Atomic Structure: Exam Practice

This lesson provides exam practice and revision strategies for the Atomic Structure and the Periodic Table topic in AQA GCSE Chemistry (Paper 1, Topic 1). This topic typically accounts for a significant portion of the marks on Paper 1. Mastering the command words, common question types, and model answer techniques covered here will help you maximise your marks.

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Key Command Words

AQA uses specific command words that tell you exactly what to do in your answer. Understanding these is crucial:

Command Word	Meaning	Marks Typically
State	Give a short, factual answer with no explanation	1 mark
Identify	Name or select the correct answer	1 mark
Describe	Say what happens — give an account of observations or a process	2–3 marks
Explain	Say WHY something happens — give reasons using scientific knowledge	2–4 marks
Compare	Identify similarities AND differences between two things	2–4 marks
Suggest	Apply your knowledge to an unfamiliar context — there may be more than one correct answer	1–3 marks
Evaluate	Consider the evidence and make a judgement, giving reasons for and against	3–6 marks
Calculate	Work out a numerical answer — always show your working	1–3 marks
Determine	Use data or information to find an answer	1–2 marks

Exam Tip: Pay very close attention to the command word. "Describe" asks WHAT happens; "Explain" asks WHY it happens. If the question says "Explain" and you only describe, you will not get full marks. Similarly, "Compare" requires both similarities AND differences — do not just describe one thing.

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Topic Overview — What You Must Know

The Atomic Structure and Periodic Table topic covers the following areas, all of which can appear on Paper 1:

Sub-Topic	Key Content
Atoms, elements and compounds	Definition of atom, element, compound; chemical formulae; conservation of atoms
Mixtures	Definition of mixture; separation techniques (filtration, distillation, chromatography, crystallisation); Rf values; formulations
Atomic structure	Sub-atomic particles; atomic number and mass number; isotopes; relative atomic mass
Electronic structure	Electron shells; configurations of first 20 elements; link to periodic table position
History of the periodic table	Newlands, Mendeleev; how the model developed over time
Group 1 — Alkali metals	Properties, reactions with water, trend in reactivity, explanation
Group 7 — Halogens	Properties, displacement reactions, trend in reactivity, halide ion tests
Group 0 — Noble gases	Full outer shell, unreactivity, uses, boiling point trend
Transition metals	Properties compared to Group 1; coloured compounds; variable oxidation states; catalysts

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Common Question Types and Model Approaches

Type 1: Calculating Sub-Atomic Particles

Typical question: "An atom has a mass number of 23 and an atomic number of 11. Calculate the number of protons, neutrons and electrons."

Model approach:

1. Protons = atomic number = 11
2. Electrons = atomic number (neutral atom) = 11
3. Neutrons = mass number - atomic number = 23 - 11 = 12

Common error: Confusing atomic number and mass number, or calculating neutrons as mass number + atomic number instead of minus.

Type 2: Relative Atomic Mass Calculations

Typical question: "Copper has two isotopes: Cu-63 (69.2% abundance) and Cu-65 (30.8% abundance). Calculate the relative atomic mass."

Model approach: Ar = (63 x 69.2 + 65 x 30.8) / 100 Ar = (4359.6 + 2002.0) / 100 Ar = 6361.6 / 100 Ar = 63.6 (to 1 decimal place)

Always show your working — even if you get the final answer wrong, you can still gain method marks.

Exam Tip: When calculating relative atomic mass, always show each step clearly. Write out the formula first, substitute the values, and show the arithmetic. If you round your answer, state the precision (e.g. "to 1 decimal place"). This approach ensures you gain all available method marks.

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Type 3: Explaining Reactivity Trends

Typical question: "Explain why potassium is more reactive than sodium."

Model answer structure:

1. Both potassium and sodium are in Group 1, so both have 1 electron in their outer shell.
2. Potassium has more electron shells than sodium (4 shells vs 3 shells).
3. The outer electron in potassium is further from the nucleus.
4. There is greater electron shielding from the inner shells.
5. The attraction between the nucleus and the outer electron is weaker.
6. Therefore, the outer electron is easier to remove (less energy needed).
7. This makes potassium more reactive than sodium.

graph TD
    A["Reactivity Comparison Question"] --> B["Identify the group and what they have in common"]
    B --> C["State the difference in electron shells"]
    C --> D["Explain distance from nucleus"]
    D --> E["Mention electron shielding"]
    E --> F["Link to strength of attraction"]
    F --> G["Conclude: easier/harder to lose or gain electron"]
    G --> H["Therefore more/less reactive"]

    style A fill:#2c3e50,color:#fff
    style D fill:#e74c3c,color:#fff
    style E fill:#e74c3c,color:#fff
    style H fill:#27ae60,color:#fff

Exam Tip: For reactivity trend explanations, always include these four elements: (1) number of shells, (2) distance from nucleus, (3) electron shielding, and (4) ease of losing/gaining the electron. Missing any one of these will cost you marks. Also remember: Group 1 — losing electrons, reactivity increases down; Group 7 — gaining electrons, reactivity decreases down.

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Type 4: History of the Periodic Table (6-Mark Extended Response)

Typical question: "Describe how the periodic table has developed over time. Refer to the work of Newlands and Mendeleev."

Model answer structure:

Paragraph 1 — Newlands:

• Arranged elements by atomic weight in 1864.
• Noticed every eighth element had similar properties — Law of Octaves.
• Problems: only worked for the first 16 elements; forced elements into wrong groups; did not leave gaps; not accepted by the scientific community.

Paragraph 2 — Mendeleev:

• Also arranged by atomic weight but left gaps for undiscovered elements.
• Swapped the order of some elements so those with similar properties were in the same group.
• Predicted the properties of undiscovered elements (e.g. eka-silicon/germanium).
• Table was accepted when predictions were confirmed.

Paragraph 3 — Modern table:

• Elements now arranged by atomic number (proton number) rather than atomic weight.
• This resolved anomalies in Mendeleev's table (e.g. tellurium and iodine).
• Discovery of noble gases added a new group.

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Type 5: Displacement Reactions of Halogens

Typical question: "Chlorine water is added to potassium bromide solution. Describe what you would observe and write a balanced equation."

Model answer:

• Observation: The solution changes from colourless to orange/brown as bromine is produced.
• Explanation: Chlorine is more reactive than bromine, so chlorine displaces bromine from potassium bromide.
• Equation: Cl2 + 2KBr → 2KCl + Br2

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Type 6: Chromatography and Rf Values

Typical question: "A spot of dye travels 4.2 cm. The solvent front travels 6.0 cm. Calculate the Rf value."

Model answer: Rf = distance moved by substance / distance moved by solvent Rf = 4.2 / 6.0 Rf = 0.70

Key points: Rf has no units; measure from the baseline; answer to 2 decimal places.

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Required Practicals Linked to This Topic

AQA requires you to have completed specific practical work. For this topic, the relevant required practical is:

Required Practical	What You Did
Chromatography	Investigated the composition of inks/food colourings using paper chromatography. Calculated Rf values.

You may be asked to:

• Describe the method for paper chromatography.
• Explain why the baseline is drawn in pencil (pencil is insoluble and will not dissolve in the solvent).
• Explain why the solvent level must be below the baseline (so the spots do not dissolve directly into the solvent).
• Calculate and interpret Rf values.
• Identify substances by comparing Rf values with known reference substances.

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