Your revision route
What you’ll learn
- Describe atomic structure, scale, isotopes, ions and electron energy levels.
- Explain how scattering evidence changed the accepted atomic model.
- Compare alpha, beta and gamma radiation and select a source for a stated use.
- Balance single alpha- and beta-decay equations using mass and atomic numbers.
- Explain random decay, determine half-life and, at Higher Tier, calculate a net decline ratio.
- Compare irradiation and contamination, including suitable source precautions and peer review.
- Separate Physics: explain background radiation, half-life hazards and medical uses.
- Separate Physics: describe fission, chain reactions and fusion.
Build the big picture
Key ideas
The atom is mostly room
An atom's radius is about 1 × 10⁻¹⁰ m, while its nucleus is less than one ten-thousandth of that radius.
- The positively charged nucleus contains protons and neutrons and holds almost all the atom's mass.
- Negatively charged electrons sit in energy levels, each with a characteristic distance from the nucleus.
- Absorbing electromagnetic radiation can move an electron farther from the nucleus to a higher energy level; emission can move it closer to a lower level.
The punchline: Tiny nucleus, nearly all the mass; large electron region, nearly all the volume.
Two numbers identify the nucleus
Atomic number counts protons; mass number counts protons plus neutrons. Their difference gives the neutron count.
- A neutral atom has equal numbers of protons and electrons. All atoms of one element have the same proton number.
- Isotopes keep an element's proton count but have different neutron counts, so their mass numbers differ.
- An atom becomes a positive ion by losing one or more outer electrons; its nucleus is unchanged.
- In conventional notation, the symbol and lower-left Z identify the element, upper-left A identifies its isotope, and the upper-right superscript gives ion charge.
The punchline: Protons set the element; neutrons set the isotope; electrons set the ion's charge.
A few rebounding particles overturned a model
Most alpha particles crossed thin foil, but a few rebounded: the plum pudding model could not explain both. Learn what the later evidence changed, not Bohr's or Chadwick's procedures.
- Most particles passing through showed atoms are mostly empty space; rare large deflections put positive charge and most mass in a tiny nucleus.
- The nuclear model replaced plum pudding: diffuse positive charge became a tiny charged centre containing nearly all the mass.
- Bohr put electrons at fixed distances, and his model passed a crucial test: its predictions matched experimental measurements.
- Later evidence showed nuclear charge came in identical positive units—protons. Roughly two decades after scientists accepted the nucleus, Chadwick's results revealed neutrons.
The punchline: Write observation → inference → model change. That chain is the science.
Evidence kept shrinking the atom's centre
Each model survived only until an observation demanded a better one. Science changed the explanation rather than ignoring the awkward result.
- Solid sphereBefore electrons were known, atoms were modelled as tiny indivisible spheres.
- Plum puddingThe electron's discovery placed negative electrons inside a spread-out positive charge.
- Nuclear modelRare large alpha deflections revealed a tiny, charged centre containing most of the mass.
- Energy levelsBohr's fixed electron distances agreed with observations; later evidence added protons and neutrons.
One decay is unpredictable; a large sample has a pattern
The decay time of one unstable nucleus cannot be predicted or changed by ordinary physical conditions, but large samples show measurable patterns.
- Activity is the decay rate of a source in becquerels; 1 Bq means one decay per second.
- Count rate is detector counts per second and depends on detector efficiency and geometry. Separate Physics: background radiation also contributes.
- Alpha is two protons plus two neutrons; beta-minus is a fast electron from the nucleus; gamma is an electromagnetic wave from the nucleus.
- A neutron may also be emitted, but required comparisons of penetration, range and ionisation focus on alpha, beta and gamma.
The punchline: Activity belongs to the source; count rate belongs to the detector reading.
Nuclear equations keep two ledgers
Balance the mass numbers across one line, then balance the atomic numbers across a second line.
- Alpha emission lowers mass number by 4 and atomic number by 2.
- Beta-minus emission leaves mass number unchanged and raises atomic number by 1.
- Gamma emission changes neither number.
- Recognise and use common alpha (⁴₂He) and beta-minus (⁰₋₁e) symbols when balancing one decay; the exact worked examples need not be recalled, and daughter elements need not be identified.
The punchline: Never add mass number and atomic number together; conserve each column separately.
Make the model move
Interactive checkpoint
Touch the science. Change a state, build a route or test a relationship.
Balance the nucleus
Match each emission to the nuclear change
Pair the emission with its effect on mass number and atomic number.
Nuclear equations conserve both totals. Alpha carries away two protons and two neutrons; beta-minus decay converts a neutron to a proton; gamma carries no charge or nucleons.
Watch the crowd halve
Explore repeated half-lives
Choose how many half-lives have passed. Track the expected fraction remaining, not the fate of one nucleus.
The initial number of unstable nuclei, activity or count rate is the reference value.
1 of 4 states explored
Individual decays are random, but a large sample follows a predictable statistical pattern. After n half-lives, the expected fraction remaining is (1/2)ⁿ.
Half-life describes a population, not one nucleus
One nucleus has no predictable decay time, but a large population loses the same fraction during each half-life interval.
