AQA A-Level Chemistry Paper 3, June 2018: Question 6

1 mark · Easy difficulty · Multiple Choice

Identify the diagram that shows the correct bonding and bond polarity in a molecule of oxygen difluoride (OF2).

Practise this question

Question

Question 06 asks: 'Which diagram shows the correct bonding and correct bond polarity in a molecule of oxygen difluoride?' Four options are given, each showing a non-linear F-O-F arrangement: A shows regular single covalent bonds with delta negative on each fluorine and delta positive on oxygen; B shows dative covalent bonds pointing from oxygen to fluorines with delta positive on fluorine and delta negative on oxygen; C shows dative covalent bonds pointing from fluorines to oxygen with delta negative on fluorine and delta positive on oxygen; D shows regular single covalent bonds with delta positive on fluorine and delta negative on oxygen.
Question text

06 Which diagram shows the correct bonding and correct bond polarity in a molecule of

oxygen difluoride?

[1 mark]

A

B

C

D

Mark scheme

Show the mark scheme Mark scheme table shows that for question 06, the correct answer is option A.

06 A

How to answer it

Bonding & Polarity in Oxygen Difluoride (OF₂)

📋 What this question tests

Understanding covalent vs coordinate (dative) bonding, applying periodic trends in electronegativity to determine bond dipole moments, and identifying partial charges (δ⁺ / δ⁻) in simple non-metal molecules.

Question 06 Analysis

Multiple Choice (1 Mark)

✅ Correct Option: A

Diagram A shows standard single covalent bonds (represented by solid lines) and correct partial charges:

  • Fluorine atoms carry a partial negative charge (δ⁻).
  • The oxygen atom carries a partial positive charge (δ⁺).
  • Bonds are normal shared pairs of electrons, not coordinate (dative) bonds.

📐 Step-by-Step Logic Breakdown

  1. Determine the bond type:
    Oxygen has 6 outer shell electrons and shares 1 electron with each of the two fluorine atoms. Each F atom shares 1 electron. Both atoms contribute one electron to each single bond. Therefore, these are standard covalent bonds (solid lines: — ), eliminating B and C (which incorrectly use coordinate arrows → ).
  2. Compare electronegativities:
    Electronegativity increases across Period 2 ( C < N < O < F ). Fluorine is the most electronegative element on the Pauling scale (approx. 4.0), while oxygen is second (approx. 3.4).
  3. Assign partial charges:
    Because F is more electronegative than O, the electron density in each O–F bond is drawn towards the fluorine atoms, giving each F atom a δ⁻ charge and the O atom a δ⁺ (or 2δ⁺) charge. This eliminates D.

💡 Key Knowledge

  • Electronegativity definition: The power of an atom to attract the shared pair of electrons in a covalent bond towards itself.
  • Fluorine's supremacy: Fluorine is always the most electronegative atom in any compound it forms. Oxygen is only ever partially positive (or in a positive oxidation state like +2 in OF₂) when bonded to fluorine.
  • Coordinate (dative) bond convention: An arrow ( A → B ) indicates that both shared electrons originated from atom A. In OF₂, each atom provides one electron to each bond, so simple lines must be used.

❌ Common Errors & Misconceptions

  • The "Oxygen is always δ⁻" reflex: Students are so accustomed to water (H₂O), alcohols, and carbonyls that they instinctively assign δ⁻ to oxygen (selecting option D). Remember to check which element is further right/up in the Periodic Table!
  • Confusing dipole arrows with coordinate bonds: In physics and some general chemistry contexts, arrows denote dipole vectors. However, in standard AQA skeletal/displayed structural representations, arrows denote dative covalent bonds.

🧠 Exam Technique: Two-Filter Elimination

For combined questions ("correct bonding and correct polarity"), split the decision into two independent filters to eliminate options rapidly under exam pressure:

  • Filter 1 (Bonding representation): Do the bonds involve coordinate donation? No, oxygen needs 2 electrons to complete its octet, each F needs 1. Each bond is mutually shared. → Cross out B and C instantly.
  • Filter 2 (Dipole direction): Which atom is more electronegative: O or F? F is to the right of O, so F is more electronegative → F must be δ⁻. → Cross out D. Only A remains.
Mark Allocation: [1 mark] strictly awarded for selecting A. No partial marks available.

Topics

Physical Chemistry · 3.1.3 Bonding

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.