AQA AS Level Physics Paper 2, June 2025: Question 28

1 mark · Easy difficulty · Multiple Choice

Determine the number of down quarks and up quarks in a fluorine-19 nucleus.

Practise this question

Question

Question 28 asks: 'What is the number of down quarks and the number of up quarks in a nucleus of 19 9 F?' A multiple-choice table is shown with columns 'Number of down quarks' and 'Number of up quarks'. Option A has 28 down and 29 up; Option B has 29 down and 28 up; Option C has 37 down and 47 up; Option D has 47 down and 37 up. Each row contains a tick box on the right.

Mark scheme

Show the mark scheme Mark scheme for question 28 shows the correct answer is B, corresponding to 29 down quarks and 28 up quarks, testing AO1.

How to answer it

Determining Quarks in a Fluorine-19 Nucleus

What this question tests

This question evaluates your understanding of nuclear notation (proton and nucleon numbers), the quark composition of nucleons (protons and neutrons), and your ability to correctly tabulate and calculate composite subatomic particles without falling for column-inversion traps.

Question 28 • Multiple Choice • 1 Mark

Quark Count in ¹⁹₉F

AQA AS Physics • Particles and Radiation • Assessment Objective: AO1

✅ Correct Answer

B   (29 down quarks, 28 up quarks)

Mark Scheme Breakdown:
• 1 mark: Correctly selecting option B (AO1 recall & basic application).

💡 Key Knowledge

  • Nuclear structure of ¹⁹₉F:
    Proton number (atomic number, Z ) = 9
    Mass number (nucleon number, A ) = 19
    Neutron number ( N = A − Z ) = 19 − 9 = 10
  • Baryon quark structure:
    Proton = uud (2 up, 1 down)
    Neutron = udd (1 up, 2 down)

📐 Step-by-Step Calculation

  1. Determine the number of each nucleon:
    Protons = 9
    Neutrons = 19 − 9 = 10
  2. Calculate total down quarks ( d ):
    From 9 protons: 9 × 1 = 9 down quarks
    From 10 neutrons: 10 × 2 = 20 down quarks
    Total down quarks = 9 + 20 = 29
  3. Calculate total up quarks ( u ):
    From 9 protons: 9 × 2 = 18 up quarks
    From 10 neutrons: 10 × 1 = 10 up quarks
    Total up quarks = 18 + 10 = 28
  4. Sanity check:
    Each of the 19 nucleons contains 3 quarks: 19 × 3 = 57 total quarks.
    Check sum: 29 + 28 = 57 quarks. This immediately eliminates options C and D (which sum to 84)!

❌ Common Traps & Mistakes

  • Column Order Inversion (Option A): Many students calculate up quarks first (28) and naturally write it down in the first column, accidentally choosing A instead of B. Always read table headings carefully!
  • Swapping Quark Flavours: Confusing a proton ( uud ) with a neutron ( udd ) will reverse your results completely.
  • Misinterpreting Mass Number: Taking 19 to be the number of neutrons rather than the total number of nucleons leads to bizarre numbers like options C and D.

🧠 Exam Technique & Examiner Insight

  • Use the "Total Quarks" trick: Every baryon has 3 quarks. For an atom with mass number 19, the total quarks must be 19 × 3 = 57. Looking at the options:
    • A: 28 + 29 = 57
    • B: 29 + 28 = 57
    • C: 37 + 47 = 84 (impossible)
    • D: 47 + 37 = 84 (impossible)
    This eliminates 50% of choices in under 5 seconds!
  • Highlight column headings: In multiple-choice grids, underline the specific heading (down quarks first, up quarks second) to avoid careless slips under time pressure.

Topics

Physics · 3.2 Particles and radiation

Question and mark scheme from the AQA AS Level Physics examination, Paper 2, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.