AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Higher), June 2025: Question 7

✅ Acceptable Answers (Any 2 for 1 mark each)

• Model A does not show the 3D arrangement of ions / particles (or only shows a 2D arrangement).
• Model A does not show the giant lattice arrangement (only shows two isolated ions / a single pair).
• Model A does not show the relative sizes of the ions.

💡 Key Knowledge: Why Use Different Models?

• Dot-and-cross (Model A): Excellent for showing electron transfer and where valence electrons originate, but misleadingly shows only a single pair of ions.
• 3D space-filling (Model B): Shows how ions pack tightly in a continuous repeating 3D lattice, and shows the relative sizes of Na⁺ vs Cl⁻, but hides inner electrons.

🧠 Exam Technique: "Compared With"

Always focus strictly on what Model A fails to show that Model B successfully shows. If you simply write "Model A has brackets", that is a description, not a scientific limitation of how the material really exists.

❌ Common Errors

• Vaguely stating "Model A is small" instead of "does not show the giant lattice / only shows two ions".
• Writing "Model A shows molecules" — sodium chloride is ionic, never molecular!

📐 Step-by-Step Formula Deduction

1. Identify ion charges: Aluminium = Al³⁺ , Sulfate = SO₄²⁻
2. Find the lowest common multiple: Lowest common multiple of 3 and 2 is 6.
3. Balance charges to equal zero:
   Two Al³⁺ ions: 2 × (+3) = +6
   Three SO₄²⁻ ions: 3 × (−2) = −6
   Overall charge: +6 + (−6) = 0
4. Combine with brackets for compound ion: Al₂(SO₄)₃

✅ Correct Answer

Al₂(SO₄)₃

❌ Common Formula Traps

• Forgetting brackets: Writing Al₂SO₄₃ (which means 43 oxygen atoms!). Whenever you have more than one polyatomic ion, brackets are compulsory.
• Incorrect cases: Writing AL instead of Al , or so₄ instead of SO₄ .
• Leaving charges in the final formula: Do not write Al₂³⁺(SO₄)₃²⁻ .

✅ Correct Answer (1 Mark)

Aluminium has more delocalised electrons per atom than sodium.

Also allowed: Aluminium has more outer shell (valence) electrons per atom / Aluminium releases 3 electrons per atom compared to 1 for sodium.

💡 Key Science: Group Number & Delocalised Electrons

• Sodium (Na): Group 1 metal → 1 outer electron per atom donated to the "sea" of delocalised electrons (forms Na⁺ ions).
• Aluminium (Al): Group 3 metal → 3 outer electrons per atom donated to the "sea" of delocalised electrons (forms Al³⁺ ions).
• Current: Flow of electrical charge. More delocalised electrons per unit volume = higher electrical conductivity.

❌ Examiner Warning

• Do not just say "aluminium has more electrons". You must specify delocalised electrons or outer-shell / valence electrons.
• Do not confuse electron movement with ion movement. In solid metals, the positive ions are fixed in a lattice and do not move!

💡 Comparison Breakdown

🧠 How to Secure Level 2 (3–4 Marks)

This is a Level of Response question. To hit Level 2:

• Identify both similarities and differences.
• Address both structure AND bonding.
• Refer to magnitude / scale (e.g. giant network vs individual long-chain molecules with intermolecular forces).

✅ Model 4-Mark Response

"Similarities: Both substances contain covalent bonds formed by shared pairs of electrons. Furthermore, both contain atoms forming 4 covalent bonds: each silicon atom forms 4 bonds in silicon dioxide, while each carbon atom forms 4 bonds in poly(ethene)."

"Differences: Silicon dioxide is a giant covalent structure with strong covalent bonds extending in all directions. In contrast, poly(ethene) consists of large, individual polymer molecules. While silicon dioxide contains only covalent bonds, poly(ethene) also has intermolecular forces between its polymer chains. Additionally, oxygen forms 2 bonds in SiO₂, whereas hydrogen forms only 1 bond in poly(ethene)."

❌ Common Misconceptions to Avoid

• Thinking polymers have weak covalent bonds: Covalent bonds inside poly(ethene) are very strong; it is only the intermolecular forces between the chains that are relatively weak.
• Calling silicon dioxide a simple molecule: SiO₂ does not exist as small individual molecules like CO₂; it is a giant network lattice (like diamond).
• Listing without comparing: Writing everything about SiO₂ in one paragraph and everything about poly(ethene) in another without explicitly stating what is similar and what is different.