AQA A-Level Chemistry AS Paper 1, June 2023: Question 14
1 mark · Easy difficulty · Multiple Choice
Identify which molecule can accept an electron pair during the formation of a coordinate bond.
Practise this questionQuestion
Question text
14 Which molecule can accept an electron pair during the formation of a coordinate
bond?
[1 mark]
A NH3
B AlCl3
C SiH4
D PCl3
Mark scheme
Show the mark scheme
14 B 1 (AO1) AlCl3
How to answer it
Coordinate (Dative Covalent) Bonding: Electron Pair Acceptors
This question tests your understanding of coordinate (dative covalent) bonding, outer-shell electronic configurations, and identifying electron-deficient species (Lewis acids) versus lone-pair donors (Lewis bases).
- Coordinate bond requirement: One species must have a vacant valence orbital to accept a lone pair; the other species must possess a lone pair of electrons to donate.
- Octet expansion and deficiency: Group 13 elements (such as B and Al) form neutral molecules with only 6 outer electrons, leaving a vacant orbital.
Multiple Choice: Identify the Electron Pair Acceptor
Question Breakdown & Mark Scheme
✅ Correct Answer
B: AlCl₃
Aluminium is in Group 13 and has 3 valence electrons. In AlCl₃, it forms 3 single covalent bonds, leaving it with only 6 electrons in its outer shell (an incomplete octet). It has an empty valence orbital and readily accepts an electron pair to form a coordinate bond (e.g. in Al₂Cl₆ or AlCl₄⁻).
💡 Key Knowledge
- Coordinate Bond: A shared pair of electrons where both electrons originate from the same atom.
- Acceptor (Lewis acid): Must possess an empty orbital to accommodate 2 incoming electrons.
- Donor (Lewis base): Must possess a non-bonding lone pair of electrons.
- Dimerisation example: In gaseous Al₂Cl₆, a chlorine atom on one AlCl₃ unit donates a lone pair to the electron-deficient Al atom of another AlCl₃ unit ( Al ← :Cl ).
📐 Electronic Structure Analysis of All Options
| Molecule | Outer Electrons on Central Atom | Bonding Pairs | Lone Pairs | Role in Coordinate Bonding |
|---|---|---|---|---|
| A: NH₃ | 5 (Nitrogen, Group 15) | 3 (N–H) | 1 | Donor (donates its lone pair, e.g. to H⁺ to form NH₄⁺) |
| B: AlCl₃ | 3 (Aluminium, Group 13) | 3 (Al–Cl) | 0 (has vacant orbital) | Acceptor (has only 6 outer electrons; incomplete octet) |
| C: SiH₄ | 4 (Silicon, Group 14) | 4 (Si–H) | 0 | Neither (complete stable octet of 8 electrons; tetrahedral) |
| D: PCl₃ | 5 (Phosphorus, Group 15) | 3 (P–Cl) | 1 | Donor (has a non-bonding lone pair, similar to NH₃) |
❌ Common Errors & Traps
- Confusing Donor and Acceptor: Many students confuse "accepting" with "donating". NH₃ and PCl₃ are famous donors because of their lone pairs. Choosing A is the most frequent blunder.
- Assuming all Group 13/15 compounds behave identically: Notice the distinction: Group 15 (N, P) have 5 valence electrons (3 bonds + 1 lone pair), while Group 13 (B, Al) have 3 valence electrons (3 bonds + empty orbital).
- Assuming non-metals always obey the octet rule: Forgetting that BF₃ and AlCl₃ are stable with only 6 outer-shell electrons makes it harder to spot their electron deficiency.
🧠 Exam Technique: 10-Second Elimination Strategy
- Read carefully: Highlight the word "accept".
- Identify lone-pair rich options: NH₃ and PCl₃ both belong to Group 15 and have lone pairs — they are donors, immediately cross them out.
- Check saturated options: SiH₄ has 4 single bonds (8 electrons), leaving no vacant orbital.
- Verify Group 13 deficiency: AlCl₃ only has 3 bonding pairs = 6 outer electrons. An empty orbital is present, which must be the acceptor. Select B.
Topics
Physical Chemistry · 3.1.3 Bonding
Question and mark scheme from the AQA A-Level Chemistry examination, AS Paper 1, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.