decay is random. The number of nuclei that decay in each interval varies around an average.
9. Explain why an alpha source is usually low risk when held outside the body but can be very damaging if swallowed.
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Alpha particles are strongly ionising, so they lose their energy within a few centimetres of air and are stopped by skin or clothing. Outside the body, they do not reach sensitive tissue. Inside the body, the source is in contact with tissue, so all the energy is deposited in a very small volume, causing concentrated ionisation and damage.
10. The background count rate is 15 counts/min. A source gives 620 counts/min with no absorber, 617 with paper, 590 with 3 mm aluminium and 70 with 5 cm of lead. Identify the radiation and justify your answer.
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Paper and thin aluminium barely reduce it, which rules out alpha and beta. Thick lead reduces the corrected rate from 620 − 15 = 605 to 70 − 15 = 55 counts/min, about 9% of the original. It is reduced a lot but not reduced to zero. This matches gamma.
11. Uranium-238, 23892U, emits an alpha particle, and the nucleus formed then emits a beta particle. Find the proton number and nucleon number of the final nucleus.
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Step 1, alpha: A = 238 − 4 = 234, Z = 92 − 2 = 90. This is 23490Th.
Step 2, beta: A stays 234, Z = 90 + 1 = 91. This is 23491Pa.
Z = 91 and A = 234.
12. A student says: “If I heat the sample, its half-life will be shorter, because the atoms move faster.” Explain what is wrong with this.
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Radioactive decay is a change in the nucleus, which is not affected by heating. Heating changes how fast atoms move, not the chance of a nucleus decaying. The half-life of a given isotope stays constant, and the decay remains random.
If you got these wrong
- Questions 1, 2 or 12: you may be mixing atomic and nuclear changes, or treating changes in the electrons as nuclear. Go to distinguish atomic and nuclear changes.
- Questions 3, 4, 5 or 11: the upper or lower numbers are slipping. Go to balance a simple nuclear equation, and write both sums every time.
- Questions 6 or 10: the properties are not yet sorted. Go to compare radiation properties descriptively.
- Questions 7 or 8: check whether you subtracted background and whether you halved from corrected values. Go to interpret random count-rate data.
- Questions 9 or 10: ionisation and penetration may be merged. Go to separate ionising behaviour from penetration.
Record each slip in the mistake log and retest queue with its error type, so a fresh question can be retested later. For a wider plan, return to the topic overview.
If the same type of error keeps returning, that is often the moment when a teacher watching your working helps most in online one-to-one Physics tuition.