OCR A-Level Chemistry Unified chemistry (03), June 2018: Question 1

13 marks · Medium difficulty · Structured Questions

Identify specific elements from the first three periods based on physical and chemical properties, and explain the bonding and structure properties of Period 3 chlorides.

Practise this question

Question

Question 1 asks students to select elements from the first three periods of the Periodic Table based on eight descriptions, including ion electron configurations, successive ionisation energies, and compound properties, followed by a table of Period 3 chlorides showing their electrical conductivity and melting points, which must be explained in terms of bonding and structure.
Question text

1 This question refers to the elements in the first three periods (H Ar) of the Periodic Table.

(a) Select an element from the first three periods that fits each of the following descriptions.

(i) The element that forms a 1– ion with the same electron configuration as helium.

… [1]

(ii) The element with the highest first ionisation energy.

… [1]

(iii) The element in Period 3 which has the successive ionisation energies shown below.

Ionisation number 1st 2nd 3rd 4th

Ionisation energy/kJ mol–1 738 1451 7733 10541

… [1]

(iv) The element which forms a compound with fluorine that has octahedral molecules.

… [1]

(v) An element which reacts with water to form an acidic solution.

… [1]

(vi) The element X, which forms a compound with hydrogen, XH3, with a molar mass of

34.0 g mol–1.

… [1]

(vii) An element which forms a compound with hydrogen in which the element has an

oxidation number of –4.

… [1]

(viii) The element which has a density of 1.33 × 10–3 g cm–3 at room temperature and pressure.

… [1]

(b) Table 1.1 shows some properties of Period 3 chlorides.

Group 1 2 14 (4) 15 (5) 16 (6)

Chloride NaCl MgCl2 SiCl4 PCl3 SCl2

Electrical Solid poor poor poor poor poor

conductivity Liquid good good poor poor poor

Melting point high high low low low

Table 1.1

Explain the properties shown in Table 1.1 in terms of bonding and structure.

… [5]

Mark scheme

Show the mark scheme The mark scheme lists the correct element symbols for each of the eight parts in section (a) and provides detailed marking points for section (b), allocating marks for giant ionic lattices, simple molecular structures, mobile ions in liquids, London forces, and a comparison of bond strengths.

Question Answer Marks Guidance

1 (a) (i) Hydrogen/H 1 ALLOW H2

(ii) Helium/He 1

(iii) Magnesium/Mg 1

(iv) Sulfur/S 1 ALLOW sulphur; S8

(v) Chlorine/Cl OR fluorine/F 1 ALLOW Cl2 OR F2

(vi) Phosphorus/P 1 ALLOW P4

(vii) Carbon/C 1 ALLOW silicon/Si

(viii) Oxygen/O 1 ALLOW O2

(b) NaCl OR MgCl2 2 marks 5

Giant ionic OR ionic lattice

Ions are mobile in liquid state IGNORE aqueous/dissolved ions are mobile

IGNORE ‘free ions’

AND ‘ions are free to carry current’

--------------------------------------------------------------------------------- ---------------------------------------------------------------

SiCl4 OR PCl3 OR SCl2 2 marks

(Simple) molecular OR simple covalent (lattice) ALLOW ‘are molecules’

Induced dipole(–dipole) forces/interactions IGNORE

OR London forces permanent dipole(–dipole) forces

IDID and LDF

van der Waals

------------------------------------------------------------------------------ ------------------------------------------------

Comparison of bond strengths 1 mark

Ionic bonds are stronger than London forces ALLOW attraction between ions for ionic bonds

OR ALLOW intermolecular forces for London forces

Ionic bonds need more energy to break than London forces ALLOW overcome for break

ALLOW indirect comparison, i.e.

Ionic bonds are strong AND London forces are

weak

OR

Ionic bonds need a large amount of energy to

break AND London forces need little energy to

break

Total 13

How to answer it

Periodic Table Elements & Period 3 Chlorides Study Guide

What this question tests

This question assesses your recall of periodic trends, electron configurations, successive ionisation energies, bonding types, and physical properties (melting points and electrical conductivity) across the first three periods of the Periodic Table (H to Ar).

Question 1(a) - Element Identification

Identifying Elements from Descriptions

✅ Correct Answers (Parts i–iv)

  • (i) Hydrogen (or H / H₂ ): Forms a hydride ion (H⁻) with 2 electrons, matching helium.
  • (ii) Helium (or He ): Has the highest first ionisation energy due to its small atomic radius and lack of shielding.
  • (iii) Magnesium (or Mg ): Successive IEs show a massive jump between 2nd (1451) and 3rd (7733) kJ mol⁻¹, indicating removal from a principal quantum shell closer to the nucleus.
  • (iv) Sulfur (or S / S₈ ): Reacts with fluorine to form SF₆, which has an octahedral molecular geometry.

✅ Correct Answers (Parts v–viii)

  • (v) Chlorine or Fluorine (or Cl₂ / F₂ ): Reacts with water to form acidic solutions (e.g., HCl and HClO).
  • (vi) Phosphorus (or P / P₄ ): Forms PH₃ with a molar mass of (31.0 + 3.0) = 34.0 g mol⁻¹.
  • (vii) Carbon (or C ): Forms methane (CH₄) where carbon has an oxidation number of −4.
  • (viii) Oxygen (or O₂ ): Calculate gas density using ideal gas law or standard molar volume at rtp (24.0 dm³ mol⁻¹): density of O₂ is roughly 32 g / 24 dm³ = 1.33 × 10⁻³ g cm⁻³.

🧠 Exam Technique & Common Errors

Examiner Tip: Always check whether the question asks for the element name or its formula. For maximum safety, write both (e.g., "Magnesium / Mg"). Avoid common errors like misinterpreting successive ionisation energy jumps—always count valence electrons by looking for the huge spike in energy.

Question 1(b) - Period 3 Chlorides

Explaining Physical Properties of Chlorides

💡 Key Knowledge: Bonding & Structure

  • NaCl and MgCl₂: Giant ionic lattices. High melting points because large amounts of energy are required to overcome strong electrostatic forces of attraction between oppositely charged ions. Conduct electricity only when molten/liquid because ions become mobile (free to move).
  • SiCl₄, PCl₃, and SCl₂: Simple molecular (or simple covalent) structures. Low melting points because little energy is needed to overcome weak intermolecular forces (induced dipole-dipole / London forces) between molecules. Poor electrical conductivity in both solid and liquid states as there are no mobile charged particles.

❌ Common Errors to Avoid

  • Saying ions are "free to move" in aqueous solutions when referring to conductivity of a liquid (stick strictly to liquid or molten state as shown in the table).
  • Using outdated terminology like "van der Waals forces" broadly without specifying London forces or induced dipole-dipole interactions .
  • Confusing intermolecular forces with ionic bonds when discussing melting point comparisons.
Mark Scheme Breakdown (5 Marks Total):
  • 2 marks for NaCl/MgCl₂: Identifies giant ionic structure + explains ion mobility in liquid state.
  • 2 marks for SiCl₄/PCl₃/SCl₂: Identifies simple molecular structure + names induced dipole-dipole / London forces.
  • 1 mark for Bond Strength Comparison: Explicitly states ionic bonds are stronger than London forces (or require more energy to break).

Topics

Module 2: Foundations in chemistry · Module 3: Periodic table and energy · 2.2 Electrons, bonding and structure · 3.1 The periodic table

Question and mark scheme from the OCR A-Level Chemistry examination, Unified chemistry (03), June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.