AQA GCSE Physics Physics Paper 1 (Foundation), June 2025: Question 3

11 marks · Low Demand difficulty · Short Answer

Answer questions on alpha decay, half-life graphs, nuclear structure (protons, neutrons, isotopes), and atomic models.

Practise this question

Question

Question 3 spans several sub-questions: 03.1 presents three decay equations for uranium decaying into thorium by alpha emission to choose from. Figure 4 shows a decay curve of percentage of uranium atoms against time in days, followed by 03.2 and 03.3 asking for the time for 50% to decay and the half-life. Figure 5 compares decay curves of three isotopes A, B, and C, with 03.4 asking which is most unstable and why. Figure 6 shows four diagrammatic nuclei (A, B, C, D) made of shaded circles representing protons and unshaded circles representing neutrons, followed by questions 03.5 to 03.7 regarding mass number, atomic number, and identifying isotopes. Finally, 03.8 and 03.9 ask multiple-choice questions about the development of the nuclear model.

Mark scheme

Show the mark scheme Mark scheme for Question 3 with 11 marks total. 03.1 gives 230/92 U -> 226/90 Th + 4/2 He (1 mark). 03.2 gives 20 days (1 mark). 03.3 gives 20 days, allowing error carried forward (1 mark). 03.4 awards 1 mark for isotope A and 1 mark for shortest half-life / decays quickest / steepest initial graph. 03.5 gives A (1 mark). 03.6 gives D (1 mark). 03.7 awards 1 mark for B and C, and 1 mark for having the same number of protons and different number of neutrons. 03.8 gives plum pudding model (1 mark). 03.9 gives the mass of the atom is concentrated in the nucleus (1 mark).

How to answer it

Radioactive Decay, Half-Life & Atomic Models

WHAT THIS QUESTION TESTS

This question assesses your fundamental knowledge across AQA Topic 4.4 (Atomic Structure and Radiation):

  • Writing and identifying balanced alpha decay equations.
  • Extracting half-life values accurately from radioactive decay curves.
  • Connecting nuclear instability to the rate of decay and half-life length.
  • Deducing atomic number, mass number, and identifying isotopes from particle diagrams.
  • Recalling key milestones in the development of the nuclear model of the atom.

Part 03.1: Alpha Decay Equation

Balancing mass and atomic numbers in radioactive decay

✅ Correct Answer

Tick the first box:

²³⁰₉₂U → ²²⁶₉₀Th + ⁴₂He

💡 Key Knowledge

  • An alpha particle (α) is a helium nucleus: ⁴₂He (2 protons, 2 neutrons).
  • In any nuclear equation, both the mass numbers (top) and the atomic numbers (bottom) must balance on both sides:
    Top: 230 = 226 + 4
    Bottom: 92 = 90 + 2

❌ Common Errors

  • Confusing decay with fusion (e.g. picking ²³⁰U + ²²⁶Th → ⁴He).
  • Reversing parent and daughter nuclei (²²⁶Th → ²³⁰U + ⁴He), which would require creating mass from nowhere.
Mark Scheme: 1 mark for selecting the correct equation (²³⁰₉₂U → ²²⁶₉₀Th + ⁴₂He).

Parts 03.2 & 03.3: Reading Half-Life from a Graph

Using Figure 4 to find half-life

📐 Step-by-Step Method (Part 03.2)

  1. Find the starting value: The graph begins at 100% uranium atoms at time t = 0.
  2. Find 50% decay: A 50% drop means 50% of the original atoms remain. Locate 50% on the vertical y-axis.
  3. Read horizontally to the curve: Follow the grid line across from 50% until it hits the curve.
  4. Read down to the x-axis: Drop straight down to read the time on the horizontal axis: exactly 20 days.

✅ Correct Answers

03.2 Time taken: 20 days

03.3 Half-life: 20 days

Note: Error carried forward (ecf) is allowed from 03.2 into 03.3. If you wrote 20 days in 03.2, you must write 20 days in 03.3.

💡 Key Definition

The half-life of a radioactive isotope is the time it takes for:

  • the number of nuclei of the isotope in a sample to halve, OR
  • the count rate (or activity) from a sample to fall to half its initial level.

