AQA GCSE (9-1) Physics: Atomic Structure & Radioactivity

Key Takeaways for Atomic Structure & Radioactivity

1. Atomic Structure

Key Components

• Protons: Positively charged (+1), found in the nucleus.
• Neutrons: Neutral (0 charge), found in the nucleus.
• Electrons: Negatively charged (-1), orbit the nucleus.
• Nucleus: Contains protons and neutrons; radius ≈ 10−1510−15 m.
• Atom: Overall neutral (protons = electrons).

Atomic & Mass Numbers

• Atomic Number (ZZ): Number of protons (defines the element).
  • E.g., Carbon: Z=6Z=6 → 6 protons.
• Mass Number (AA): Protons + neutrons.
  • E.g., Carbon-14: A=14A=14 → 6 protons + 8 neutrons.

Isotopes

• Atoms of the same element with different neutrons.
• E.g., Carbon-12 (126C126​C) vs. Carbon-14 (146C146​C).

Example Calculation:

• Lithium atom: 3 protons, 4 neutrons.
  • Atomic number (ZZ) = 3.
  • Mass number (AA) = 3 + 4 = 7.

2. Nuclear Radiation

Types & Properties

Decay Equations

• Alpha Decay:
  92238U→90234Th+24He92238​U→90234​Th+24​He
  • Mass ↓4, Atomic ↓2.
• Beta Decay:
  614C→714N+−10e614​C→714​N+−10​e
  • Mass same, Atomic ↑1 (neutron → proton + electron).
• Gamma Decay: No change in mass/atomic number.

Tip: Balance equations by ensuring mass and atomic numbers are equal on both sides.

3. Half-Life & Radioactive Decay

• Half-Life (t1/2t1/2​): Time for half the nuclei in a sample to decay.
• Random Process: Cannot predict when a single nucleus will decay.

Example Calculation:

• A sample has an initial activity of 160 Bq. After 2 half-lives:
  • 160→80→40160→80→40 Bq.

Graph Tip: Plot count rate vs. time. The half-life is the time taken for the count rate to halve.

4. Uses of Radioactivity

• Medical Tracers:
  • E.g., Technetium-99 (γ emitter, t1/2t1/2​ = 6 hours) for imaging organs.
• Radiotherapy:
  • Gamma rays target cancerous cells.
• Sterilisation:
  • Gamma rays kill bacteria on medical equipment.

Safety: Use tongs, lead shielding, and monitor doses with film badges.

5. Hazards & Safety

• Ionisation: Radiation removes electrons from atoms, damaging cells.
• Dose Measurement: In sieverts (Sv).
  • 1 mSv = 1100010001​ Sv.
• Background Radiation: Natural sources (radon gas, cosmic rays) contribute ~2.4 mSv/year in the UK.

Key Risk: Alpha particles are most dangerous if ingested (e.g., radon gas in lungs).

6. Nuclear Fission & Fusion

Fission

• Process: Splitting heavy nuclei (e.g., 92235U92235​U) → energy + neutrons.
  92235U+01n→56141Ba+3692Kr+301n+energy92235​U+01​n→56141​Ba+3692​Kr+301​n+energy
• Chain Reaction: Neutrons trigger further fission (controlled in reactors).

Fusion

• Process: Combining light nuclei (e.g., hydrogen → helium).
  • Occurs in stars.
    211H→12H+energy211​H→12​H+energy

Key Difference:

• Fission releases energy from heavy atoms; fusion from light atoms.

Exam Tips

1. Equations: Always balance mass and atomic numbers.
2. Half-Life: Use Remaining=Initial×(12)nRemaining=Initial×(21​)n, where nn = number of half-lives.
3. Radiation Properties: Memorise penetration/ionising power (alpha = paper, beta = aluminium, gamma = lead).
4. Safety: Explain shielding methods (e.g., lead for gamma).

Common Mistake: Confusing neutron emission (mass ↓1, atomic same) with beta decay (atomic ↑1).

