AQA GCSE Physics 8463: Atomic structure – Revision Notes

These notes condense section 4.4 Atomic structure of the AQA GCSE Physics (8463) specification, Version 1.1 (30 September 2019), for teaching from September 2016 and exams from 2018 onwards. They cover 4.4.1 to 4.4.4. The topic is examined on Paper 1, at Foundation and Higher Tier; the single (HT only) point is marked Higher tier only. For full explanations and worked examples, read the Atomic structure study guide first.

When you are ready, test yourself with the practice questions. The course hub is AQA GCSE Physics and the printable checklist lists every point. The free diagnostics help you find weak topics across the course.

4.4.1 Atoms and isotopes

Key facts

• Atom radius: about 1 × 10⁻¹⁰ m.
• Nucleus radius: less than 1/10 000 of the atom’s radius.
• Nucleus: protons (positive) and neutrons (neutral). Holds most of the mass.
• Electrons: negative, at set distances (energy levels).
• Absorb EM radiation → electron moves further out (higher level). Emit EM radiation → electron moves closer (lower level).
• Atom: electrons = protons, so no overall charge.
• Lose outer electron(s) → positive ion.

Definitions

Term	Meaning
Atomic number	Number of protons. Same for every atom of one element.
Mass number	Protons + neutrons.
Isotopes	Atoms of the same element with different numbers of neutrons.
Neutrons	Mass number − atomic number.

Method: reading nuclear notation

Step 1  top number = mass number;  bottom number = atomic number (protons)
Step 2  neutrons = top − bottom
Step 3  electrons = protons for an atom; subtract one per + charge for an ion

Reminder: ²³₁₁Na has 11 protons, 12 neutrons, 11 electrons. Na⁺ has 10 electrons.

Model of the atom: the timeline

1. Indivisible tiny spheres.
2. Electron discovered → plum pudding (ball of positive charge, electrons embedded).
3. Alpha scattering → mass and charge concentrated in a tiny nucleus → nuclear model.
4. Bohr: electrons orbit at specific distances; calculations matched experiments.
5. Positive charge split into whole numbers of identical particles → protons.
6. Chadwick: evidence for neutrons, about 20 years after the nucleus was accepted.

Scattering evidence, point by point: most alpha particles went straight through (mostly empty space); some deflected (charged centre); very few bounced back (tiny, dense centre holding most of the mass).

4.4.2 Atoms and nuclear radiation

Emissions

	Alpha α	Beta β	Gamma γ
Nature	2p + 2n (helium nucleus)	Fast electron from nucleus (n → p)	EM radiation from nucleus
Symbol	⁴₂He	⁰₋₁e	γ
Stopped by	Paper	Thin aluminium	Thick lead/concrete (reduced)
Range in air	A few cm	Further	Very far
Ionising	Most	Middle	Least

A nucleus may also emit a neutron (n).

• Activity: decays per second of a source, in becquerel (Bq).
• Count-rate: decays recorded per second by a detector (e.g. Geiger-Muller tube).

Decay rules

Decay	Mass number	Atomic number	Charge of nucleus
Alpha	− 4	− 2	Decreases
Beta	No change	+ 1	Increases
Gamma	No change	No change	No change

Method: balancing a decay equation

Step 1  write the parent nucleus and the emitted particle (⁴₂He or ⁰₋₁e)
Step 2  top numbers: parent = daughter + particle
Step 3  bottom numbers: parent = daughter + particle
Step 4  use the daughter name you are given

Reminder: ²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂He (226 = 222 + 4; 88 = 86 + 2).

Half-life

• Decay is random: you cannot say which nucleus decays next, or when.
• Half-life: time for the number of nuclei of the isotope to halve, or for the count-rate (activity) to fall to half.
• From a graph: pick a value, find when it has halved, read the time difference. Repeat and average.

Method: activity after whole half-lives

Step 1  number of half-lives = time ÷ half-life
Step 2  halve the starting value that many times

Reminder: 800 Bq, half-life 2 days, after 6 days → 3 half-lives → 800 → 400 → 200 → 100 Bq.

Higher tier only: net decline as a ratio after n half-lives = 1 : 2ⁿ. After 4 half-lives it is 1 : 16.

Contamination vs irradiation

Contamination	Irradiation
Radioactive atoms get on or into an object	Object is exposed to radiation from outside
Hazard continues while the atoms decay	Stops when the source is removed
Object now contains a radioactive source	Object does not become radioactive

Studies of radiation’s effects on people must be published and shared for peer review.

