AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Higher), 2024: Question 7

✅ Correct answer

Each calcium atom loses two electrons.

Each chlorine atom gains one electron.

So one calcium atom reacts with two chlorine atoms.

This forms Ca²⁺ ions and Cl⁻ ions.

💡 Key knowledge

• Calcium is in Group 2, so it forms a 2+ ion.
• Chlorine is in Group 7, so each atom gains 1 electron to form a 1− ion.
• Two chlorine atoms are needed to take the two electrons lost by one calcium atom.
• The mark scheme also allows wording about full outer shells.

🧠 Exam technique

• For 4 marks, give 4 separate points rather than one vague sentence.
• Name both substances: what calcium does and what chlorine does.
• Include the ratio: one calcium to two chlorine atoms.
• Finish with the ions formed to secure the final mark.

Examiner insight: students often got some credit for saying calcium loses electrons and chlorine gains electrons, but lost marks by missing the numbers: two electrons for calcium and one electron per chlorine atom.

❌ Common errors

• Saying chlorine gains two electrons.
• Forgetting that two chlorine atoms are needed.
• Writing the wrong ions, such as Ca⁺ or Cl²⁻ .
• Talking generally about “sharing electrons” — this is ionic bonding, so electrons are transferred, not shared.

✅ Correct answer

Chlorine has fewer shells than bromine.

So the outer shell is closer to the nucleus.

There is a stronger attraction between the nucleus and an incoming electron.

So chlorine gains an electron more easily and is more reactive.

💡 Key knowledge

• Halogens react by gaining one electron.
• Down Group 7, atoms have more shells.
• More shells means the outer shell is further from the nucleus.
• This causes more shielding and weaker attraction for an incoming electron.

The mark scheme allows alternatives such as: chlorine is a smaller atom, or there is less shielding.

🧠 Exam technique

• This is a chained explanation: fewer shells → closer outer shell → stronger attraction → gains electron more easily.
• Use the word because or so to link the ideas.
• Make sure your answer explains reactivity, not just atomic size.

Examiner insight: stronger answers linked the size/shell idea directly to gaining an electron. Weaker answers only said “chlorine is smaller” and stopped there.

❌ Common errors

• Saying chlorine is more reactive because it has more shells — it has fewer than bromine.
• Talking about losing electrons. Halogens become reactive by gaining electrons.
• Saying chlorine has a stronger attraction because it has more protons, without linking to distance and shielding.
• Giving the trend the wrong way round and saying reactivity increases down Group 7.

✅ Correct answer

As the relative formula mass increases, the boiling points of the hydrogen halides increase.

However, HF does not follow this trend because HF has the highest boiling point.

💡 Key knowledge

• Look for the overall pattern first.
• From HCl to HI, boiling point increases.
• HF is the exception or anomalous result.
• For this question, you only need to describe the trend, not explain it.

🧠 Exam technique

• For 2 marks, give both the trend and the exception.
• Use the word however or but to show the break in pattern.
• Do not just list the values — turn them into a sentence about the pattern.

Examiner insight: many students gained 1 mark for spotting that boiling point increases with relative formula mass, but missed the second mark by not mentioning that HF does not follow the trend.

❌ Common errors

• Saying all four values increase smoothly — they do not, because HF is much higher.
• Describing the trend as “decreasing” because of the jump from HF to HCl.
• Trying to explain instead of describe, and then forgetting to state the actual trend.

✅ Mark breakdown

• 07.1 = 4 marks
  • calcium loses two electrons
  • each chlorine gains one electron
  • one calcium reacts with two chlorine atoms
  • Ca²⁺ and Cl⁻ ions formed
• 07.2 = 4 marks
  • chlorine has fewer shells / is smaller
  • outer shell closer to nucleus / less shielding
  • stronger attraction for incoming electron
  • gains electron more easily
• 07.3 = 2 marks
  • boiling point increases as relative formula mass increases
  • HF is the exception / HF has the highest boiling point

🧠 What top answers did well

• Used precise words like loses, gains, ions, shells, and attraction.
• Included the correct numbers: 2 electrons, 1 electron, 2 chlorine atoms.
• Recognised that a data trend can have an exception.
• Answered exactly what the question asked: explain for 07.1 and 07.2, describe for 07.3.