Atomic Structure: Exam Practice

This lesson brings together all the key topics from the AQA GCSE Physics Atomic Structure unit (specification 4.4) and provides exam-focused revision, common question types, model answers, and strategies for maximising your marks. Use this lesson to consolidate your understanding and practise the skills you will need in the exam.

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Key Topics to Revise

The Atomic Structure unit covers:

Topic	Key Content
Structure of the atom	Nucleus (protons + neutrons), electrons in shells, relative charges and masses
Mass number and atomic number	A = protons + neutrons; Z = protons; neutrons = A - Z
Isotopes	Same protons, different neutrons; same chemical properties, different physical properties
Development of the atomic model	Dalton, Thomson (plum pudding), Rutherford (nuclear model), Bohr (electron shells), Chadwick (neutron)
Radioactive decay	Random and spontaneous; alpha, beta, gamma emission
Properties of radiation	Ionising ability, penetrating power, charge, mass
Half-life	Time for activity/undecayed nuclei to halve; decay curves; calculations
Nuclear equations [H]	Balancing mass number and atomic number in alpha and beta decay
Uses of radiation	Medical tracers, radiotherapy, smoke detectors, thickness monitoring, carbon dating
Irradiation vs contamination	Exposure to radiation vs radioactive material on/in body
Fission and fusion	Splitting large nuclei vs joining small nuclei; chain reactions; nuclear power

Exam Tip: Before the exam, make sure you can define every bold term in the table above from memory. Definitions are easy marks that students often lose because they are vaguely worded. Be precise and use the exact scientific terminology.

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Common Question Types

Type 1: Define and Describe

These questions ask you to state a definition or describe a concept.

Example: Define the term "isotope." (2 marks)

Model answer: Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons. (They therefore have the same atomic number but different mass numbers.)

Key point: Always mention BOTH "same protons" AND "different neutrons."

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Type 2: Compare

These questions ask you to compare two things (e.g., alpha vs beta, fission vs fusion).

Example: Compare the properties of alpha and gamma radiation. (4 marks)

Model answer:

Property	Alpha	Gamma
Nature	Particle (2 protons + 2 neutrons)	Electromagnetic wave
Charge	+2	0
Ionising ability	Strongly ionising	Weakly ionising
Penetrating power	Low — stopped by paper	High — reduced by thick lead

Key point: Use a structured comparison with specific contrasts for each property.

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Type 3: Explain with Reference to Evidence

These questions ask you to link experimental evidence to a model or conclusion.

Example: Explain how the results of the alpha particle scattering experiment led to the development of the nuclear model of the atom. (6 marks)

Model answer plan:

1. Geiger and Marsden fired alpha particles at thin gold foil.
2. Most alpha particles passed straight through — showing the atom is mostly empty space.
3. Some alpha particles were deflected at small angles — showing the nucleus has a positive charge that repels the positive alpha particles.
4. A very small number bounced straight back — showing the nucleus is very small and very dense, with a concentrated positive charge.
5. The plum pudding model could not explain why some particles bounced back (if positive charge were spread out, there would be nothing to cause such strong repulsion).
6. Rutherford concluded the atom has a small, dense, positively charged nucleus with electrons orbiting at a distance — the nuclear model.

Exam Tip: For 6-mark questions on the alpha scattering experiment, you need THREE observations linked to THREE conclusions. Each observation-conclusion pair is worth 2 marks. Use the structure: "They observed that... This showed that..."

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Type 4: Calculate Half-Life

Example: A radioactive source has an initial activity of 1600 Bq. After 40 minutes, the activity is 100 Bq. Calculate the half-life.

