AQA A-Level Physics Paper 2, June 2025: Question 27

✅ Correct Answer

Option D: ¹²³₅₃I + e⁻ → ¹²³₅₂Te + νₑ

💡 Key Knowledge: Why Electron Capture Causes Excitation

• Mechanism: A proton-rich nucleus absorbs an inner orbital electron (typically from the innermost K-shell):
  p + e⁻ → n + νₑ
• Vacancy Creation: The captured electron leaves an empty space (a vacancy) in an inner atomic energy level.
• Excited State: Because the daughter atom ( Te ) still possesses outer-shell electrons while having an empty inner-shell slot, its electron configuration is unstable and at a higher energy—it is in an electronically excited state.
• De-excitation: Outer electrons subsequently drop into the inner vacancy, emitting characteristic X-ray photons.

📐 Process Breakdown (Step-by-Step)

1. Identify Decay Types:
   A: Alpha (α) decay
   B: Beta-minus (β⁻) decay
   C: Beta-plus (β⁺) decay
   D: Electron capture
2. Examine the Electron Shells:
   In options A, B, and C, decay occurs purely inside the nucleus, ejecting particles away from the atom. They do not selectively remove an inner orbital electron.
3. Deduce Energy States:
   Only electron capture takes an electron directly from an inner shell ( n = 1 or n = 2 ).
4. Conclusion:
   The daughter atom has higher-level electrons remaining with a hole underneath, which defines an electronically excited atom.

❌ Common Errors & Traps

• Confusing Nuclear vs Atomic Excitation: Many students confuse an excited nucleus (which emits γ rays) with an excited atom / electron state (which emits characteristic X-rays or UV/visible photons). All decays can leave a nucleus excited, but electron capture uniquely leaves the electron cloud excited.
• Misting β⁻ Decay for Gaining an Electron: In β⁻ decay (Option B), the emitted electron is ejected from the nucleus at high speed; it is not captured into atomic orbitals.
• Ignoring the Reactants: Seeing + e⁻ on the left-hand side is the dead giveaway for orbital electron capture.