Example Question:
“A sample of iodine-131 (t1/2t1/2​ = 8 days) has an activity of 800 Bq. What is its activity after 24 days?”
Solution:
24 days = 3 half-lives.
800→400→200→100800→400→200→100 Bq.

50 GCSE Questions on Atomic Structure & Radioactivity

Questions

1. Atomic Structure:
   a) Name the three subatomic particles in an atom and state their charges.
   b) What is the approximate radius of an atom?
   c) How does the radius of a nucleus compare to the radius of an atom?
2. Isotopes:
   a) Define the term isotope.
   b) Carbon-12 and Carbon-14 are isotopes. How do their atomic structures differ?
3. Atomic & Mass Numbers:
   a) An atom has 6 protons and 8 neutrons. Calculate its mass number.
   b) The atomic number of sodium is 11. How many electrons does a neutral sodium atom have?
4. Nuclear Radiation:
   a) List the four types of nuclear radiation.
   b) Which type of radiation has the highest ionising power?
5. Alpha Decay:
   a) Write the equation for the alpha decay of Uranium-238 (92238U92238​U).
   b) How do the mass and atomic numbers change during alpha decay?
6. Beta Decay:
   a) Write the equation for the beta decay of Carbon-14 (614C614​C).
   b) Explain why beta decay increases the atomic number by 1.
7. Gamma Radiation:
   a) Why does gamma decay not change the mass or atomic number of a nucleus?
   b) State one use of gamma radiation in medicine.
8. Neutron Emission:
   a) Write the equation for the neutron emission of Helium-5 (25He25​He).
   b) How does neutron emission affect the mass number?
9. Ionisation:
   a) Explain how alpha particles ionise atoms.
   b) Why is ionisation harmful to living cells?
10. Geiger-Müller Tube:
    a) What is the purpose of a Geiger-Müller (GM) tube?
    b) Define background radiation.
11. Penetration Power:
    a) Which material stops gamma radiation: paper, aluminium, or lead?
    b) Why can alpha particles be stopped by paper?
12. Half-Life:
    a) Define half-life.
    b) A sample has a half-life of 5 years. What fraction remains after 15 years?
13. Carbon Dating:
    a) Explain how carbon-14 is used to determine the age of ancient artifacts.
    b) Why is carbon dating unreliable for objects older than 50,000 years?
14. Radiation Dose:
    a) What unit is used to measure radiation dose?
    b) A dose of 4 Sv is considered fatal. Convert 4000 mSv to Sv.
15. Medical Tracers:
    a) Why is technetium-99 used as a medical tracer?
    b) State two properties that make it suitable for this use.
16. Radiotherapy:
    a) How are gamma rays used in cancer treatment?
    b) Why is a high dose of radiation dangerous even in targeted therapy?
17. Irradiation vs Contamination:
    a) Distinguish between irradiation and contamination.
    b) Why is contamination more hazardous than irradiation?
18. Nuclear Fission:
    a) Define nuclear fission.
    b) Write the equation for the fission of Uranium-235 (92235U92235​U) into Barium-141 (56141Ba56141​Ba) and Krypton-92 (3692Kr3692​Kr).
19. Chain Reaction:
    a) What is a chain reaction in nuclear fission?
    b) How do control rods in a nuclear reactor regulate this process?
20. Nuclear Fusion:
    a) Define nuclear fusion.
    b) Why is fusion difficult to achieve on Earth?
21. Rutherford’s Experiment:
    a) Describe how Rutherford’s gold foil experiment disproved the plum pudding model.
    b) What conclusion did Rutherford draw about the structure of the atom?
22. Bohr Model:
    a) How did Bohr’s model improve upon Rutherford’s?
    b) What happens when an electron absorbs electromagnetic radiation?
23. Plum Pudding Model:
    a) Describe the plum pudding model of the atom.
    b) Why was it replaced?
24. Background Radiation:
    a) List three natural sources of background radiation.
    b) Why is radon gas a significant hazard?
25. Radiation Protection:
    a) Why do nuclear workers wear film badges?
    b) What safety precautions should be taken when handling alpha sources?

