AQA A-Level Chemistry Paper 3, 2024: Question 10

AQA A-Level Chemistry • Bonding (Dative covalent)

Spotting a Dative (Coordinate) Bond in Diagrams

What this question tests

Knowledge

Definition of a dative covalent bond: both electrons in the shared pair come from the same atom.
Recognising lone pair donor → electron-deficient acceptor (e.g. H⁺, BF₃).
Arrow convention: arrow points from the atom donating the lone pair to the atom accepting it.

Skill / AO

AO2: selecting the correct diagram based on bonding logic (not just memorising examples).

1 mark Multiple choice Coordinate bond arrows

Part (a) — Which diagram shows the formation of a dative covalent bond? [1 mark]

✅ Correct answer (from mark scheme)

Mark breakdown: 1 mark for selecting option C only.

💡 Key knowledge (how to recognise it fast)

Donor: species with a lone pair (e.g. O, N, NH₃, H₂O).
Acceptor: species with an empty orbital / electron deficiency (e.g. H⁺, AlCl₃, BF₃, metal cations).
Arrow direction: donor → acceptor (from lone pair atom to the electron-deficient species).
After bond formation, the bond is just a covalent bond, but we use an arrow to show its origin.

🧠 Exam technique (what to check in the diagram)

Look for an H⁺ (or other cation) being attached: H⁺ can only form a bond by accepting a lone pair.
Confirm the arrow is pointing to the species that needs electrons (often the positively charged one).
If the diagram already shows a normal single bond with no donor/acceptor logic, it’s probably not “formation of a dative bond”.

Examiner insight: the 1 mark is “all-or-nothing” — no working is credited, so be decisive.

Why option C is correct

In option C, H⁺ is shown approaching the oxygen in water. Oxygen has lone pairs and can donate a lone pair to H⁺ to form a coordinate bond (forming H₃O⁺ overall).

The arrow indicates electron-pair donation ( O → H ), which is exactly what “formation of a dative covalent bond” means.

❌ Common errors (why students lose this mark)

Arrow the wrong way: drawing/choosing an arrow pointing from H⁺ to O (H⁺ has no electrons to donate).
Confusing polarity with dative bonding: thinking a δ⁺/δ⁻ attraction means coordinate bond formation.
Picking “familiar-looking” ions without checking the arrow: e.g. seeing NH₄⁺ or complexes and guessing.
Forgetting what “formation” means: the diagram should show a donor lone pair being used to make a new bond to an acceptor.

Quick elimination of the other options (examiner-style reasoning)

A: Shows H⁺ near NH₃, but the diagram does not clearly indicate the correct coordinate bond formation with arrow convention as required here (mark scheme does not accept it).
B: N → B looks like a classic dative bond (BF₃ is an acceptor), but the question asks for the diagram that shows formation as presented; in this paper the credited option is C.
D: Shows bonding involving Ag⁺ between two NH₃ groups; the arrowing/representation is not the accepted “formation of a dative covalent bond” diagram for this question.

Important: In multiple choice, you must answer according to the exact conventions used in the paper/mark scheme.

✅ What to draw/write if this were not multiple choice

If asked to show the formation of the bond in option C, you would:

Draw H₂O with a lone pair on O (even as “:” next to O).
Draw H⁺ separately.
Draw an arrow from the O lone pair to H⁺ (O → H).
Show the product as H₃O⁺ with an overall + charge.

Mini checklist to secure the mark next time

💡 Dative bond checklist

Is there a lone pair donor (N/O/halide, etc.)?
Is there an electron-pair acceptor (H⁺, BF₃, AlCl₃, metal ion)?
Does the arrow go from donor to acceptor?

❌ Last-second traps

Choosing based on “looks right” rather than arrow direction.
Mixing up dipole arrows with coordinate bond arrows (they mean different things).