- Over one half-life, the unstable-nucleus population—or the source activity or count rate attributable to that sample—drops to 50% of its starting value.
- On a graph, choose a value, halve it, read both times and subtract; repeat elsewhere to check consistency.
- Separate Physics: subtract background count rate before using detector data when a background value is supplied.
- Higher Tier, both routes: after n half-lives the remaining fraction is (1/2)ⁿ; net decline is the initial amount minus the remainder.
The punchline: Halve vertically, measure horizontally, then repeat the check.
Near a source is not the same as carrying it away
Irradiation ends when exposure ends; contamination leaves radioactive atoms on or inside an object, where they keep decaying.
- For the alpha-, beta- and gamma-source situations here, irradiation neither transfers radioactive material nor leaves the object radioactive; use shielding, distance and short exposure.
- Contamination is unwanted radioactive material. Internal alpha contamination can be especially hazardous because alpha is strongly ionising.
- Follow trained staff, school or workplace rules; never handle, open or improvise with a radioactive source.
- Publishing human radiation studies allows methods and conclusions to be checked through peer review.
The punchline: Name whether the radioactive material is merely nearby or has been transferred.
Three emissions, three different journeys
Penetration and ionising power pull in opposite directions. The best source depends on what the radiation must pass through and affect.
- AlphaA helium nucleus: very strongly ionising, short range in air and stopped by paper or skin.
- BetaA fast electron from the nucleus: moderately ionising, travels about a metre in air and is stopped by a few millimetres of aluminium.
- GammaElectromagnetic radiation from the nucleus: weakly ionising, has a very long range in air and is reduced by thick lead or metres of concrete.
Separate Physics: one fission can seed the next
A causal model of induced fission becoming a chain reaction. Labelled circles show nuclei and neutrons; arrows show what travels where.
Separate Physics: dose depends on place, work and source
Ambient ionising radiation is present before a test source is added; its level changes with location and occupation.
- Sources include rocks, cosmic rays, weapons-test fallout and nuclear accidents. Dose is measured in Sv; 1000 mSv = 1 Sv, though the unit need not be recalled.
- A long half-life source stays active for longer but usually has a lower activity for the same number of nuclei; a short half-life source decays faster.
- Introduce a gamma-emitting tracer with a suitably short half-life; gamma escapes the body, so an external detector maps its distribution to show organ function.
- Target unwanted tissue with radiation; ionisation damages or kills its cells. Weigh control or destruction against healthy-cell damage and increased future cancer risk using supplied data.
The punchline: Radiation can increase risk; the size and type of dose matter.
Separate Physics: nuclei can split or join
Fission breaks a heavy nucleus; fusion joins light nuclei. Both rearrange nuclei and release energy, but by different routes.
- Neutron capture destabilises a uranium or plutonium nucleus. Two similar-sized lighter nuclei form, releasing two or three neutrons and gamma radiation; spontaneous fission is rare.
- The products have kinetic energy. Released neutrons can trigger further fissions, forming a chain reaction.
- A reactor controls the chain reaction; a nuclear weapon involves an uncontrolled chain reaction.
- When two light nuclei combine, they make a heavier product; a small loss of mass appears as radiation energy.
The punchline: Fission: one heavy nucleus splits. Fusion: two light nuclei join.
Words worth knowing
Key definitions
- isotope
- One version of an element: it keeps that element's proton count but has a different neutron count.
- activity
- How many nuclear decays occur each second in a source; its unit is the becquerel (Bq).
- count rate
- How many ionising-radiation events a detector records each second.
- radioactive decay
- An unstable nucleus releases nuclear radiation and moves towards a more stable state; the instant it happens is random.
- half-life
- The interval in which a sample's unstable-nucleus population—or its activity or count rate attributable to that sample—drops to 50% of its starting value.
- irradiation
- Alpha, beta or gamma reaches an object without radioactive atoms being deposited; in these GCSE source situations the object is not made radioactive.
- radioactive contamination
- Radioactive atoms have been transferred onto or into material where they are not meant to be.
- Separate Physics: background radiation
- The ever-present ionising radiation around us, arising from natural processes and human activity.
- Separate Physics: nuclear fission
- A heavy unstable nucleus breaks apart, producing two lighter nuclei plus neutrons, gamma rays and energy.
- Separate Physics: nuclear fusion
- Two light nuclei combine into one heavier product; a small amount of mass can emerge as radiation energy.
Calculate with confidence
Equations
Alpha decay example
²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂He
Radium-226 emits a helium-4 nucleus, leaving radon-222; mass number falls by 4 and atomic number falls by 2.
Exam tip: Recognise and use the common alpha symbol ⁴₂He when balancing; this exact worked example need not be recalled, and the daughter element need not be identified.
Beta-minus decay example
²⁴₁₁Na → ²⁴₁₂Mg + ⁰₋₁e
Sodium-24 emits a fast electron and becomes magnesium-24; mass number stays fixed while atomic number rises by 1.
Exam tip: Recognise and use the common beta-minus symbol ⁰₋₁e when balancing; this exact worked example need not be recalled, and the daughter element need not be identified.
