AQA AS Level Physics Paper 2, June 2025: Question 32

1 mark · Easy difficulty · Multiple Choice

Identify the number of protons and neutrons in the resulting nucleus after oxygen-15 emits a beta-plus particle.

Practise this question

Question

Question 32 asks: 'A nucleus of oxygen-15 (mass number 15, atomic number 8) emits a beta-plus particle. Which row shows the number of protons and the number of neutrons in the resulting nucleus?' A table lists four options: Row A has 9 protons and 7 neutrons; Row B has 9 protons and 6 neutrons; Row C has 7 protons and 8 neutrons; Row D has 7 protons and 7 neutrons.

Mark scheme

Show the mark scheme Mark scheme for question 32 indicates the correct answer is C, with 7 protons and 8 neutrons, allocated 1 mark under assessment objective AO1.

How to answer it

Beta-Plus (β⁺) Decay of Oxygen-15

📋 What this question tests

This question assesses your understanding of positron emission (β⁺ decay) at the nuclear and nucleon levels, requiring you to determine nucleon numbers (proton number Z and neutron number N) following radioactive transmutation according to the conservation of nucleon and proton numbers.

Multiple Choice · Question 32 [1 Mark]

Nuclear Changes in Positron Emission

Identifying Proton and Neutron Numbers in the Daughter Nucleus

✅ Correct Answer

C: 7 protons, 8 neutrons

Mark Scheme: C (AO1 recall and application of radioactive decay laws). Award 1 mark for selecting C.

💡 Key Knowledge

  • Beta-plus (β⁺) particle: A positron, represented as ⁰₊₁e or ⁰₊₁β .
  • Nucleon change: Inside the nucleus, a proton converts into a neutron, releasing a positron and an electron neutrino:
    p → n + e⁺ + νₑ
  • Conservation laws:
    • Total nucleon number (A) remains constant (ΔA = 0).
    • Atomic number (Z) decreases by 1 (Z → Z − 1).
    • Neutron number (N) increases by 1 (N → N + 1).

📐 Step-by-Step Calculation

  1. Analyse parent nucleus (¹⁵₈O):
    • Protons: Z = 8
    • Nucleons: A = 15
    • Initial neutrons: N = A − Z = 15 − 8 = 7
  2. Write the nuclear equation:
    ¹⁵₈O → ¹⁵₇N + ⁰₊₁β + νₑ
  3. Determine daughter nucleus values:
    • Number of protons = 8 − 1 = 7
    • Number of neutrons = 15 − 7 = 8 (or 7 + 1 = 8 )
  4. Match to options:
    Row with Protons = 7 and Neutrons = 8 is C.

❌ Common Errors & Traps

  • Confusing β⁺ and β⁻ decay: Students often mistakenly treat this as β⁻ decay, where a neutron becomes a proton, leading to row A (9 protons, 7 neutrons) or B.
  • Forgetting mass number stays unchanged: Students selecting D (7 protons, 7 neutrons) incorrectly subtracted 1 from both the proton and nucleon counts (total A = 14 instead of 15).
  • Reading "neutrons" as "mass number": Mistaking the second column for mass number A rather than neutron number N = A − Z.

🧠 Exam Technique & Quick Check

Whenever you see a multiple-choice nuclear decay question, quickly write down a quick check table:

Particle / State Protons (Z) Neutrons (N) Mass Number (A)
Parent (¹⁵₈O) 8 7 15
Change (β⁺ emission) −1 +1 0
Daughter (¹⁵₇N) 7 8 15

Examiner Tip: Always verify that Protons + Neutrons = 15 for the daughter nucleus. In row D, 7 + 7 = 14 ≠ 15, instantly ruling it out!

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.