Detailed Answers

1. Atomic Structure:
   a) Protons (+1), neutrons (0), electrons (-1).
   b) Radius ≈ 10−1010−10 m.
   c) The nucleus is about 110,00010,0001​ the size of the atom.
2. Isotopes:
   a) Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
   b) Carbon-12 has 6 neutrons; Carbon-14 has 8 neutrons.
3. Atomic & Mass Numbers:
   a) Mass number = 6 + 8 = 14.
   b) Electrons = protons = 11.
4. Nuclear Radiation:
   a) Alpha, beta, gamma, neutron.
   b) Alpha particles (high ionisation due to large mass and charge).
5. Alpha Decay:
   a) 92238U→90234Th+24He92238​U→90234​Th+24​He
   b) Mass ↓4, atomic ↓2.
6. Beta Decay:
   a) 614C→714N+−10e614​C→714​N+−10​e
   b) A neutron converts to a proton (↑ atomic number by 1) and emits an electron.
7. Gamma Radiation:
   a) Gamma rays are electromagnetic waves; they only remove energy, not particles.
   b) Sterilising medical equipment or cancer treatment.
8. Neutron Emission:
   a) 25He→24He+01n25​He→24​He+01​n
   b) Mass number ↓1; atomic number unchanged.
9. Ionisation:
   a) Alpha particles collide with atoms, knocking off electrons and creating ions.
   b) Ions disrupt chemical reactions in cells, causing mutations or cell death.
10. Geiger-Müller Tube:
    a) Detects ionising radiation by counting particles entering the tube.
    b) Background radiation is natural radiation from rocks, cosmic rays, etc.
11. Penetration Power:
    a) Lead.
    b) Alpha particles are large and highly charged, losing energy quickly.
12. Half-Life:
    a) Time taken for half the radioactive nuclei in a sample to decay.
    b) 1881​ remains (3 half-lives: 12→14→1821​→41​→81​).
13. Carbon Dating:
    a) Living organisms absorb carbon-14. After death, it decays. By measuring remaining 14C14C, age is calculated.
    b) Too little 14C14C remains to measure accurately.
14. Radiation Dose:
    a) Sieverts (Sv).
    b) 4000 mSv=4 Sv4000mSv=4Sv.
15. Medical Tracers:
    a) Emits gamma rays (detectable outside the body) and has a short half-life (6 hours).
    b) Low ionisation, short half-life.
16. Radiotherapy:
    a) Gamma rays destroy cancer cells by ionising their DNA.
    b) High doses damage healthy cells, causing side effects.
17. Irradiation vs Contamination:
    a) Irradiation: Exposure to radiation externally. Contamination: Radioactive material enters the body.
    b) Contamination exposes internal organs to prolonged radiation.
18. Nuclear Fission:
    a) Splitting a heavy nucleus into smaller nuclei, releasing energy.
    b) 92235U+01n→56141Ba+3692Kr+301n+energy92235​U+01​n→56141​Ba+3692​Kr+301​n+energy
19. Chain Reaction:
    a) Neutrons from fission trigger further fission events.
    b) Control rods absorb excess neutrons to slow the reaction.
20. Nuclear Fusion:
    a) Combining light nuclei to form a heavier nucleus, releasing energy.
    b) Requires extremely high temperatures/pressure (like in stars).
21. Rutherford’s Experiment:
    a) Most alpha particles passed through, but some deflected, indicating a small, dense nucleus.
    b) Atoms have a tiny, positively charged nucleus surrounded by empty space.
22. Bohr Model:
    a) Electrons orbit in fixed energy levels, explaining atomic spectra.
    b) Electron moves to a higher energy level (excited state).
23. Plum Pudding Model:
    a) A “sea” of positive charge with electrons embedded.
    b) It couldn’t explain alpha particle scattering results.
24. Background Radiation:
    a) Radon gas, cosmic rays, rocks, food.
    b) Radon emits alpha particles; inhaled, it damages lung tissue.
25. Radiation Protection:
    a) To monitor cumulative radiation exposure.
    b) Use tongs, avoid ingestion, store in lead containers.