Follow it step by step
Processes to remember
How to balance a nuclear equation
- Write the mass number above each symbol and the atomic number below it.
- Add the mass numbers on each side and fill the missing mass number.
- Add the atomic numbers on each side and fill the missing atomic number.
- Check that the emission is alpha or beta and that both totals balance.
Exam tip: Do not try to balance ordinary chemical formulae; this is a nucleus ledger.
How to choose radiation for a use
- State what the radiation must penetrate or where it must be absorbed.
- Match the required range and penetration to alpha, beta or gamma.
- Consider ionising power and whether the source is outside or inside the body.
- Choose a half-life long enough for the task but not needlessly long, then state a precaution.
Exam tip: A named radiation without a property-to-use link earns less than a justified choice.
Separate Physics: how to sketch a fission chain reaction
- Draw a neutron entering one large unstable nucleus.
- Show two smaller nuclei and two or three outgoing neutrons, with energy released.
- Send at least two outgoing neutrons towards other large nuclei.
- Repeat one more split so the growing chain is unmistakable.
Exam tip: Separate Physics: label the incoming neutron, daughter nuclei, outgoing neutrons and released energy.
See the thinking
Worked example
Worked example: half-life and net decline
A sample's count rate is 640 counts/s. Its half-life is 3 hours. Find the count rate after 9 hours and, at Higher Tier, the net decline as a ratio of the initial count rate.
- Find the number of half-lives: 9 h ÷ 3 h = 3.
- Halve once: 640 → 320 counts/s.
- Halve twice more: 320 → 160 → 80 counts/s.
- The remaining ratio is 80/640 = 1/8.
- Higher Tier: net decline = 1 − 1/8 = 7/8 of the initial count rate.
Answer: After 9 hours the count rate is 80 counts/s. The Higher Tier net decline ratio is 7/8.
Nine hours contains three half-lives, so multiply by one half three times. The remaining fraction is 1/8; net decline asks for the fraction removed, so subtract it from 1.
Protect the marks
Common mistakes
Watch out: Saying most alpha particles bounced back in the foil experiment.
Do this instead: Most passed through; only a very small fraction made large deflections.
Watch out: Describing beta as an electron from an atomic energy level.
Do this instead: Beta-minus is a fast electron emitted from the nucleus when a neutron becomes a proton.
Watch out: Predicting exactly when one nucleus will decay.
Do this instead: Individual decay is random; half-life predicts the statistical behaviour of a large sample.
Watch out: Halving the time instead of the activity.
Do this instead: Advance by one half-life along the time axis, then halve activity or count rate.
Watch out: Claiming irradiation makes an object radioactive.
Do this instead: Irradiation is exposure; contamination transfers radioactive material.
Watch out: Calling all radiation exposure certain to cause cancer.
Do this instead: Ionising radiation can increase risk; evaluate type, dose and evidence.
Try it before you move on
Quick check
Say your answer first, then open the card to check it.
An atom has atomic number 11 and mass number 23. How many neutrons does it have?
Answer: 12 neutrons
Neutrons = mass number − atomic number = 23 − 11.
What did the few large alpha-particle deflections reveal?
Answer: Positive charge and most atomic mass are concentrated in a tiny nucleus.
A spread-out positive charge could not exert the intense force needed for the rare large deflections.
How do mass number and atomic number change in beta-minus decay?
Answer: Mass number stays the same; atomic number increases by 1.
A neutron changes into a proton while an electron is emitted from the nucleus.
What fraction of a large sample is expected to remain after four half-lives?
Answer: 1/16
Halve four times: 1 → 1/2 → 1/4 → 1/8 → 1/16.
Why can alpha contamination inside the body be hazardous despite alpha's short range?
Answer: Alpha is strongly ionising and deposits energy over a short distance in nearby tissue.
Outside the body, skin stops alpha; inside, the source is already beside living cells.
Separate Physics: what allows one fission event to trigger others?
Answer: Two or three emitted neutrons can be absorbed by other unstable nuclei.
Those later fissions release more neutrons, so a chain reaction can grow.
Good questions, clear answers
Frequently asked questions
Are atoms mostly empty space?
In the nuclear model, the nucleus is less than one ten-thousandth of the atom's radius, while electrons occupy the surrounding region. ‘Empty’ describes the large scale difference, not an absence of electric fields.
Does beta radiation come from the electron shells?
No. In beta-minus decay, a neutron in the nucleus changes into a proton and a fast electron is emitted from the nucleus.
Why is radioactive decay called random if half-life is predictable?
The moment one nucleus decays is unpredictable. In a large sample, the probability produces a stable pattern, so the time for the population or activity to halve is measurable.
Does an irradiated object stay radioactive?
Not in the alpha-, beta- and gamma-source situations considered here. Irradiation does not transfer radioactive material or leave the object radioactive; contamination is the unwanted transfer of radioactive atoms.
Which parts are Separate Physics only?
Background radiation, half-life hazards, medical uses, fission and fusion are Separate Physics only here. The net-decline ratio is Higher Tier for both Combined Science and Separate Physics.
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