AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Higher), 2018: Question 4

✅ Correct answer

Cl₂ (g)

So the state symbol is g.

💡 Key knowledge

• Chlorine is a gas at room temperature.
• State symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous.

🧠 Exam technique

• Write the symbol exactly as g.
• The mark scheme does not accept upper case G.
• Do not write the word “gas” if a state symbol is asked for.

❌ Common errors

• Using G instead of g.
• Leaving the blank empty.
• Writing (l) or (s) by mistake.

✅ Correct answer

• Draw one shared pair of electrons in the overlap between the two fluorine atoms.
• Show 6 other outer-shell electrons on each fluorine atom.

What should be drawn: two overlapping circles labelled F and F, with one electron from each atom in the overlap to make the covalent bond, and three lone pairs on each atom outside the overlap.

💡 Key knowledge

• Fluorine is in Group 7, so each atom has 7 outer-shell electrons.
• In F₂, each atom shares 1 electron to make a single covalent bond.
• This leaves 6 outer electrons on each fluorine atom as lone pairs.

🧠 Exam technique

• The exact choice of dots and crosses is flexible.
• You can use dots only, crosses only, or a mixture — the key is to show the shared pair clearly.
• The mark scheme says to ignore inner-shell electrons, so only draw outer-shell electrons.

❌ Common errors

• Showing two shared pairs instead of one.
• Putting all 7 electrons in the overlap.
• Forgetting the 6 other electrons on each atom.
• Drawing inner-shell electrons when they are not needed.

✅ Correct answer

18

💡 Key knowledge

• The atomic number of fluorine is 9.
• A neutral fluorine atom has 9 electrons.
• F₂ has two fluorine atoms, so 9 + 9 = 18 electrons.

🧠 Exam technique

• Use the atomic number, not the mass number.
• In this question, the number of electrons is the same as the number of protons because fluorine atoms are neutral.

❌ Common errors

• Answering 9 because you forgot it is a molecule.
• Using the mass number 19 by mistake.
• Giving 14 or 38 from unrelated counting errors.

✅ Correct answer

2 Al + 3 Br₂ → 2 AlBr₃

The product formula is AlBr₃.

💡 Key knowledge

• Aluminium forms Al³⁺ ions.
• Bromine forms Br⁻ ions.
• So the compound must contain 1 aluminium and 3 bromine atoms: AlBr₃.

🧠 Exam technique

• First write the correct formula for the product, then balance the equation.
• The mark scheme allows a range of equivalent formatting, including brackets as Al(Br)₃ , but the standard answer is AlBr₃.
• State symbols and charges are ignored here.

❌ Common errors

• Writing the wrong formula, e.g. AlBr or AlBr₂.
• Forgetting bromine is Br₂ in its elemental form.
• Not balancing atoms on both sides.

✅ Correct answer

• Chlorine is a smaller atom than bromine, or it has fewer energy levels.
• It has less shielding.
• So there is a greater attraction between the nucleus and an incoming electron, meaning chlorine can gain an electron more easily.

💡 Key knowledge

• Group 7 reactivity decreases down the group.
• Top halogens are more reactive because they attract an extra electron more strongly.
• In the reaction shown, chlorine displaces bromine from potassium bromide because chlorine is more reactive.

🧠 Exam technique

• To get full marks, build the explanation in a chain:
• smaller atom / fewer shells → less shielding → stronger attraction → gains electron more easily.
• Use chlorine and bromine correctly. The “it” in the question refers to chlorine.
• If you do not mention outer shell or energy levels, the mark scheme limits the marks you can gain.

❌ Common errors

• Saying chlorine is more reactive because it has more electrons — this is not the mark point.
• Referring to the wrong ion, such as chloride atom.
• Giving a vague answer like “chlorine is stronger” without explaining why.

💡 Top responses did these things

• Used the exact state symbol (g).
• In the dot-and-cross diagram, showed one shared pair and the correct number of outer electrons.
• Used the atomic number to count electrons correctly in F₂.
• Wrote the correct formula AlBr₃ before balancing.
• Explained reactivity using size, shielding and attraction rather than just saying “it is more reactive”.

❌ Where students lost marks

• Not knowing chlorine is a gas.
• Drawing the fluorine molecule with the wrong number of shared electrons.
• Using 19 instead of 9 to count electrons.
• Forgetting bromine exists as Br₂.
• Giving a trend statement with no explanation of outer shell distance or shielding.