Nuclear Decay — Flashcards & Quiz

Key Points

• Alpha (α) decay: emits a helium-4 nucleus. Mass number decreases by 4, atomic number by 2.
• Beta-minus (β⁻) decay: a neutron converts to a proton, emitting an electron and antineutrino. Mass number unchanged, atomic number +1.
• Beta-plus (β⁺) decay: a proton converts to a neutron, emitting a positron and neutrino. Mass number unchanged, atomic number –1.
• Gamma (γ) emission: high-energy photon released from an excited nucleus. Mass and atomic numbers both unchanged.
• Half-life (t½): time for half the radioactive nuclei to decay. N = N₀ · (½)^(t/t½) or N = N₀ e^(–λt) where λ = ln2/t½.
• Penetration: alpha stopped by paper; beta by thin aluminium; gamma needs lead or thick concrete. Ionising ability: alpha > beta > gamma.

Common Mistakes to Avoid

1. Writing alpha decay with the wrong mass number change — it decreases by 4 and atomic number by 2.
2. Confusing beta-minus (electron + antineutrino) with beta-plus (positron + neutrino).
3. Claiming gamma emission changes the element — it doesn't; only energy is released.
4. Forgetting that half-life is constant for each isotope — it does not depend on environment or amount.
5. Using the wrong form of the decay equation — exponential (N = N₀e⁻⁻ᵗ) vs half-life (N = N₀ · (½)^(t/t½)).

Exam Strategy

SACE Stage 2 nuclear decay questions ask you to (1) write decay equations, (2) calculate remaining nuclei or activity, or (3) compare radiation types. Method: balance mass numbers and atomic numbers on both sides of the equation, then apply half-life formulas for quantitative questions. For penetration/ionisation questions, remember the trade-off: alpha ionises heavily but travels short distances.

Revision Tip

Decay equations are pattern recognition — drill a Revizi deck with 10+ parent isotopes asking you to write alpha and beta decay products.