This mini-lesson walks you through the key ideas of Edexcel Topic 6 — Radioactivity: the structure of the atom, how our model of the atom was built, the types of radiation, nuclear equations, activity and half-life, the difference between irradiation and contamination, and finally fission and fusion.
Make a guess: what is the inside of an atom like?
Mission 1What must the inside of an atom be like?
Mission 2How can the easiest-stopped radiation be worst?
Mission 3Where did the radioactivity go?
Mission 4How is a chain reaction kept steady?
Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Press Start when you're ready.
The atom
Inside the atom
An atom is a tiny positive nucleus of protons and neutrons, surrounded by negative electrons orbiting at fixed distances. Almost all the mass sits in the nucleus, yet the nucleus is tiny compared with the whole atom.
A labelled atom (lithium-7: 3 protons, 4 neutrons, 3 electrons). If the atom were a stadium, the nucleus would be a pea in the centre — the rest is mostly empty space.
Proton: charge +1, relative mass 1
Neutron: charge 0, relative mass 1
Electron: charge −1, relative mass ≈ 1/1836 (about 0.0005)
Positron: charge +1, relative mass ≈ 1/1836 (same as an electron)
Sense of scale: the nuclear radius (~10⁻¹⁴ m) is about 10 000 times smaller than the atomic radius (~10⁻¹⁰ m). Atoms and small molecules are of the order of 10⁻¹⁰ m across.
Quick check
Which particle?
?Which subatomic particle has a relative charge of −1 and a relative mass of only about 0.0005?
Notation & isotopes
Atomic number, mass number, isotopes
Every nucleus is described by two numbers, written around its symbol:
Carbon-14: A = 14, Z = 6, so it has 6 protons and 14 − 6 = 8 neutrons.
Atomic number Z = number of protons (this fixes which element it is).
Mass number A = number of protons + neutrons.
Number of neutrons = A − Z.
Isotopes are atoms of the same element (same Z, same protons) with different numbers of neutrons — so they have a different mass number A. Example: carbon-12, carbon-13 and carbon-14 all have 6 protons.
Calculate
Your turn — counting neutrons
1A uranium nucleus is written as uranium-235 with atomic number 92. How many neutrons does it contain?
neutrons
Hint: neutrons = mass number − atomic number = 235 − 92.
Development of the model
How the model of the atom changed
Edexcel wants you to know how and why the atomic model was rebuilt as new evidence arrived:
Dalton (1803): matter is made of tiny, solid, indivisible atoms.
Thomson (1897): discovered the electron → the plum-pudding model (a ball of positive charge with electrons dotted through it).
Rutherford (1909–11): the alpha-scattering experiment → most of the atom is empty space, with a tiny, dense, positive nucleus (the nuclear model).
Bohr (1913): electrons orbit the nucleus in fixed energy levels (shells) at set distances — this fixed a flaw in Rutherford's model.
Chadwick (1932): discovered the neutron, completing the picture of the nucleus.
Why it changed: each new model was proposed only because an experiment produced results the old model could not explain — this is how science works.
Key experiment
The alpha-scattering experiment
Geiger and Marsden (directed by Rutherford) fired alpha particles at a very thin gold foil and recorded where they went:
The results and what each one told Rutherford.
Most went straight through → the atom is mostly empty space.
Some were deflected → the centre is positively charged (it repels the positive alpha).
A very few bounced back > 90° → the mass and positive charge are concentrated in a tiny, dense nucleus.
This destroyed the plum-pudding model: if charge were spread out evenly, the alpha particles would never have bounced straight back.
Quick check
Reading the experiment
?In the alpha-scattering experiment, what does the fact that a very small number of alpha particles bounced straight back tell us?
Mission 1 check
What must the inside of an atom be like?
?A model atom is drawn to scale, as wide as a football pitch. Which description of the inside of the atom is correct?
Mission 2 of 4
How can the easiest-stopped radiation be worst?
Make a guess: how can radiation stopped by paper be the most harmful?
Types of radiation
Alpha, beta, gamma — and more
An unstable nucleus becomes more stable by emitting ionising radiation. Edexcel lists five emissions:
Alpha (α) — a helium nucleus (2 protons + 2 neutrons). Charge +2. Most ionising, least penetrating.
Beta-minus (β⁻) — a fast electron from the nucleus (a neutron turns into a proton). Charge −1. Medium ionising / medium penetrating.