🧠 Exam Technique

Always draw pencil lines directly onto the exam paper graph: one line horizontally from 50% to the curve, and a vertical line down to the time axis. This prevents misreading grid squares under exam pressure.

Mark Scheme: 03.2: 1 mark for 20 (days). 03.3: 1 mark for 20 (days) [allow ecf from 03.2].

Part 03.4: Comparing Nuclear Stability

Linking half-life and rate of decay to instability

✅ Correct Answer

Tick box: Isotope A

Reason (any one):

  • It has the shortest half-life (its 50% level is reached at ~4 days, compared to ~10 days for C and >20 days for B).
  • It decays the quickest / fastest.
  • Its graph is the steepest initially.
  • It has the smallest percentage of atoms left at any given time after t = 0.

💡 Key Knowledge

Instability = Rapid Decay: An unstable nucleus has a high probability of decaying in a given time. Therefore, the more unstable an isotope is, the more rapidly it decays and the shorter its half-life.

❌ Common Errors & Examiner Notes

  • Dependent Mark: Mark 2 (the reason) depends on correctly choosing Isotope A. If you tick B or C, you score 0 marks even if your explanation mentions half-life.
  • Confusing stable with unstable: Students often incorrectly choose B because it "lasts the longest" – but lasting longest means it is the most stable, not the most unstable!
Mark Scheme: 1 mark for ticking Isotope A; 1 mark for valid reason (e.g. shortest half-life / decays quickest).

Parts 03.5 – 03.7: Subatomic Particles & Isotopes

Interpreting nuclear diagrams (Figure 6)

Particle breakdown from Figure 6:
• Nucleus A: 1 proton (dark) + 2 neutrons (light) → Mass number = 3, Atomic number = 1
• Nucleus B: 3 protons (dark) + 2 neutrons (light) → Mass number = 5, Atomic number = 3
• Nucleus C: 3 protons (dark) + 3 neutrons (light) → Mass number = 6, Atomic number = 3
• Nucleus D: 4 protons (dark) + 3 neutrons (light) → Mass number = 7, Atomic number = 4

✅ Correct Answers

03.5 Smallest mass number: A

03.6 Atomic number of 4: D

03.7 Isotopes of same element: Nucleus B and Nucleus C

03.7 Reason: They have the same number of protons (or same atomic number), but a different number of neutrons.

💡 Key Knowledge

  • Atomic number: Total number of protons (dark circles). Defines which element it is.
  • Mass number: Total number of protons + neutrons (all circles combined).
  • Isotopes: Atoms of the same element (same proton number) with different numbers of neutrons (different mass numbers).

❌ Common Errors

  • Mixing up the key: counting white circles as protons instead of dark circles.
  • Mentioning electrons in 03.7: The question specifically shows nuclei; there are no electrons present. The mark scheme explicitly states: "ignore electrons".
  • Giving an incomplete definition: stating only "they have different neutrons" without confirming they have the same number of protons.
Mark Scheme:
• 03.5: 1 mark for A.
• 03.6: 1 mark for D.
• 03.7: 1 mark for B and C; 1 mark for reason (same number of protons / same atomic number).

Parts 03.8 & 03.9: Atomic Models History

Development of the model of the atom

✅ Correct Answers

03.8 Model replaced by the nuclear model:

Plum pudding model

03.9 Mass of the atom in the nuclear model:

The mass of the atom is concentrated in the nucleus.

💡 Key Timeline & Discoveries

  1. Dalton: Tiny spheres that cannot be divided.
  2. JJ Thomson (Plum Pudding): Ball of positive charge with negative electrons embedded throughout.
  3. Rutherford (Alpha Scattering Experiment): Replaced plum pudding with the nuclear model — discovered that mass and positive charge are concentrated in a tiny central nucleus.
  4. Bohr: Adapted the nuclear model by showing electrons orbit at specific distances (shells).

🧠 Exam Technique: Timeline Trap

Notice the order: The plum pudding model was replaced by the nuclear model. The Bohr model came after the nuclear model to refine it. Do not confuse which model replaced which!

Mark Scheme:
• 03.8: 1 mark for "plum pudding model".
• 03.9: 1 mark for "The mass of the atom is concentrated in the nucleus."

Topics

Physics · P4: Atomic Structure

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