This mini-lesson walks you through the whole of CCEA Atomic & Nuclear Physics: the structure of the atom, isotopes and nuclide notation, how the atomic model developed, the three types of nuclear radiation, background radiation, decay equations, half-life, the uses and dangers of radioactivity, and fission & 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.
Structure of the atom
Inside the atom
Every atom has a tiny central nucleus containing protons and neutrons, with electrons orbiting in shells. The nucleus is around 10,000 times smaller than the whole atom, yet holds nearly all of its mass.
Protons (red) and neutrons (blue) sit in the nucleus; electrons orbit in shells.
The three particles have relative charges and relative masses you must learn:
proton +1, mass 1 · neutron 0, mass 1 · electron −1, mass ≈ 1/2000An atom is neutral overall: number of protons = number of electrons.
Quick check
Which particle?
?Which subatomic particle has a relative charge of 0 and a relative mass of 1?
Atomic number, mass number & isotopes
Counting the nucleus
Atomic (proton) number, Z — the number of protons. It defines the element.
Mass (nucleon) number, A — the total number of protons and neutrons.
Number of neutrons = A − Z.
Carbon-12 and carbon-14 are isotopes: same protons, different neutrons.
Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons — so they have the same atomic number but different mass numbers.
Calculate
Your turn — counting neutrons
1A nucleus of strontium is written as ⁹⁰₃₈Sr. How many neutrons does it contain?
neutrons
Hint: neutrons = mass number − atomic number = 90 − 38.
Development of the atomic model
How the model changed
Our picture of the atom was rebuilt by experiment:
Plum pudding (Thomson): a ball of positive charge with electrons dotted through it — no nucleus.
Rutherford's alpha-scattering: alpha particles fired at thin gold foil. Most passed straight through, a few were deflected and a tiny number bounced almost straight back.
Nuclear model: the results showed the atom is mostly empty space with a tiny, dense, positively charged nucleus at the centre — replacing the plum pudding model.
Alpha-scattering: rebounds reveal a tiny, dense, positive nucleus.Quick check
Reading the experiment
?In Rutherford's experiment, a tiny number of alpha particles bounced almost straight back. What did this show?
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?
Three types of nuclear radiation
Alpha, beta and gamma
Unstable nuclei become more stable by emitting radiation. CCEA needs all three:
Alpha (α) — a helium nucleus (2 protons + 2 neutrons), charge +2. Most ionising, but least penetrating.
Beta (β) — a fast electron from the nucleus, charge −1. Moderately ionising and penetrating.
Gamma (γ) — a high-energy electromagnetic wave, no charge, no mass. Least ionising, but most penetrating.
Penetration: α stopped by paper, β by aluminium, γ needs thick lead (or concrete).
Watch out — a classic trap: alpha is the most ionising but the least penetrating. Being big and highly charged, it collides quickly and is stopped fast. Don't mix "ionising" up with "penetrating" — they run in opposite order.
Sort it
Name the radiation
Tap the type of radiation each clue describes.
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?
Charge & deflection
Deflecting the radiation
Because α and β are charged, they are deflected by electric and magnetic fields — in opposite directions, because their charges are opposite. Gamma has no charge, so it is not deflected at all.
α bends one way, β the opposite way (and turns more, being lighter); γ is undeflected.
Beta bends more than alpha in the same field because a beta particle has a much smaller mass, even though its charge is smaller.
Background radiation & detection
The radiation around us
Background radiation is the low-level radiation that is around us all the time. Its main sources are:
Radon gas from rocks and soil (the largest source in many areas).
Rocks & buildings (e.g. granite) and food & drink.
Cosmic rays from space and the Sun.
Medical sources (X-rays, scans) and a small amount from nuclear weapons testing and the nuclear industry.
A Geiger-Müller (G-M) tube clicks faster when more radiation reaches it.
Always subtract the background: a detector reads the source plus background. To find the true count rate of a sample you must measure the background separately and take it away from your reading. This is the corrected count rate.
Calculate
Your turn — corrected count rate
2A sample is measured at 540 counts/minute. With the sample removed, the background reads 30 counts/minute. What is the corrected count rate of the sample?