Beta-plus (β⁺) — a positron (a proton turns into a neutron). Charge +1.
Gamma (γ) — high-energy electromagnetic radiation (no mass, no charge). Least ionising, most penetrating.
Neutron (n) — a neutral particle, can also be emitted from an unstable nucleus.
The big trade-off: the radiation that ionises the most (alpha) penetrates the least, because it dumps all its energy quickly. Gamma ionises least but travels furthest.
Penetration & range
How far each one gets
Penetration increases α → β → γ. Ionising power runs the other way: α > β > γ.
Alpha: stopped by paper / a few cm of air. Range in air: a few cm.
Beta: stopped by a few mm of aluminium. Range in air: tens of cm.
Gamma: only reduced by thick lead or concrete — never fully stopped. Range: very large.
Sort it
Which radiation is it?
Tap the radiation that matches each clue.
Quick check
Mind the misconception
?A student says: "Gamma must be the most dangerous because it travels furthest, so it must be the most ionising too." Which statement is correct?
Mission 2 check
How can the easiest-stopped radiation be worst?
?A lab source gives out only alpha radiation. A single sheet of paper stops all of it. Yet technicians handle it with great care. What makes alpha so harmful where it is absorbed?
Nuclear equations
What decay does to the numbers
In any nuclear equation, the mass numbers must balance and the atomic numbers must balance on both sides.
α decay: A → A − 4, Z → Z − 2an alpha particle is ⁴₂He, so the nucleus loses 2 protons and 2 neutrons
β⁻ decay: A stays, Z → Z + 1a neutron becomes a proton, emitting an electron (₋₁⁰e)
Beta-plus (β⁺): a proton becomes a neutron and a positron (⁰₊₁e) is emitted, so A stays the same and Z → Z − 1. Gamma emission carries away energy only — A and Z are unchanged; it often follows alpha or beta decay as the new nucleus rearranges. Neutron emission: A → A − 1, Z unchanged.
Calculate
Your turn — balancing alpha decay
2Polonium-210 (atomic number 84) emits an alpha particle. What is the mass number of the new nucleus formed?
(mass number)
Hint: alpha decay lowers the mass number by 4 → 210 − 4.
Mission 3 of 4
Where did the radioactivity go?
Make a guess: where did the tracer's radioactivity go?
Activity & detection
Activity, becquerels and the GM tube
Radioactive decay is a random process — you cannot predict which nucleus will decay next, or exactly when. But with huge numbers of nuclei the behaviour is very predictable.
Activity is the number of nuclear decays per second in a source. It is measured in becquerels (Bq), where 1 Bq = 1 decay per second.
A Geiger–Müller (GM) tube clicks each time it absorbs radiation; the count rate measures how active the source is. Photographic film (as in a film badge) also detects radiation: it darkens when exposed.
Analogy for randomness: radioactive decay is like popcorn in a pan — you know roughly how many kernels will pop each second, but never which one pops next. Same as rolling thousands of dice and removing every six.
Half-life
Half-life
The half-life is the time taken for the number of undecayed nuclei (or the activity) to fall to half its starting value. It is constant for a given isotope.
Each half-life the amount halves: 100% → 50% → 25% → 12.5% … (never quite reaching zero).
Worked example — reading off a value
A sample starts with an activity of 800 Bq. Its half-life is 6 hours.
After 6 h → 400 Bq; after 12 h → 200 Bq; after 18 h → 100 Bq (that's 3 half-lives).
Higher Tier: you may be asked for the net decline as a ratio — e.g. after 3 half-lives, 800 → 100 Bq, so the net decline is (800 − 100)/800 = 7/8.
Calculate
Your turn — half-life
3A radioactive source has a half-life of 5 years and a starting activity of 240 Bq. What will its activity be after 15 years?
Bq
Hint: 15 years = 3 half-lives, so halve three times: 240 → 120 → 60 → ?
Calculate
Your turn — reading a half-life
4From a decay graph, a sample's activity falls from 2000 Bq to 250 Bq in 30 hours. What is its half-life?
hours
Hint: 2000 → 1000 → 500 → 250 is 3 halvings. 30 h ÷ 3 = ?
Background radiation
Background radiation
Background radiation is the low-level radiation that is around us all the time. It comes from both natural and man-made sources. Most of it is natural — the largest single source in the UK is radon gas from rocks.