Solution:

Count the halvings:

• 1600 --> 800 (1 half-life)
• 800 --> 400 (2 half-lives)
• 400 --> 200 (3 half-lives)
• 200 --> 100 (4 half-lives)

4 half-lives = 40 minutes Half-life = 40 / 4 = 10 minutes

Further Half-Life Practice

Initial Activity	Final Activity	Time Elapsed	Half-Lives	Half-Life
8000 Bq	1000 Bq	24 hours	3	8 hours
1200 Bq	150 Bq	30 minutes	3	10 minutes
6400 Bq	400 Bq	20 years	4	5 years
2000 Bq	250 Bq	9 days	3	3 days

Check: 8000 --> 4000 --> 2000 --> 1000 = 3 halvings in 24 hours, so half-life = 8 hours. Correct.

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Type 5: Nuclear Equations [H]

Example: Americium-241 (Z = 95) undergoes alpha decay. Write the nuclear equation for this decay. (3 marks)

Solution:

Am (241, 95) --> Np (237, 93) + He (4, 2)

Working:

• Mass number of daughter = 241 - 4 = 237
• Atomic number of daughter = 95 - 2 = 93
• Element with Z = 93 is neptunium (Np)

Check: 241 = 237 + 4 (mass numbers balance). 95 = 93 + 2 (atomic numbers balance).

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Type 6: Explain a Use of Radiation

Example: Explain how beta radiation is used to monitor the thickness of paper in a factory. (4 marks)

Model answer:

1. A beta source is placed on one side of the paper, and a detector on the other side.
2. Beta radiation passes through the paper to the detector.
3. If the paper is too thick, less beta radiation reaches the detector, and the count rate decreases.
4. If the paper is too thin, more beta radiation reaches the detector, and the count rate increases.
5. The machinery automatically adjusts the rollers to keep the paper at the correct thickness.
6. Beta is used because it has the right penetrating power — it is partially absorbed by paper (unlike gamma, which would pass straight through, or alpha, which would be completely stopped).

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Exam Command Words

Command Word	What It Means	What To Do
State	Give a brief answer	No explanation needed
Define	Give the meaning of a term	Use precise scientific language
Describe	Say what happens	Give a step-by-step account
Explain	Say what happens AND why	Include reasons using scientific concepts
Compare	Identify similarities and differences	Make direct comparisons (use "whereas" or a table)
Calculate	Work out a numerical answer	Show all working, include units
Evaluate	Weigh up evidence and make a judgement	Give advantages and disadvantages, then a conclusion
Suggest	Apply your knowledge to an unfamiliar context	Use what you know to make a reasonable answer

Exam Tip: Pay close attention to the command word. "Describe" and "explain" are different. "Describe" means say what happens (e.g., "the activity decreases over time"). "Explain" means say what happens and why (e.g., "the activity decreases over time because the number of undecayed nuclei decreases, so fewer decays occur per second").

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Common 6-Mark Question Topics

The following topics frequently appear as 6-mark extended response questions:

1. Describe and explain the alpha scattering experiment and how it changed the atomic model.
2. Compare the properties of alpha, beta, and gamma radiation.
3. Explain how a specific application of radiation works and why that type is suitable.
4. Compare fission and fusion, including conditions, products, and where they occur.
5. Explain the development of the atomic model from Dalton to Chadwick.
6. Describe how a nuclear power station works, including the role of each component.

Tips for 6-Mark Questions

• Plan your answer before writing. Spend 1-2 minutes listing key points.
• Use paragraphs — do not write one continuous block of text.
• Include scientific terminology — use the correct names for particles, processes, and units.
• Give specific examples where possible (e.g., name specific isotopes, give specific half-lives).
• Write in a logical order (chronological for history questions, step-by-step for process questions).
• Check you have covered all aspects of the question.

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Key Equations and Values to Remember

Item	Value
Radius of an atom	~1 x 10^-10 m
Radius of a nucleus	~1 x 10^-14 m
Relative mass of a proton	1
Relative mass of a neutron	1
Relative mass of an electron	Very small (~1/1836)
Charge of a proton	+1
Charge of a neutron	0
Charge of an electron	-1
Activity unit	Becquerel (Bq) = 1 decay per second
Half-life of carbon-14	5,730 years
Half-life of uranium-238	4.5 billion years

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Revision Checklist

Use this checklist to ensure you have covered everything: