AQA A-Level Chemistry AS Paper 1, November 2021: Question 13
1 mark · Easy difficulty · Multiple Choice
Identify which given molecule is not able to form a co-ordinate bond with another species
Practise this questionQuestion
Question text
13 Which molecule is not able to form a co-ordinate bond with another species?
[1 mark]
A BH3
B CH4
C NH3
D H2O
Mark scheme
Show the mark scheme
13 B 1 CH4
How to answer it
Identifying Co-ordinate Bonding Capability
What this question tests
This question assesses your understanding of co-ordinate (dative covalent) bonding. Specifically, it tests your ability to recognize the structural requirements for a molecule to act either as an electron-pair donor (possessing a lone pair) or an electron-pair acceptor (possessing an electron-deficient central atom with an incomplete octet).
Exam Breakdown
✅ Correct Answer
B (CH₄)
💡 Key Knowledge
- Co-ordinate bond: Formed when one atom provides both of the shared electrons in a covalent bond.
- Donor requirement: The molecule must have a lone pair of electrons (e.g., NH₃ , H₂O ).
- Acceptor requirement: The molecule must be electron-deficient with a vacant orbital (e.g., BH₃ ).
🧠 Exam Technique
Quickly sketch or visualize the outer-shell electrons of the central atom for each option:
- C (in CH₄): Group 4, forms 4 single covalent bonds, full octet, zero lone pairs, no vacant orbitals. Completely saturated and stable.
❌ Common Errors
Students sometimes confuse general covalent bonding stability with reactivity or mistakenly assume molecules with polar bonds can automatically form dative bonds. Always check for lone pairs or electron deficiency explicitly!
Detailed Analysis of All Options
- A ( BH₃ ): Boron has 3 outer electrons and forms 3 single bonds, leaving it with only 6 electrons in its outer shell. It is electron-deficient and acts as an electron-pair acceptor (e.g., forming H₃N-BH₃ ).
- B ( CH₄ ): Carbon has 4 outer electrons and shares 4 pairs with hydrogen, achieving a stable octet. It has no lone pairs and no vacant orbitals, meaning it cannot donate or accept a pair to form a co-ordinate bond. (Correct choice)
- C ( NH₃ ): Nitrogen has a lone pair of electrons and acts as a dative covalent donor (e.g., forming the ammonium ion NH₄⁺ ).
- D ( H₂O ): Oxygen has two lone pairs of electrons and acts as a dative covalent donor (e.g., coordinating with H⁺ to form H₃O⁺ ).
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.