AQA A-Level Chemistry AS Paper 1, June 2023: Question 5

6 marks · Medium difficulty · Long Answer

Discuss the difference between the shapes and bond angles of CF4 and XeF4.

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Question text

05 This question is about the shapes of molecules.

Discuss the difference between the shapes of CF4 and XeF4

In your answer you should:

• name the shape of each molecule

• explain the shape of each molecule

• explain the bond angle(s) in each molecule.

[6 marks]

Mark scheme

Show the mark scheme

Question Marking guidance Additional Comments/Guidelines Mark

This question is marked using levels of response. Refer to the Mark Stage 1: Name of each shape

Scheme Instructions for Examiners for guidance on how to mark this

question. 1a: CF4 = Tetrahedral

Level 3:

All stages are covered and the explanation of each stage is 1b: XeF4 = Square Planar

generally correct and virtually complete.

5-6 Stage 2: Explanation of shape / bond angle

Answer is well structured with no repetition or irrelevant

points. in CF4

Accurate and clear expression of ideas with no errors in use 2a: four bonding pairs (and zero lone pairs)

of technical terms.

Level 2: 2b: electron pairs repel each other to be as far

All stages are covered but the explanation of each stage apart as possible / electron pairs repel

may be incomplete or may contain inaccuracies OR two each other equally

stages are covered and the explanations are generally

correct and virtually complete. 3-4 2c: so bond angle is 109.5 °

05 Answer shows some attempt at structure.

(6 x AO2)

Ideas are expressed with reasonable clarity with, perhaps, Stage 3: Explanation of shape / bond angle

some repetition or some irrelevant points. in XeF4

Some minor errors in use of technical terms.

Level 1: 3a: four bonding pairs

Two stages are covered but the explanation of each stage 3b two lone pairs

may be incomplete or may contain inaccuracies, OR only

one stage is covered but the explanation is generally correct 3c: so bond angle is 90 °

and virtually complete.

1-2 3d: lone pairs repel more than bonding pairs,

Answer includes isolated statements but these are not

presented in a logical order or show some confusion.

Answer may contain valid points which are not clearly linked 3e: so lp as far apart as possible / so lone

to an argument structure. Errors in the use of technical pairs are opposite each other /

terms. 180 ° apart

Level 0:

Insufficient correct chemistry to gain a mark.

How to answer it

Shapes of Molecules: CF₄ vs XeF₄

📌 What this question tests

This 6-mark extended-response question assesses your understanding of Valence Shell Electron Pair Repulsion (VSEPR) Theory applied to standard and expanded-octet molecules:

Deducing electron pair geometry: total pairs, bonding pairs (bp), and lone pairs (lp).
Naming 3D molecular geometries accurately ( CF₄ vs XeF₄ ).
Explaining shapes using electron pair repulsion rules (lone pair–lone pair > lone pair–bonding pair > bonding pair–bonding pair).
Predicting and justifying bond angles ( 109.5° vs 90° / 180° ).

⚖️ Levels of Response Breakdown

AQA 6-Mark Structure: The 3 Stages

This question is marked using a 3-stage criteria. To achieve Level 3 (5–6 marks), all three stages must be covered with accurate technical language and clear logic.

Stage 1: Naming the Shapes

CF₄: Tetrahedral
XeF₄: Square planar

Stage 2: Explanation of CF₄

4 bonding pairs, 0 lone pairs around C.
Electron pairs repel equally to get as far apart as possible.
Bond angle is 109.5°.

Stage 3: Explanation of XeF₄

4 bonding pairs, 2 lone pairs around Xe (total 6 pairs; octahedral arrangement).
Lone pairs repel more than bonding pairs.
Lone pairs position opposite each other (180° apart) to minimise repulsion.
F–Xe–F bond angle is 90°.

🔍 Stage-by-Stage Analysis

Detailed Explanations & Model Answers

✅ Molecule 1: CF₄ (Carbon Tetrafluoride)

1. Electron Count:

Carbon is in Group 4 → 4 outer electrons.
4 fluorine atoms contribute 1 electron each → 4 + 4 = 8 electrons = 4 pairs.
Forms 4 bonding pairs, 0 lone pairs.

2. Shape & Bond Angle:

Name: Tetrahedral.
Bond angle: 109.5°.

3. Justification:

Electron pairs repel each other to be as far apart as possible. Since all 4 pairs are bonding pairs, they repel each other equally, giving a symmetrical tetrahedral structure.

✅ Molecule 2: XeF₄ (Xenon Tetrafluoride)

1. Electron Count:

Xenon is a noble gas (Group 0/8) → 8 outer electrons.
4 fluorine atoms contribute 1 electron each → 8 + 4 = 12 electrons = 6 pairs.
Forms 4 bonding pairs and 2 lone pairs.

2. Shape & Bond Angle:

Name: Square planar.
F–Xe–F bond angle: 90° (and 180° across opposite F atoms).

3. Justification:

Total of 6 electron pairs adopt an underlying octahedral geometry. Lone pairs repel more than bonding pairs. To minimise lone pair–lone pair repulsion, the 2 lone pairs sit opposite each other (180° apart, above and below the plane). The 4 bonding pairs remain in a single plane at 90° to one another.

📐 How to Draw the Molecules (Examiner Visual Advice)

CF₄: Central C atom with two standard lines in the plane of the paper (angle ~109.5°), one wedged bond pointing forwards, and one dashed/hashed bond pointing backwards, each ending in an F atom.
XeF₄: Central Xe atom with 4 F atoms drawn in a flat square/cross in the plane (or viewed in perspective with wedges/dashes). Two lone pairs drawn as electron lobes with two dots each, located directly opposite each other along the vertical axis (above and below the Xe atom).

🧠 Examiner Insight

Exam Technique vs Common Traps

🧠 Top Exam Technique for 6/6

Use Subheadings: Clearly separate your answer into CF₄ and XeF₄ . This guarantees the examiner spots all three stages easily.
Always state pair types explicitly: Do not just say "there are 4 pairs" — state "4 bonding pairs and 0 lone pairs".
Quote the repulsion rule: Always include the general VSEPR principle: "electron pairs repel as far apart as possible" and "lone pairs repel more than bonding pairs".
Explain lone pair positions: For XeF₄ , explain that the lone pairs reside opposite each other (180°) to minimise lp-lp repulsions.

❌ Common Student Errors

Calling XeF₄ "tetrahedral": Students see 4 fluorine atoms on both molecules and assume both are tetrahedral, forgetting xenon's 2 lone pairs.
Forgetting Xenon's valence electrons: Xenon has 8 valence electrons, not 0.
Vague repulsion statements: Writing "atoms repel" or "bonds repel" instead of electron pairs repel. Marks are routinely lost for imprecise wording.
Giving incorrect bond angles: Stating 109.5° or 104.5° for XeF₄ instead of 90°. Because the lone pairs cancel out symmetrically, the equatorial F–Xe–F angle remains exactly 90°.

Examiner Summary: A student who writes "both have 4 bonds so both are tetrahedral" receives 1 or 2 marks at best (Level 1). High-scoring scripts clearly demonstrate counting valence electrons to show why XeF₄ has 6 pairs (4 bp + 2 lp) while CF₄ has 4 pairs (4 bp + 0 lp), explicitly naming both shapes and bond angles.

Topics

Physical Chemistry · 3.1.3 Bonding

Question and mark scheme from the AQA A-Level Chemistry examination, AS Paper 1, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.