When a nucleus decays, the total mass number and total atomic number are conserved (the same before and after). That lets you work out the new nucleus.
Alpha decay: AX → A−4Y + 4₂αThe nucleus loses 2 protons & 2 neutrons: mass number −4, atomic number −2.
Beta decay: AX → AY + 0₋₁βA neutron becomes a proton: mass number unchanged, atomic number +1.
Check both numbers balance: the top numbers (mass) must add up, and the bottom numbers (atomic) must add up, on each side.
Calculate
Your turn — balance the decay
3Carbon-14 undergoes beta decay to nitrogen: ¹⁴₆C → ?₇N + ⁰₋₁β. What is the mass number of the nitrogen nucleus?
mass number
Hint: in beta decay the mass number is unchanged; the beta particle has mass number 0.
Mission 3 of 4
Where did the radioactivity go?
Make a guess: where did the tracer's radioactivity go?
Half-life
Half-life
Decay is random — you can't say when a single nucleus will decay. But for a large sample the rate is predictable:
Half-life = time for half the undecayed nuclei to decay(equivalently, the time for the count rate to fall to half its value)
Measured count rates include background radiation, which is around us all the time, so subtract the background count before finding the half-life.
Each 2 hours the count rate halves (800 → 400 → 200 → 100). Here the half-life is 2 hours.
Half-life is probabilistic: after one half-life roughly half the nuclei have decayed, but you can never say which ones. The decay curve never quite reaches zero — it just keeps halving.
Calculate
Your turn — read the half-life
4A source starts at 2400 Bq. After 15 minutes it has dropped to 300 Bq. What is its half-life?
minutes
Hint: 2400 → 1200 → 600 → 300 is 3 halvings in 15 min. Half-life = 15 ÷ 3.
Calculate
Your turn — after n half-lives
5An isotope has a half-life of 6 days. A sample starts with an activity of 3200 Bq. What is its activity after 24 days?
Uses of radioactivity match the half-life and penetration to the job:
Smoke detectors — a weak alpha source (alpha is easily blocked, so it is safe in the home).
Medical tracers & treatment — gamma sources to image inside the body or to destroy cancer cells.
Carbon dating — the known half-life of carbon-14 dates ancient once-living material; other isotopes date rocks.
Thickness gauges (beta for paper) and sterilising equipment with gamma rays.
Dangers: radiation is ionising, so it can damage or kill cells, cause mutations and cancer. To stay safe: keep shielded (lead/concrete), keep your distance, limit time of exposure, and use tongs/handle sources remotely.
Irradiation vs contamination: irradiation is being exposed to radiation from a source (it stops when the source is removed); contamination is getting radioactive material on or in you, which keeps irradiating you until it is removed.
Match up
Match the radiation to the job
Tap a use on the left, then its best-suited radiation on the right.
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
Splitting heavy nuclei
Nuclear fission is the splitting of a large, unstable nucleus (such as uranium-235) when it absorbs a neutron. It splits into two smaller "daughter" nuclei, releases energy, and throws out 2 or 3 more neutrons.
Each fission releases neutrons that split more nuclei — a chain reaction.
In a reactor, control rods absorb spare neutrons to keep the chain reaction steady (one neutron causing the next fission), while a moderator slows neutrons down so they are more likely to be absorbed by uranium-235 nuclei and cause fission.
Quick check
Controlling the reactor
?What is the job of the control rods in a nuclear reactor?
Nuclear fusion
Joining light nuclei
Nuclear fusion is the joining of two light nuclei (e.g. hydrogen isotopes) to make a heavier nucleus, releasing huge amounts of energy. It is the process that powers the Sun and other stars.
Two hydrogen nuclei fuse into helium, releasing energy — fusing hydrogen into helium powers the Sun.
Don't confuse them:fission splits a heavy nucleus; fusion joins light nuclei. Fusion needs enormous temperatures and pressures, which is why it is so hard to use on Earth.
Quick check
Fission or fusion?
?Two small nuclei join together to form a larger nucleus, releasing energy. Which process is this, and where does it naturally occur?
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?
Recap
The big ideas to know
Atom: nucleus (protons +1, neutrons 0) + electrons (−1); Z = protons, A = nucleons, neutrons = A − Z.