4.4.3 Hazards and uses

• Background radiation: natural (rocks, cosmic rays) and man-made (weapons-testing fallout, nuclear accidents). Dose varies with occupation and location.
• Dose unit: sievert (Sv); 1000 mSv = 1 Sv (unit not needed from memory).
• Short half-life: intense at first, falls quickly. Long half-life: weaker, but hazardous for a long time.
• Medicine: exploring internal organs (tracers) and controlling or destroying unwanted tissue.
• Tracer choice: gamma emitter; half-life of hours, not seconds or years.
• Destroying unwanted tissue: the radiation must reach the tissue and damage it, while the dose to healthy tissue is kept as low as possible.

Method: choosing a source for a job

Step 1  what must the radiation do? (pass through, be stopped, ionise)
Step 2  pick alpha, beta or gamma from penetration, range and ionising power
Step 3  pick a half-life: long enough for the job, short enough to limit the hazard
Step 4  give a reason for rejecting each other option

Reminder: a thickness gauge for thin card uses beta. Alpha is stopped by the card; gamma passes through almost unchanged, so small thickness changes would not show.

Method: evaluating a risk from data

Step 1  quote the dose or activity from the data, with its unit
Step 2  compare it with a reference value given (e.g. yearly background dose)
Step 3  weigh the benefit (diagnosis, treatment) against the risk
Step 4  give a conclusion that follows from the numbers

Reminder: a scan dose of 6.0 mSv against a yearly background dose of 2.0 mSv is 6.0 ÷ 2.0 = 3 years’ worth of background.

4.4.4 Fission and fusion

• Fission: large unstable nucleus (uranium, plutonium) absorbs a neutron → two smaller nuclei of roughly equal size + 2 or 3 neutrons + gamma rays + energy. All products have kinetic energy.
• Spontaneous fission is rare.
• Chain reaction: released neutrons cause more fissions. Controlled in a reactor; uncontrolled in a nuclear weapon.
• Fusion: two light nuclei join to make a heavier nucleus; some mass may be converted to the energy of radiation.

Chain reaction in words

One neutron → one fission → 2 or 3 neutrons → more fissions → more neutrons. If every neutron caused a new fission, the number of fissions would multiply in each generation. In a reactor the process is kept steady; in a weapon it runs out of control.

Must-know distinctions

• Atomic number vs mass number: protons only vs protons + neutrons.
• Isotope vs ion: different neutrons vs different electrons.
• Activity vs count-rate: what the source does vs what the detector records.
• Contamination vs irradiation: radioactive atoms present vs just exposed.
• Fission vs fusion: heavy nucleus splits vs light nuclei join.
• Plum pudding vs nuclear: charge spread out vs concentrated in a tiny nucleus.

Quick self-test

1. What is the approximate radius of an atom?
2. How many protons and neutrons are in ⁵⁶₂₆Fe?
3. What is a beta particle, and where does it come from?
4. How does alpha decay change the mass number and atomic number?
5. How does gamma emission change the nucleus’s mass and charge?
6. Which emission is the most ionising, and which the most penetrating?
7. A source has an activity of 3200 Bq and a half-life of 5 days. What is its activity after 20 days?
8. (Higher tier only) What is the net decline, as a ratio, after 3 half-lives?
9. Convert 0.25 Sv into millisieverts.
10. Does an irradiated apple become radioactive?
11. What usually has to happen to a uranium-235 nucleus before it undergoes fission?
12. Define nuclear fusion.

Answers

1. About 1 × 10⁻¹⁰ m.
2. 26 protons; 56 − 26 = 30 neutrons.
3. A high-speed electron ejected from the nucleus as a neutron turns into a proton.
4. Mass number falls by 4; atomic number falls by 2.
5. Neither changes.
6. Alpha is the most ionising; gamma is the most penetrating.
7. 20 ÷ 5 = 4 half-lives: 3200 → 1600 → 800 → 400 → 200 Bq.
8. 1 : 8.
9. 0.25 × 1000 = 250 mSv.
10. No. Irradiation does not make an object radioactive.
11. It must absorb a neutron.
12. Two light nuclei join to form a heavier nucleus.

Where marks are usually lost

• Writing “the nucleus is 1/10 000 of the atom” instead of “less than 1/10 000 of the radius”.
• Describing the plum pudding model as having a nucleus.
• Explaining alpha scattering without linking each observation (straight through, deflected, bounced back) to a conclusion.
• Leaving out the charge on the beta symbol, or writing it as +1.
• Stating that half-life is “half the time for the source to decay completely”.
• Counting half-lives wrongly: after 3 half-lives, a sample is at 1/8, not 1/6.
• Saying irradiated food or equipment “becomes radioactive”.
• Choosing an alpha source for a medical tracer: it cannot get out of the body to the detector.
• Forgetting that fission needs a neutron to be absorbed first, or that it also releases gamma rays.
• Calling fusion “splitting” or giving uranium as a fusion fuel.

Official syllabus

AQA GCSE Physics (8463) specification, Version 1.1, 30 September 2019, for teaching from September 2016 and exams from 2018 onwards (AQA), section 4.4 Atomic structure.