AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Higher), 2021: Question 1

✅ Correct answer

Group 0
or noble gases

💡 Key knowledge

• Mendeleev arranged elements mostly by atomic mass, before all modern groups were known.
• The noble gases were discovered later because they are very unreactive and not easy to spot in reactions.

🧠 Exam technique

• If a question asks for a group, write the group number or the family name.
• For AQA, Group 0 and noble gases are both accepted.

❌ Common errors

• Writing Group 7 or halogens — these were already known.
• Giving a specific element instead of a group.

✅ Correct answer

B

💡 Key knowledge

• In the plum pudding model, electrons are embedded in a sphere of positive charge.
• It has no nucleus.

🧠 Exam technique

• Look for a model with electrons scattered through a positive “blob”.
• AQA often tests recognition of atomic models by their visual features.

❌ Common errors

• Choosing the nuclear model with electrons orbiting a central nucleus.
• Choosing the solid sphere model, which is Dalton’s model.

✅ Correct answer

A

💡 Key knowledge

• Chadwick discovered the neutron.
• This led to the modern nuclear model with a nucleus containing protons and neutrons.

🧠 Exam technique

• Match the model to the scientist’s contribution:
• Dalton = solid sphere
• Thomson = plum pudding
• Rutherford = nuclear model
• Chadwick = nucleus with neutrons

❌ Common errors

• Picking the model with electrons in fixed shells but no clear nucleus detail.
• Confusing Rutherford and Chadwick.

✅ Correct answer

Atoms of the same element with the same number of protons but different numbers of neutrons.

💡 Key knowledge

• Isotopes are still the same element because the proton number is the same.
• Different neutrons change the mass number, not the element identity.
• Atomic number = number of protons.

🧠 Exam technique

• Use both parts of the definition for full marks.
• Best answers mention protons and neutrons.
• You can also say “same atomic number” for same protons.

❌ Common errors

• Saying “same number of electrons” — not needed and not enough.
• Saying “different relative atomic mass” — this is not accepted as the definition.
• Mixing up mass number and atomic number.

📐 Calculation: step by step

1. Multiply each mass number by its percentage abundance.
2. Add the two products.
3. Divide by 100.
4. Round to 1 decimal place.

(39 × 93.1) + (41 × 6.9) = 363.9 + 282.9 = 3918? No — carefully calculate:

Correct working:
(39 × 93.1) = 3630.9
(41 × 6.9) = 282.9
3630.9 + 282.9 = 3913.8
3913.8 ÷ 100 = 39.138
Rounded to 1 d.p. = 39.1

✅ Correct answer

39.1

This is the weighted mean of the two isotopes.

💡 Key knowledge

• Relative atomic mass is a weighted average based on isotope abundance.
• More abundant isotopes have a bigger effect on the final answer.
• Potassium-39 is much more common than potassium-41, so the answer is close to 39.

🧠 Exam technique

• Always include the divide by 100 because the abundances are percentages.
• Write units carefully if needed, but relative atomic mass is usually unitless.
• If your arithmetic is slightly wrong but you use all values and round correctly, you may still gain method marks.

❌ Common errors

• Adding 39 + 41 and dividing by 2 — this ignores abundance.
• Forgetting to divide by 100.
• Rounding too early.
• Using 93.1 and 6.9 as decimal fractions without adjusting the calculation.
• Arithmetic slip: the products must be 3630.9 and 282.9, not 363.9 and 282.9.

💡 Mark breakdown

• 01.1: 1 mark for Group 0 / noble gases
• 01.2: 1 mark for B
• 01.3: 1 mark for A
• 01.4: 2 marks for a full isotope definition
• 01.5: 3 marks for correct method and answer

🧠 What top answers did well

• Used precise scientific vocabulary: protons, neutrons, atomic number.
• Recognised atomic models from their features.
• Showed a clear weighted mean calculation rather than a guess.

❌ Where students lost marks

• Confusing isotopes with ions or different elements.
• Mixing up the plum pudding model and the nuclear model.
• Making an arithmetic slip in the potassium calculation.