Overview
You should be able to:
- Describe the structure of an atom and the nucleus.
- Know charge and relative mass of protons, neutrons and electrons.
- Explain isotopes and radioactive decay as a random process.
- Compare α, β and γ radiation – charge, mass, penetration, ionisation.
- Write simple nuclear equations for α and β decay.
- Explain half-life and read half-life graphs.
- Describe background radiation, uses and dangers of radioactivity.
- Outline nuclear fission in power stations and the idea of fusion in stars.
Most marks here are for clear explanations and being organised with your facts.
1. Atomic Structure & Isotopes
Inside the Atom
- The atom has a tiny, dense nucleus in the centre, surrounded by electrons.
- The nucleus contains protons and neutrons.
Relative charge:
- proton: +1
- neutron: 0
- electron: −1
Relative mass:
- proton: 1
- neutron: 1
- electron: ~0 (very small compared to proton/neutron)
Atomic number Z = number of protons. Mass number A = number of protons + neutrons.
Isotopes
Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons.
Example: carbon-12 and carbon-14 both have 6 protons, but 6 vs 8 neutrons.
Atom Diagram
In nuclear notation: AZX – A = mass number, Z = atomic number.
Quick Check
Q1. An atom of sodium has A = 23 and Z = 11. How many protons, neutrons and electrons does it have?
Show answer
Protons = 11 (Z). Neutrons = A − Z = 23 − 11 = 12. Electrons = 11 (neutral atom).
2. Radioactivity & Types of Radiation
Radioactive Decay
- Some nuclei are unstable – they can emit radiation and change into different nuclei.
- This is called radioactive decay.
- Decay is random – you cannot predict exactly when one nucleus will decay.
The 3 Main Types (IGCSE)
- Alpha (α)
- Helium nucleus: 2 protons, 2 neutrons.
- Charge: +2, relatively heavy, strongly ionising.
- Very short range in air (a few cm), stopped by paper or skin.
- Beta (β⁻)
- High-speed electron from nucleus.
- Charge: −1, light, moderately ionising.
- Range: several tens of cm in air, stopped by thin aluminium.
- Gamma (γ)
- High-energy electromagnetic wave (no mass, no charge).
- Weakly ionising, but very penetrating.
- Reduced by thick lead or many cm of concrete (but not completely stopped).
Penetration Diagram
α: stopped by paper. β: stopped by aluminium. γ: passes paper & Al, but is absorbed inside thick lead.
Nuclear Equations (Structure Only)
For alpha emission:
For beta-minus emission (neutron → proton + electron):
Quick Check
Q2. A nucleus of 21084Po emits an alpha particle. What is the new nucleus?
Show answer
New A = 210 − 4 = 206, new Z = 84 − 2 = 82 → 20682Pb.
3. Detecting Radiation & Background
Detecting Radiation
- Geiger–Müller (GM) tube + counter – clicks/counts each time radiation enters the tube.
- Photographic film – darkens when exposed (used in old film badges).
- Scintillation detectors – crystals that flash when hit by radiation.
In exam questions, you must always subtract the background count from your readings.
Background Radiation
Even with no source, there is always some radiation around us from:
- Cosmic rays from space.
- Rocks and soil (e.g. radon gas).
- Medical uses (X-rays, treatments).
- Past nuclear tests and nuclear industry.
Quick Check
Q3. A GM tube records 48 counts in 1 minute with a source present. Background is 12 counts per minute. What is the count from the source alone?
Show answer
48 − 12 = 36 counts per minute.
4. Half-life
What Is Half-life?
- Half-life is the time taken for:
- the number of radioactive nuclei (or mass of the sample) to halve, or
- the activity (count rate) to halve.
- Decay is random but predictable for large numbers of nuclei.
It is not “half the time until it all decays” – the sample never really reaches zero.
Half-life Graph (Halving Every Time Unit)
Simple Half-life Calculations
Q4. A source has a count rate of 800 counts/min. After one half-life it will be?
Show answer
800 → 400 counts/min.
Q5. A source has a half-life of 5 days. After 15 days its activity is 40 Bq. What was the original activity?
Show answer
15 days = 3 half-lives. So activity: start → ½ → ¼ → ⅛. 40 Bq = (1/8) of original → original = 40 × 8 = 320 Bq.
5. Uses & Dangers of Radioactivity
Useful Applications
- Medical tracers – radioactive isotopes injected or swallowed, used in imaging (must have short half-life and emit γ).
- Cancer treatment (radiotherapy) – β or γ used to kill cancer cells.
- Smoke alarms – α source ionises air; smoke reduces current, triggering alarm.
- Thickness monitoring – β source and detector used to control thickness of paper or metal foil.
- Sterilising equipment – γ used to kill bacteria on medical instruments and food.
Dangers & Safety
- Ionising radiation can damage cells and DNA, causing mutations and cancer.
- Irradiation – being exposed to radiation from an external source (body not made radioactive).
- Contamination – getting radioactive material inside or on you (dust, liquid, etc.).
- Contamination is dangerous because the source stays with you and keeps emitting.
Safety: use tongs, keep distance, limit time exposed, shielding (lead), wear film badges, follow strict protocols.
Quick Check
Q6. Explain the difference between irradiation and contamination in one or two sentences.
Show answer
Irradiation is exposure to radiation from outside the body; the body is not radioactive. Contamination is when radioactive material is on or inside the body, so it keeps exposing you.
6. Nuclear Fission & Fusion
Nuclear Fission (Splitting)
- In a nuclear reactor, a large nucleus (e.g. uranium-235) absorbs a neutron and becomes unstable.
- It splits into two smaller nuclei, releasing:
- energy,
- 2 or 3 more neutrons.
- These neutrons can cause more fissions → chain reaction.
- Control rods absorb neutrons to keep the reaction steady and safe.
Fission Diagram
Nuclear Fusion (Joining)
- Fusion is when two light nuclei join to form a heavier nucleus, releasing energy.
- Happens in stars (e.g. hydrogen nuclei fuse to make helium).
- Requires extremely high temperature and pressure to overcome repulsion between positive nuclei.
- On Earth, controlled fusion is still experimental (fusion reactors).
Fusion Diagram
Quick Check
Q7. Why do nuclear power stations use fission instead of fusion at the moment?
Show answer
Controlled fission in reactors is already practical; controlled fusion needs extremely high temperature and pressure and is not yet available as a reliable power source.
What Next?
To make this topic safe for the exam:
- Memorise the comparison table for α, β and γ (charge, mass, range, ionising ability, penetration).
- Practise 3–4 nuclear equations for alpha and beta decay.
- Do half-life graph questions from past papers until they feel automatic.
- Be ready to explain background radiation and the difference between irradiation and contamination in words.