Atomic structure
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AQA 6.4 / PHYSICS
Atomic structure
How did we discover what is inside an atom, and how can we use radiation safely? Follow the evidence, read decay graphs and explain the science.
Supported GCSE version · Clear examples, worked calculations and optional challenges.
Foundation & Higher39 main + 3 challenge questionsSave and return any time
Understand the science
01 / 03Words to know
Nucleus
The small central region of an atom containing protons and neutrons.
Isotope
Atoms of the same element with different numbers of neutrons.
Ion
An atom or group of atoms with an overall electrical charge.
Energy level
An allowed electron energy, associated with a particular distance from the nucleus.
Radioactive decay
A random change in an unstable nucleus that emits radiation.
Activity
The rate of nuclear decay, measured in becquerels.
Becquerel
One nuclear decay per second; symbol Bq.
Count rate
The number of counts a radiation detector records per unit time.
Ionisation
Removing electrons from atoms, leaving charged ions.
Half-life
Time for the number of radioactive nuclei, activity or corrected count rate to halve.
Irradiation
Exposure to nuclear radiation without necessarily transferring radioactive material.
Contamination
Unwanted radioactive material on or inside something.
Peer review
Checking research methods, evidence and conclusions by other scientists.
1. A tiny atom with a much tinier centre
An atom has a positive nucleus surrounded by negative electrons. The nucleus contains protons and neutrons.
| Particle | Relative charge | Relative mass |
|---|---|---|
| Proton | +1 | 1 |
| Neutron | 0 | 1 |
| Electron | −1 | Very small |
Scale check: an atom’s radius is about 1 × 10⁻¹⁰ m. A nucleus has a radius less than one ten-thousandth of this. Most atomic mass is packed into the nucleus.
2. Electrons move between energy levels
Electrons occupy particular energy levels at different distances from the nucleus. They can move between these levels when energy is transferred.
- Absorb electromagnetic radiation: an electron can move to a higher energy level, farther from the nucleus.
- Emit electromagnetic radiation: an electron moves to a lower energy level, closer to the nucleus.
This changes an electron’s energy. Radioactive decay changes the nucleus. Do not mix up the two.
3. Read the numbers around the symbol
The atomic number counts protons. The mass number counts protons plus neutrons. In the notation 2311Na, 23 is the mass number and 11 is the atomic number.
- Protons = atomic number.
- Neutrons = mass number − atomic number.
- For a neutral atom, electrons = protons.
Worked example: 2311Na has 11 protons, 23 − 11 = 12 neutrons and 11 electrons. Na⁺ has lost one electron, so it has 10 electrons. Its nucleus has not changed.
Isotopes are atoms of the same element with different numbers of neutrons. A positive ion forms when an atom loses one or more outer electrons.
4. Why scientists changed their picture of an atom
A scientific model must fit the evidence. New results can force scientists to change or replace it.
| Evidence or development | How the model changed |
|---|---|
| Early idea | Atoms were tiny spheres that could not be divided. |
| Electrons discovered | Atoms had smaller parts; plum pudding placed negative electrons in a ball of positive charge. |
| Alpha scattering | A small, dense, charged nucleus replaced the spread-out positive charge. |
| Bohr’s calculations matched observations | Electrons occupy specific energy levels. |
| Later experiments | Nuclear positive charge occurs in whole units: protons. |
| Chadwick’s evidence | Uncharged neutrons are also in the nucleus. |
5. The few surprising particles mattered
Positive alpha particles were directed at thin metal foil. Most passed straight through. Some were deflected, and very few turned back.
| Observation | What it suggests |
|---|---|
| Most pass straight through | Most of an atom is empty space. |
| Very few are strongly deflected | The nucleus is very small. |
| Some positive particles turn back | Mass and positive charge are concentrated in a small, dense centre. |
The old spread-out positive charge could not explain the rare large deflections. That is why new evidence changed the model.
6. An unstable nucleus can change
Some nuclei are unstable. They emit radiation as they change to become more stable. This is radioactive decay.
- Random: we cannot predict exactly when one particular nucleus will decay.
- Activity: the number of nuclear decays per second. The unit is the becquerel, Bq; 1 Bq means one decay per second.
- Count rate: the number of counts a detector records per second. It usually detects only some of the emissions.
A detector can record radiation from the surroundings too. Measure this background count rate with the source absent, using the same counting time.
Worked example: 540 counts in 60 s gives 540 ÷ 60 = 9 counts/s. If background is 2 counts/s, corrected source count rate = 9 − 2 = 7 counts/s. Count rate is not automatically equal to source activity.
