AQA GCSE Physics Physics Paper 1 (Higher), June 2025: Question 7

10 marks · Standard Demand difficulty · Short Answer

Answer questions on atomic models, irradiation and contamination, alpha decay nuclear equations, half-life calculations, and isotope activity comparison.

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Question

Question 7 consists of five parts about atomic physics. 07.1 asks to select the correct reason why scientists change models of atomic structure from four options. 07.2 asks for the difference between irradiation and radioactive contamination. 07.3 presents an alpha decay nuclear equation for polonium-210 into lead and helium-4, with boxes to fill in the missing atomic number of polonium and mass number of lead. 07.4 asks to calculate the time in days for 256 000 atoms of polonium-210 (half-life 138 days) to reach 16 000 atoms. 07.5 asks to compare the activity of polonium-209 (half-life 125 years) with polonium-210 (half-life 138 days) for equal numbers of atoms.

Mark scheme

Show the mark scheme Mark scheme for Question 7. 07.1: 'new evidence is discovered that existing models cannot explain' (1 mark). 07.2: 'irradiation is the exposure to radiation' (1 mark), and '(radioactive) contamination is the (unwanted) presence of radioactive material on / inside a person' (1 mark). 07.3: Po atomic number is 84 (1 mark), Pb mass number is 206 (1 mark). 07.4: number of half-lives = 4 (1 mark), time = 4 × 138 (1 mark), time = 552 days (1 mark). 07.5: polonium-209 has a lower activity than polonium-210 (1 mark), because polonium-209 has a longer half-life (1 mark). Total: 10 marks.

How to answer it

AQA GCSE Physics • Topic 4: Atomic Structure

Radioactivity: Models, Safety, and Decay

What this question tests

This 10-mark question assesses foundational and quantitative skills in radioactivity:

  • Development of the atom: Why scientific models change over time when new experimental evidence arises.
  • Hazards of radiation: The distinct scientific definitions of irradiation vs contamination.
  • Nuclear equations: Balancing mass numbers and atomic numbers during alpha (α) decay.
  • Half-life calculations: Determining total time from an initial to a final count rate/number of nuclei.
  • Linking half-life to activity: Explaining why a longer half-life results in a lower rate of radioactive emission for equal sample sizes.

Part 07.1: Scientific Models

Multiple Choice (1 Mark)

✅ Correct Answer

"New evidence is discovered that existing models cannot explain."

Award 1 mark for the correct box ticked.

🧠 Exam Technique

  • Science is evidence-driven. Models are never replaced simply because of time elapsed (e.g. "every 100 years") or because they are "old-fashioned".
  • Always link changes in scientific consensus to new experimental data (such as Rutherford's alpha-scattering experiment challenging Thomson's plum pudding model).

Part 07.2: Irradiation vs Contamination

Distinguishing Key Radiation Concepts (2 Marks)

✅ Correct Answer

  • Irradiation: Exposure of an object or person to radiation. [1 mark]
  • Contamination: The unwanted presence of radioactive material/atoms on or inside an object or person. [1 mark]

💡 Key Knowledge

  • Irradiation stops as soon as the source is removed or shielded. The irradiated object does not become radioactive.
  • Contamination means the radioactive source itself is physically on you (e.g. dust on skin or swallowed), continuously emitting radiation until cleaned or decayed.

❌ Common Misconception

Many students incorrectly write that "irradiation makes an object radioactive". It does not! Only direct contamination or specific neutron capture makes something radioactive.

Part 07.3: Balancing an Alpha Decay Equation

Nuclear Equation Completion (2 Marks)

²¹⁰[ 84 ]Po  ⟶  [ 206 ]₈₂Pb  +  ⁴₂He

📐 Step-by-Step Balancing

  1. Find the missing atomic number (bottom left for Po):
    Total charge must balance: 82 + 2 = 84 . [1 mark]
  2. Find the missing mass number (top right for Pb):
    Total mass number must balance: 210 - 4 = 206 . [1 mark]

🧠 Exam Technique

Treat the arrow like an equals sign ( = ):

  • Top row: 210 = Mass(Pb) + 4  →  Mass(Pb) = 206
  • Bottom row: Atomic(Po) = 82 + 2  →  Atomic(Po) = 84

Part 07.4: Half-Life Calculation

Calculating Total Decay Time (3 Marks)

📐 Step-by-Step Solution

  1. Determine the number of half-lives:
    Halve the starting number of atoms until reaching 16 000:
    256 000 ⟶ (1) 128 000 ⟶ (2) 64 000 ⟶ (3) 32 000 ⟶ (4) 16 000
    Number of half-lives = 4 [1 mark]
  2. Calculate the total elapsed time:
    Total time = number of half-lives × length of one half-life
    Time = 4 × 138 [1 mark]
  3. Final answer:
    Time = 552 days [1 mark]

❌ Common Errors

  • Dividing the starting number directly: Dividing 256 000 by 16 000 gives 16 (the reduction factor, 2⁴), not the number of half-lives (4).
  • Off-by-one counting: Counting the arrow stages incorrectly (e.g. counting 5 stages instead of 4 divisions). Write out each halving step clearly.

Part 07.5: Half-Life and Activity Comparison

Explaining Radioactive Activity (2 Marks)

✅ Correct Answer

  • Polonium-209 has a lower activity than polonium-210. [1 mark]
  • (Because) polonium-209 has a longer half-life (or Po-210 decays much faster). [1 mark]
Note: Mark 2 is dependent on correctly awarding Mark 1.

💡 Key Concept: Activity vs Half-Life

  • Activity is the number of decays per second (measured in Becquerels, Bq).
  • For an equal number of unstable nuclei, an isotope with a shorter half-life decays much more frequently each second, meaning higher activity.
  • Po-209 takes 125 years to halve, while Po-210 takes only 138 days. Po-210 nuclei decay far more rapidly!

🧠 Examiner Insight

The mark scheme explicitly states: "ignore description of rate of decay". You must explicitly state whether the activity is higher or lower and link it directly to the duration of the half-life.

Topics

Physics · P4: Atomic Structure

Question and mark scheme from the AQA GCSE Physics examination, Physics Paper 1 (Higher), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.