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CognitoAtomic structure and radioactivity
Key facts & equations
Structure of the atom
- Protons and neutrons in the nucleus. Electrons orbit in shells.
- Proton: +1 charge, mass 1. Neutron: 0 charge, mass 1. Electron: -1 charge, negligible mass.
- Atomic number (Z) = number of protons. Mass number (A) = protons + neutrons.
- Isotopes: same element, same protons, different neutrons. Same chemical properties.
- Strong nuclear force holds nucleus together (very short range — overcomes electrostatic repulsion).
Types of radioactive decay
- Alpha (α): helium nucleus (2 protons + 2 neutrons). Charge +2. Symbol ⁴₂He.
- Beta minus (β⁻): high-speed electron. Charge -1. Neutron → proton + electron + antineutrino.
- Gamma (γ): electromagnetic radiation (high-energy photon). No charge, no mass.
- Beta plus (β⁺): positron (Higher). Proton → neutron + positron + neutrino.
Properties of the three radiations
- Alpha: stopped by a few cm of air or thin paper. Highly ionising. Dangerous if inhaled.
- Beta: stopped by a few mm of aluminium. Moderately ionising.
- Gamma: never fully stopped; reduced by thick lead or concrete. Weakly ionising.
- Ionising ability: α >> β >> γ. Penetrating ability: γ >> β >> α.
- All three are deflected by electric and magnetic fields (alpha and beta, not gamma in field — wait: gamma has no charge — only α and β deflect).
Nuclear equations
- Alpha decay: mass number −4, atomic number −2. ²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂He.
- Beta minus decay: mass number unchanged, atomic number +1. ¹⁴₆C → ¹⁴₇N + ⁰₋₁e.
- Gamma emission: no change to mass or atomic number — nucleus just loses energy.
- Check: top numbers (mass) and bottom numbers (atomic) must balance on each side.
Exam questions — 4 questions · 11 marks · AQA 8463 style
⚡ Show all working — the AI marks against the AQA mark scheme. Method marks awarded for correct working even if the final answer is wrong.
1 markAlpha particleAQA 8463 style
What is an alpha particle?
A A high-speed electron
B An electromagnetic wave from the nucleus
C A helium nucleus — 2 protons and 2 neutrons
D A neutron emitted from the nucleus
1 markPenetration comparisonAQA 8463 style
Which type of radiation is most penetrating?
A Alpha
B Beta
C Gamma
D All three are equally penetrating
4 marksNuclear equationAQA 8463 P2 style
Radium-226 undergoes alpha decay to form radon. Write a balanced nuclear equation. Show that atomic number and mass number are conserved. (3 marks)
Hint: Alpha decay: subtract 4 from mass number, subtract 2 from atomic number. Check both numbers balance on each side.
+40 XP
1 markBeta decay atomic numberAQA 8463 style
Carbon-14 undergoes beta minus decay. What happens to the atomic number?
A Decreases by 1
B Increases by 1
C Stays the same
D Decreases by 2
Module complete! 🎉
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