7. What can the radiation get through?
Ionisation means removing electrons from atoms, leaving ions. Ionising radiation can damage living cells.
| Radiation | Range and penetration | Relative ionising power |
|---|---|---|
| Alpha | Short range in air; stopped by paper or outer skin | Strong |
| Beta | Further in air; absorbed by thin aluminium | Medium |
| Gamma | Long range; thick lead or concrete reduces it | Weak |
Choose a source for its job. For example, a detector on the other side of a thin sheet needs radiation that can pass through some of the sheet but is affected by changes in thickness. A question will give the context: explain your choice using the properties.
8. Nuclear equations keep both numbers balanced
In a nuclear equation, balance the mass numbers at the top and the atomic numbers at the bottom. The emitted particle must be included.
- Alpha, 42He: daughter mass number is 4 smaller and atomic number is 2 smaller.
- Beta, 0−1e: mass number stays the same; atomic number increases by 1 because a neutron becomes a proton.
- Gamma: neither mass number nor nuclear charge changes.
You may be given the daughter symbol. Work out its numbers; identifying unfamiliar daughter elements from memory is not required.
9. Random nuclei, predictable patterns
The half-life is the time for half the radioactive nuclei in a sample to decay. It is also the time for its activity, or corrected count rate, to halve.
- Choose a starting activity on the graph.
- Find half that activity.
- Read the times for both values and subtract.
Worked example using different data: activity falls from 640 Bq to 320 Bq in 5 minutes and to 160 Bq after another 5 minutes. Half-life = 5 minutes each time.
A large sample produces a smooth overall trend, although individual decays are random. Actual count-rate readings fluctuate, so repeated measurements improve the estimate.
Higher only: fractions and ratios after repeated half-lives
After 1, 2 and 3 half-lives, the fractions remaining are 1/2, 1/4 and 1/8. After 3 half-lives, remaining : original = 1 : 8, and the fraction that has decayed is 7/8.
For an initial 800 Bq, three halvings give 400 → 200 → 100 Bq. Net decrease = 800 − 100 = 700 Bq. Always check whether the question asks for what remains or what has been lost.
10. Has the material moved, or only the radiation?
Irradiation is exposure to radiation. Contamination is unwanted radioactive material on or inside something.
| Situation | Why it matters |
|---|---|
| Source outside the body | Gamma can penetrate to internal tissues. Alpha is usually stopped by the outer skin. |
| Alpha-emitting material inhaled or swallowed | Strong ionisation occurs close to internal living cells; it can be very hazardous. |
| Contaminated clothing | Radioactive material may move to other surfaces and keep emitting until removed or decayed. |
| Irradiated object with no transferred material | It does not become radioactive just because it was exposed. |
A long range does not mean “most ionising”. Consider the radiation type, its location, the exposure time and whether material can spread.
11. Reduce exposure and check the evidence
Radioactive sources belong in controlled, supervised work. Students should not obtain or handle sources independently.
| Precaution | Reason |
|---|---|
| Minimise time near a source | Reduces exposure time. |
| Increase distance; use suitable handling tools | Reduces radiation reaching the person. |
| Use appropriate shielding and safe storage | Absorbs or reduces radiation. |
| Contain radioactive material; prevent spills and inhalation | Reduces contamination and its spread. |
Peer review means other scientists checking a study’s methods, evidence and conclusions. Publishing and sharing studies of radiation’s effects allows weaknesses to be identified and results to be compared. Peer review helps reliability; it does not guarantee a conclusion is perfect.
There is no numbered AQA required practical for this topic. You can still be asked to analyse detector readings, judge a method or explain suitable precautions.
Put it into practice
02 / 03Start with the 39 main questions. Higher-only questions are labelled. You can complete this topic over several sessions; your answers save as you go. Use a hint if you get stuck, then compare your work with an example answer. Your tutor can mark written answers.
Question words
State / name: give a short answer. Describe: say what happens. Explain: give a reason using “because”. Calculate: show your working and include units.
State / name: give a short answer. Describe: say what happens. Explain: give a reason using “because”. Calculate: show your working and include units.
Diagram, data and practical tasks
Read, check and explain
Use the graph to estimate the half-life twice, starting from different activities. Explain why the activity drop in the first three hours is larger than the drop in the next three hours even though both intervals are one half-life.
Show an example answer
400→200 Bq takes 0→3 h; 200→100 Bq takes 3→6 h. Both give 3 h. The same fraction halves each time, but fewer radioactive nuclei remain, so the absolute decrease is smaller.
Try a nuclear equation on paper
Write an alpha-decay equation using a parent with mass number 238 and atomic number 92. Use X for the daughter nucleus. Check the top numbers and bottom numbers separately.
Check your numbers
The daughter has mass number 234 and atomic number 90; the emitted alpha particle has mass number 4 and atomic number 2.
Practice questions
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03 / 03Your answer sheet
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One final thought
Use a scientific idea and a specific example in your reflection.