AQA A-Level Chemistry Paper 3, 2017: Question 6

1 mark · Easy difficulty · Multiple Choice

Identify the correct order of melting points for the Period 3 elements sulfur, phosphorus, chlorine, and argon.

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Question

Question 06 asks: 'Which is the correct order of melting points of these Period 3 elements? [1 mark]' followed by four multiple choice options with tick boxes: A 'phosphorus > sulfur > chlorine > argon', B 'argon > chlorine > phosphorus > sulfur', C 'sulfur > phosphorus > chlorine > argon', and D 'chlorine > phosphorus > sulfur > argon'.
Question text

06 Which is the correct order of melting points of these Period 3 elements?

[1 mark]

A phosphorus > sulfur > chlorine > argon

B argon > chlorine > phosphorus > sulfur

C sulfur > phosphorus > chlorine > argon

D chlorine > phosphorus > sulfur > argon

Mark scheme

Show the mark scheme Mark scheme table showing question number 6 with the correct answer corresponding to letter C.

6 C

How to answer it

Period 3 Periodicity: Melting Points of Non-Metals

📋 What This Question Tests

This question assesses your understanding of Period 3 trends in physical properties, specifically the melting points of molecular non-metals:

  • Identifying the molecular formula and structure of non-metallic Period 3 elements (P₄, S₈, Cl₂, Ar).
  • Relating molecular size and electron count to the strength of van der Waals (induced dipole-dipole) forces.
  • Understanding why sulfur has a higher melting point than phosphorus despite being adjacent in the periodic table.

Question 06 Breakdown

Multiple Choice: Order of Melting Points [1 mark]

✅ Correct Answer: C

sulfur > phosphorus > chlorine > argon

Mark Scheme Reference: Award 1 mark for Option C.

Sulfur exists as S₈ molecules, phosphorus as P₄, chlorine as Cl₂, and argon as individual atoms (Ar). Because melting involves breaking only weak van der Waals forces between molecules, the order directly tracks molecular size and electron count: S₈ > P₄ > Cl₂ > Ar.

💡 Key Knowledge

All four of these elements exist as simple molecular (or monoatomic) substances:

Element Formula Electrons / Particle
Sulfur S₈ 128 e⁻
Phosphorus P₄ 60 e⁻
Chlorine Cl₂ 34 e⁻
Argon Ar 18 e⁻

More electrons = larger electron cloud = greater polarisability = stronger van der Waals forces = more thermal energy needed to overcome them.

📐 Step-by-Step Comparison

  1. Classify the structures: None of these are giant lattices (unlike Si, which has giant covalent structure and the highest melting point in Period 3). They all have simple molecular/atomic structures.
  2. Identify what breaks on melting: Only intermolecular forces (van der Waals forces) are overcome, NOT covalent bonds.
  3. Compare particle sizes:
    • S₈ has 8 atoms (128 electrons) → strongest intermolecular forces.
    • P₄ has 4 atoms (60 electrons) → second strongest.
    • Cl₂ has 2 atoms (34 electrons) → third.
    • Ar is monoatomic (18 electrons) → weakest van der Waals forces.
  4. Assemble descending order: S > P > Cl > Ar.

❌ Common Errors & Misconceptions

  • Assuming a strict left-to-right trend: Many students pick Option A because phosphorus (atomic number 15) comes before sulfur (16), incorrectly assuming melting point steadily decreases across the non-metals.
  • Confusing covalent bonds with intermolecular forces: Stating that "covalent bonds break on melting". Covalent bonds inside the S₈ or P₄ molecules remain completely intact.
  • Thinking phosphorus is P₂: Nitrogen in Period 2 forms diatomic N₂, but phosphorus forms tetrahedral P₄ molecules.

🧠 Exam Technique & Examiner Insight

  • The "Peak at Sulfur" rule: In Period 3 melting point graphs, silicon forms the highest peak (giant covalent). Across the molecular non-metals, the graph dips at phosphorus (P₄), jumps back up at sulfur (S₈), and then falls steeply through Cl₂ down to Ar. Remembering the "peak at sulfur" instantly eliminates options A, B, and D!
  • Process of Elimination:
    • Argon is a monoatomic gas at room temperature, so it must have the lowest melting point of the set. This eliminates B immediately.
    • Sulfur (S₈) has a higher melting point than phosphorus (P₄). That instantly narrows the choice down to C.

Topics

Inorganic Chemistry · Physical Chemistry · 3.2.1 Periodicity · 3.1.3 Bonding

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.