The top three are natural; the bottom two are man-made.
Why it matters: when measuring a source's activity you must subtract the background count rate first to get the true reading.
Sort it
Natural or man-made?
Tap a source, then tap the box it belongs in.
🌿 Natural
🏭 Man-made
Irradiation vs contamination
Irradiation is NOT contamination
This is the most-tested misconception in Topic 6 — keep them separate:
Irradiation: an object is exposed to radiation from an outside source. The object does NOT become radioactive. It stops the moment the source is removed or shielded.
Contamination: radioactive atoms get onto or into an object (e.g. dust on skin, swallowed material). The object is now itself radioactive and keeps emitting until those atoms decay.
Left: radiation reaches the object from outside. Right: the radioactive atoms stay on it.
Protection: reduce exposure by shielding (lead/concrete), keeping your distance, and limiting the time near a source. Workers wear film badges and use tongs and lead aprons.
Quick check
Spot the difference
?Surgical instruments are sterilised by exposing them to gamma rays from a sealed source. The instruments are then safe to use immediately. Why?
Uses & dangers
Putting radiation to work — and its risks
The radiation type is chosen to match the job, using penetration and half-life:
Smoke alarms: an alpha source (americium-241) ionises air; smoke disrupts the current and sets off the alarm. Alpha is safe here because it can't escape the box.
Thickness gauges: a beta source — only beta is partly absorbed by thin paper/foil, so the detector reading shows the thickness.
Medical tracers & PET scans: a gamma (or positron) emitter with a short half-life so its activity falls quickly and the patient’s dose stays low.
Dangers: ionising radiation can kill cells or damage DNA, causing mutations that may lead to cancer. Inside the body, alpha is the most dangerous (highly ionising, all energy absorbed locally); outside the body, alpha is least dangerous because skin or paper stops it.
(The uses of radioactivity are Physics-only content, 6.28P; the dangers, 6.29, are for every student.)
Mission 3 check
Where did the radioactivity go?
?A tracer with a half-life of 6 hours is injected into a patient. One day later it gives out far less radiation, even though none has left the body. Why?
Mission 4 of 4
How is a chain reaction kept steady?
Make a guess: how is a reactor's chain reaction kept steady?
Nuclear fission
Nuclear fission & the chain reaction
Fission is the splitting of a large, unstable nucleus (e.g. uranium-235) into two smaller "daughter" nuclei, after it absorbs a neutron. It releases energy, gamma rays and two or three more neutrons.
Each fission releases neutrons that trigger further fissions — a chain reaction.
Control rods (boron) absorb excess neutrons to control the rate.
Moderator (water or graphite) slows neutrons so they can be absorbed and cause more fission.
(Fission, fusion and nuclear power are on the separate-science Physics paper, 1PH0.)
Nuclear fusion
Nuclear fusion
Fusion is the joining of two light nuclei (e.g. hydrogen isotopes) to form a heavier nucleus, releasing huge amounts of energy — the new nucleus has slightly less mass than the nuclei that formed it. Fusion is the energy source of stars, including the Sun.
Fusion needs enormous temperature and pressure to overcome the repulsion between the positive nuclei.
Why fusion is so hard on Earth: both nuclei are positive, so they repel each other. Only at the extreme temperatures and pressures found in stars can they get close enough to fuse — which is why a practical fusion power station is so difficult to build.
Mission 4 check
How is a chain reaction kept steady?
?The energy output of a nuclear reactor starts to rise too quickly. What should the operators do to bring the chain reaction back to a steady rate?
Don't mix them up
Fission vs fusion
Fission: a big nucleus splits into two smaller ones (used in today's nuclear power stations).
Fusion: two small nuclei join into a bigger one (powers the stars).
Both release energy, but in opposite directions on the chart of elements.
Memory hook: "FisSION = SPLIT in two (scission); fuSION = FUSE together." Fission starts with a neutron being absorbed; fusion needs star-like heat and pressure.
Recap
Topic 6 in one place
Atom: tiny positive nucleus (p + n), electrons in shells; nucleus ~10⁻¹⁴ m, atom ~10⁻¹⁰ m.
Numbers: Z = protons, A = protons + neutrons, neutrons = A − Z; isotopes share Z, differ in A.