OCR A-Level Chemistry AS Breadth in chemistry (01), June 2023: Question 21
7 marks · Medium difficulty · Structured Questions
Describe covalent bonding, draw dot-and-cross diagrams, predict shapes and bond angles, and explain the shapes of NF3 and BF3 molecules.
Practise this questionQuestion
Question text
21 This question is about NF3 and BF3 molecules.
(a) NF3 and BF3 contain covalent bonds.
(i) What is meant by a covalent bond?
… [1]
(ii) Draw ‘dot-and-cross’ diagrams for NF3 and BF3.
Show outer electrons only.
NF3 BF3
[2]
(b) Molecules of NF3 and BF3 have different shapes and bond angles.
(i) Predict the different shapes of, and bond angles in, NF3 and BF3 molecules.
Bond angle Name of shape
NF3
BF3
[2]
(ii) Explain why NF3 and BF3 molecules have different shapes and bond angles.
… [2]
Mark scheme
Show the mark scheme
AO
Question Answer Marks Guidance
element
21 (a) (i) Shared pair of electrons ✓ 1 AO1.1 IGNORE comments about attraction between
nuclei and shared pair OR magnetic attraction
(ii) 2 AO2.5 Use annotations with ticks, crosses ECF etc.
F F 2 for this part
F N F B
Must be ‘dot-and-cross’ using 2 different symbol
F F for electron source ONLY
IGNORE absence of circles
NF3:
3 ‘dot-and-cross’ bonds AND 1 lone pair around N ALLOW 1 mark for
AND 3 lone pairs around each of the 3 F atoms ✓ correct dot and cross diagrams for NF3 AND BF3
BUT with F lone pairs omitted
BF3:
3 ‘dot-and-cross’ bonds ONLY around B ALLOW absent symbols from circles
AND 3 lone pairs around each of the 3 F atoms ✓
A lone pair can be shown as 2 single electrons
IGNORE charges
(b) (i) Bond angles and shapes 2 AO1.2
Bond angle Name of shape For NF3
ALLOW 106–108º OR 109.5º – 2.5º
NF3 107º pyramidal ALLOW pyramid
BF 120º trigonal planar ALLOW trigonal pyramid(al)
For BF3
ALL 4 points → 2 marks ✓✓ ALLOW planar triangle
2 OR 3 points → 1 mark ✓ BUT ‘planar’ is insufficient
AO
element
(ii) 2 AO1.2
• NF3 has 3 bonded pairs AND 1 lone pair (of electrons) 2 ALLOW lp for lone pair (of electrons)
AND lone pairs repel more (than bonded pairs) ✓ bp for bonding pair (of electrons)
regions for electron pairs
ALLOW ‘bonds’ for ‘bonded pair’
• BF3 has three bonded pairs (of electrons) ✓
IGNORE ‘electrons repel’
DO NOT ALLOW ‘atoms repel’
How to answer it
Bonding, Molecular Shapes and Bond Angles (NF₃ and BF₃)
What this question tests
This question assesses your foundational understanding of covalent bonding definitions, your ability to construct accurate dot-and-cross diagrams (including non-octet and electron-deficient centres like boron), your knowledge of standard VSEPR (Valence Shell Electron Pair Repulsion) shapes and bond angles, and your ability to explain structural differences based on lone pairs and bonding pairs.
Definition of a Covalent Bond
✅ Correct Answer
A shared pair of electrons.
💡 Key Knowledge
You must memorise precise definitions for OCR AS chemistry. A covalent bond specifically forms between non-metal atoms when they share one or more pairs of electrons to achieve a stable outer shell.
❌ Common Errors
Students often lose marks by overcomplicating the definition or mentioning electrostatic attractions between nuclei and electrons (which describes bonding generally or metallic bonding). Keep it concise: shared pair of electrons .
Dot-and-Cross Diagrams for NF₃ and BF₃
✅ Correct Answer
For NF₃: Nitrogen must be shown with 3 bonding pairs shared with 3 Fluorine atoms, plus 1 lone pair on Nitrogen. Each Fluorine atom must display its remaining 3 unshared lone pairs (total 8 outer electrons per F atom).
For BF₃: Boron must be shown with 3 bonding pairs shared with 3 Fluorine atoms and no lone pairs on Boron (an electron-deficient molecule with only 6 electrons in its outer shell). Each Fluorine atom has 3 lone pairs.
🧠 Exam Technique
Use two distinct symbols (e.g., dots for one atom's electrons and crosses for the other) to represent the electron sources clearly. Make sure non-bonding outer electrons on surrounding atoms (Fluorine) are explicitly drawn to show full octets where required.
❌ Common Errors
A major trap is forcing boron (BF₃) to have an octet by adding an extra lone pair onto B. Boron is an exception to the octet rule and only accommodates 6 valence electrons around it in BF₃.
Predicting Shapes and Bond Angles
✅ Correct Answer
- NF₃ Bond Angle: 107° (Accept 106° to 108°)
- NF₃ Shape: Pyramidal (or trigonal pyramidal)
- BF₃ Bond Angle: 120°
- BF₃ Shape: Trigonal planar
💡 Key Knowledge
Standard tetrahedral geometry has an angle of 109.5°. The presence of 1 lone pair on nitrogen in NF₃ compresses this angle down to 107° due to extra lone pair-bond pair repulsion. BF₃ has 3 bonding pairs and 0 lone pairs, spreading out symmetrically into a flat trigonal planar layout at 120°.
❌ Common Errors
Writing just "planar" for BF₃ is insufficient; you must write trigonal planar to gain credit. Similarly, writing "pyramidal" without specifying "trigonal" is generally accepted, but using the full standard VSEPR terminology is safest.
Explaining Shape and Bond Angle Differences
✅ Correct Answer
- NF₃ has 3 bonded pairs and 1 lone pair of electrons, and lone pairs repel more than bonded pairs.
- BF₃ has 3 bonded pairs (and 0 lone pairs).
🧠 Exam Technique
VSEPR explanations require a strict comparative structure: state the number of bonding pairs and lone pairs for each molecule, then explicitly mention repulsion hierarchy (lone pair-bond pair repulsion > bond pair-bond pair repulsion).
❌ Common Errors
Examiners heavily penalise answers that state "atoms repel" instead of electron pairs repel. Make sure you refer strictly to electron pairs, bonded pairs, and lone pairs.
Topics
Module 2: Foundations in chemistry · 2.2 Electrons, bonding and structure
Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.