AQA A-Level Chemistry AS Paper 1, November 2021: Question 15

1 mark · Easy difficulty · Multiple Choice

Identify which bond has the most unsymmetrical electron distribution among the given options.

Practise this question

Question

Multiple choice question 15 asking 'Which bond has the most unsymmetrical electron distribution?' worth [1 mark]. Options are A: H-O, B: H-S, C: H-N, D: H-P.
Question text

15 Which bond has the most unsymmetrical electron distribution?

[1 mark]

A H–O

B H–S

C H–N

D H–P

Mark scheme

Show the mark scheme Mark scheme indicating question 15 correct answer is A, with 1 mark awarded for H-O.

15 A 1 H–O

How to answer it

Identifying the Most Unsymmetrical Electron Distribution

What this question tests: This question tests your understanding of electronegativity and polar bonds. Specifically, it assesses your ability to recognise that an "unsymmetrical electron distribution" in a covalent bond is another way of describing a polar bond with a large permanent dipole, driven by a large difference in electronegativity between the two bonded atoms.
Question 1.5

Exam Breakdown & Solution

✅ Correct Answer

A (H–O)

Awarded 1 mark for selecting option A.

💡 Key Knowledge

  • Electronegativity is the power of an atom to attract the pair of electrons in a covalent bond.
  • Oxygen has the second-highest electronegativity on the Pauling scale (approx. 3.4) compared to hydrogen (approx. 2.2).
  • The larger the difference in electronegativity, the more the bonding electrons are pulled towards one atom, creating the most "unsymmetrical" electron distribution.

🧠 Exam Technique

  • Look at the periodic trends: electronegativity increases across a period and decreases down a group.
  • Oxygen (Period 2) is far more electronegative than Sulfur (Period 3), Nitrogen (Period 2, but lower than O), and Phosphorus (Period 3).
  • When comparing H bonded to different elements, focus entirely on the electronegativity of the heteroatom.

❌ Common Errors

  • Confusing size with electronegativity: Students sometimes mistakenly choose H–S or H–P thinking larger atoms create bigger distortions, forgetting that smaller, more electronegative atoms hold bonding pairs much more tightly.
  • Mixing up N and O: While both are in Period 2, oxygen's higher nuclear charge gives it a greater electronegativity than nitrogen, making H–O more polar than H–N.

Topics

Physical Chemistry · 3.1.3 Bonding

Question and mark scheme from the AQA A-Level Chemistry examination, AS Paper 1, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.