Knowledge Organiser: Radioactive Decay
Key Terms
- Alpha (α): 2p + 2n, charge +2
- Beta (β): Electron, charge −1
- Gamma (γ): EM wave, no charge
- Ionisation: Removing electrons from atoms
Key Facts
- Alpha: stopped by paper; most ionising
- Beta: stopped by aluminium
- Gamma: reduced by thick lead
- High ionising = low penetrating
- Decay is random and spontaneous
Nuclear Equation Changes
- Alpha: A − 4, Z − 2
- Beta: A same, Z + 1
- Gamma: A same, Z same
- Alpha + beta → element changes
Exam Tips
- Balance both A and Z in equations
- Alpha particle = ⁴₂He
- Beta particle = ⁰₋₁e
- Gamma = most penetrating, least ionising
Key Equations
- Alpha decay: A decreases by 4, Z decreases by 2
- Beta decay: A unchanged, Z increases by 1
- Gamma decay: A and Z both unchanged (energy released only)
- Activity (Bq) = number of decays per second
Common Mistakes
- Not balancing nuclear equations: Both the mass numbers (top) and atomic numbers (bottom) must balance on each side of the equation — always check both separately
- Confusing ionising power and penetrating power: Alpha is most ionising but least penetrating; gamma is least ionising but most penetrating — these properties are inversely related
- Saying gamma radiation changes the nucleus: Gamma emission releases energy but does not change the proton or neutron numbers — A and Z are both unchanged after gamma emission
- Confusing beta decay with electron emission from shells: Beta particles are emitted from the nucleus (a neutron converts to a proton + electron) — not from electron shells
- Forgetting that radioactive decay is random: Radioactive decay is spontaneous and random — it cannot be triggered or prevented by chemical reactions, temperature, or pressure
Memory Aid
- Alpha: A goes down 4, Z goes down 2 (loses a "he-4" helium nucleus)
- Beta: Z goes up 1, A stays same (neutron becomes proton: +1 proton)
- Gamma: Nothing changes in nucleus (just energy release)