AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Higher), 2019: Question 4

✅ Correct answer

By increasing atomic mass (also called relative atomic mass).

💡 Key knowledge

• Early periodic tables were arranged by atomic weight / atomic mass.
• Mendeleev later improved the table by using patterns in properties, not just mass.

🧠 Exam technique

• Use the exact phrase increasing atomic mass for full credit.
• If you say “order of mass”, that is usually acceptable if the meaning is clear.

❌ Common errors

• Saying atomic number — this is the modern periodic table, not the early one.
• Saying “sorted by reactivity” — not the original arrangement.

✅ Correct answers

• He left gaps for elements not yet discovered.
• He moved some elements so that elements with similar properties were in the same group.
• He predicted properties of undiscovered elements.

💡 Key knowledge

Mendeleev noticed the pattern of properties repeated. If the mass order did not fit the properties, he sometimes changed the order so the groups made chemical sense.

🧠 Exam technique

• Say why he did it if you can: to keep similar elements together.
• “Left gaps” is the safest one-mark answer.

❌ Common errors

• Writing that he used atomic number — that was done later.
• Saying he “ignored mistakes” without explaining the method.

✅ Correct answer

Fluorine, chlorine and bromine are simple molecules made of small molecules. The forces between the molecules are weak, so little energy is needed to overcome them.

💡 Key knowledge

• These halogens exist as diatomic molecules: F₂, Cl₂, Br₂.
• Boiling point depends on intermolecular forces, not the bonds inside the molecules.

🧠 Exam technique

• For 2 marks, include both the type of structure and the reason it boils easily.
• Use the phrase weak intermolecular forces if possible.

❌ Common errors

• Saying “weak covalent bonds” — these are not broken when boiling.
• Saying “they are gases so have low boiling points” without explanation.
• Talking about reactivity instead of structure and forces.

✅ Correct answer

The boiling points increase down the group. As the molecules get larger, they have more electrons, so the intermolecular forces become stronger. Therefore, more energy is needed to separate the molecules.

💡 Key knowledge

• Fluorine has the lowest boiling point, then chlorine, then bromine.
• Down Group 7, molecules get bigger and have more electrons.
• Stronger intermolecular forces means a higher boiling point.

🧠 Exam technique

• Use the pattern in the table: F₂ < Cl₂ < Br₂.
• For full marks, link the trend to the size of molecules and the strength of forces.

❌ Common errors

• Saying boiling points increase because the atoms get “heavier” only — this is not enough by itself.
• Mixing up intermolecular forces with chemical bonds.
• Forgetting to state the trend is down the group.

✅ Correct answer

Neon has the electronic structure 2,8. Its outer shell is full, so it is stable and does not need to gain, lose or share electrons. Therefore, neon is unreactive.

💡 Key knowledge

• Neon is a noble gas.
• Noble gases are unreactive because they already have a full outer electron shell.
• For neon, the correct shell arrangement is 2,8.

🧠 Exam technique

• If asked for electronic structure, include it clearly in the answer, not just in your head.
• To get both marks, give the structure and the reason for unreactivity.

❌ Common errors

• Saying “it has eight electrons” without mentioning the shell structure.
• Writing the wrong structure, such as 2,7 or 2,8,1.
• Saying “it does not react because it is a gas” — being a gas is not the reason.

📐 Calculations: step-by-step

1. Find the number of moles:
   moles = mass ÷ Mr = 1 ÷ 40 = 0.025 mol
2. Use Avogadro’s constant:
   number of atoms = 0.025 × 6.02 × 10^23
3. Calculate:
   = 1.505 × 10^22
4. Round suitably:
   = 1.5 × 10^22 atoms

✅ Correct answer

1.5 × 10^22 atoms

Accept any answer equivalent to this, provided the method is shown clearly.

🧠 Exam technique

• Always use Mr properly: argon has Ar = 40.
• Write the formula carefully: moles = mass ÷ Mr.
• Remember to multiply by 6.02 × 10^23 to get particles/atoms.
• Give the final answer in standard form if possible.

❌ Common errors

• Using 1 ÷ 6.02 × 10^23 straight away — you must find moles first.
• Dividing by 40 and then forgetting to multiply by Avogadro’s constant.
• Using the wrong unit or forgetting that atoms are counted, not measured in g.
• Writing 0.025 atoms — that is moles, not atoms.

💡 What got marks

• Short, precise factual answers for parts (a), (b) and (e).
• Using correct science words like intermolecular forces and full outer shell.
• Showing the full calculation method in part (f).

❌ What lost marks

• Mixing up atomic mass with atomic number.
• Talking about covalent bonds instead of intermolecular forces.
• Giving only the trend but not the explanation in part (d).
• Forgetting to include the electronic structure of neon.

🧠 Top-tip for full marks

If a question asks explain, try to write two linked ideas: the science fact and the reason it matters. For calculations, show each stage so you can earn method marks even if the final number is slightly wrong.

✅ Answers at a glance

• (a) Increasing atomic mass
• (b) Left gaps / rearranged elements by properties
• (c) Simple molecules + weak intermolecular forces
• (d) Boiling points increase down Group 7 because molecules get larger and forces get stronger
• (e) Neon = 2,8 and full outer shell
• (f) 1.5 × 10^22 atoms

💡 Key phrases to remember

• atomic mass
• intermolecular forces
• full outer shell
• moles = mass ÷ Mr
• Avogadro’s constant = 6.02 × 10^23