AQA A-Level Chemistry AS Paper 1, June 2022: Question 3

3 marks · Medium difficulty · State/Explain/Describe

Draw the shapes of AsF5 and KrF2 including relevant lone pairs, and deduce the bond angles in AsF5.

Practise this question

Question

Question 03 asks to complete Table 2 by drawing the shapes of AsF5 and KrF2 molecules, showing all lone pairs of electrons that influence the shape, and deducing the bond angle(s) in AsF5. Table 2 has two columns headed AsF5 and KrF2, with two rows: 'Diagram of shape' and 'Bond angle(s)', where the bond angle cell for KrF2 is shaded out. The total mark allocation is 3 marks.
Question text

03 This question is about shapes of molecules.

Complete Table 2 by drawing the shapes of both the AsF5 and KrF2 molecules,

showing all lone pairs of electrons that influence the shape.

Deduce the bond angle(s) in AsF5

[3 marks]

Table 2

AsF5 KrF2

Diagram of shape

Bond angle(s)

Mark scheme

Show the mark scheme Mark scheme for question 03.1 shows a completed Table 2. Under AsF5: trigonal bipyramidal diagram with central As, two axial F atoms at 180 degrees, and three equatorial F atoms (one in plane, one wedged, one dashed) (M1); bond angles given as '90 and 120' (M3). Under KrF2: linear F-Kr-F arrangement with three equatorial lone pairs shown in electron lobes (M2). Guidance notes state KrF2 must show lone pairs (as lobes, crosses, or dots) and be linear, ignoring lone pairs on fluorine.

Question Marking guidance Additional Comments/Guidelines Mark

AsF5 KrF2 KrF2 must show lone pairs (either as lobes

Diagram or crosses/dots) and must be linear.

of shape 3

Ignore any lone pairs on fluorine. (3 x AO1)

03.1

M1 M2

Bond M3: 90 and 120

angle(s)

How to answer it

AQA A-Level Chemistry • Physical Chemistry

Shapes of Molecules & Bond Angles: AsF₅ and KrF₂

What this question tests

This question assesses your ability to apply Valence Shell Electron Pair Repulsion (VSEPR) theory to species that expand their octet into 5 electron pairs. Key knowledge areas include:

  • Determining the number of bonding pairs and lone pairs from group numbers and valence electrons.
  • Drawing accurate 3D representations using standard wedges, dashes, and solid lines.
  • Predicting geometry when lone pairs occupy equatorial positions to minimise 90° repulsions.
  • Recalling and assigning the correct bond angles for non-standard molecular shapes.

Question 03.1 (3 Marks)

Predicting and drawing the 3D shapes of AsF₅ and KrF₂ and deducing bond angles

📐 Step-by-Step Electron Pair Deduction

  1. For AsF₅:
    • As is in Group 15 (Group 5) → 5 outer electrons.
    • Forms 5 single bonds with 5 F atoms (+5 electrons) → total 10 electrons = 5 electron pairs.
    • 5 bonding pairs, 0 lone pairs → Trigonal bipyramidal.
  2. For KrF₂:
    • Kr is a noble gas (Group 18 / 0) → 8 outer electrons.
    • Forms 2 single bonds with 2 F atoms (+2 electrons) → total 10 electrons = 5 electron pairs.
    • 2 bonding pairs, 3 lone pairs.
    • To minimise repulsion, the 3 lone pairs adopt the equatorial positions (120° apart), leaving the 2 F atoms in axial positions (180° apart) → Linear.

✅ Correct Answer & Mark Breakdown

AsF₅ Diagram [1 Mark - M1]:
• Trigonal bipyramid arrangement around central As atom.
• Axial bonds: 2 solid straight lines drawn vertically (one F up, one F down).
• Equatorial bonds: 3 bonds in the horizontal plane (one solid line to the left, one wedge pointing forward-right, one dashed/hatched line pointing back-right).
KrF₂ Diagram [1 Mark - M2]:
• Central Kr atom with two F atoms drawn in a straight line: F—Kr—F (180°).
• 3 lone pairs clearly shown around the central Kr atom (drawn either as electron pair lobes or pairs of dots/crosses ×× ).
Note: Lone pairs on fluorine atoms are not required and are ignored.
Bond Angle(s) in AsF₅ [1 Mark - M3]:
• Must give both: 90° and 120° (both required for 1 mark; 180° is optional but cannot replace 90° or 120°).
Total: 3 Marks (AO1 - recall and application of bonding principles)

💡 Key Knowledge: Why KrF₂ is Linear

Electron pair repulsion decreases in the order:

lone pair–lone pair > lone pair–bonding pair > bonding pair–bonding pair

  • With 5 total electron pairs based on a trigonal bipyramid, lone pairs always prefer the equatorial plane because they experience only two 90° repulsions rather than three.
  • When all 3 equatorial sites are occupied by lone pairs, only the two axial sites remain for the fluorine atoms, forcing a bond angle of 180° (linear).

🧠 Exam Technique & Drawing Rules

  • Always use 3D convention: For trigonal bipyramidal shapes, draw one equatorial bond as a wedge and one as a dash. Flat "cross" shapes are penalised.
  • Check what the question asks: The rubric specifically instructed: "showing all lone pairs of electrons that influence the shape". For KrF₂, failing to draw the 3 lone pairs on Kr forfeits M2 immediately.
  • Bond angles in AsF₅: Because there are two distinct environments (axial and equatorial), there are two primary bond angles: equatorial-equatorial is 120°, axial-equatorial is 90°. Both must be stated.

❌ Common Misconceptions & Traps

  • Treating Kr as inert: Forgetting that larger noble gases (Kr, Xe) can form compounds and expand their octet using available d-orbitals.
  • Drawing KrF₂ bent: Treating KrF₂ like H₂O (2 bonds, bent) because students forget to count all 8 valence electrons of krypton, leading to missed lone pairs.
  • Only writing 90°: Stating only 90° for AsF₅ and forgetting the 120° trigonal planar equatorial angle.
  • Drawing lone pairs on F: While not penalised, spending time drawing 3 lone pairs on every fluorine atom wastes valuable exam time. Only draw lone pairs on the central atom unless specifically asked otherwise.

Topics

Physical Chemistry · 3.1.3 Bonding

Question and mark scheme from the AQA A-Level Chemistry examination, AS Paper 1, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.