AQA A-Level Chemistry AS Paper 1, June 2022: Question 15

✅ Correct Answer

A: Co-ordinate bond

Arsenic is in Group 15 and has 5 outer electrons. In arsine ( AsH₃ ), it forms 3 single covalent bonds with hydrogen, leaving one lone pair of electrons. The incoming H⁺ ion possesses an empty 1s orbital. The arsenic atom donates its lone pair to form a co-ordinate (dative) bond.

💡 Key Knowledge

• Periodic Group Analogy: Arsenic (As) is in the same group as Nitrogen (N) and Phosphorus (P). The formation of the arsonium ion ( AsH₄⁺ ) is identical to the formation of the ammonium ion ( NH₄⁺ ):
  NH₃ + H⁺ → NH₄⁺
• Definition of Co-ordinate Bond: A shared pair of electrons in which both electrons are provided by the same atom (here, the As atom).
• Once formed, a co-ordinate bond is identical in length, strength, and character to an ordinary covalent bond.

🧠 Exam Technique

• Look for the species involved: Whenever you see a neutral Group 15 molecule with a lone pair ( :NH₃ , :PH₃ , :AsH₃ ) reacting with a bare proton ( H⁺ ), the interaction is always a dative covalent / co-ordinate bond.
• Intra vs Inter: The question asks what interaction forms to create a polyatomic ion ( AsH₄⁺ ). This must be an internal chemical bond, eliminating intermolecular forces straight away.

❌ Common Errors & Distractor Analysis

• B (Dipole–dipole force): Dipole–dipole forces are weak intermolecular attractions between separate neutral polar molecules, not bonds holding an ion together.
• C (Hydrogen bond): Arsenic is not sufficiently electronegative to form hydrogen bonds (only N, O, and F do). Furthermore, H-bonding is an intermolecular force, not the bond forming a molecular cation.
• D (Ionic bond): Students often spot the + charge on H⁺ and think "ionic". An ionic bond is an electrostatic attraction between oppositely charged ions (e.g., Na⁺ and Cl⁻ ), not the addition of a proton to a neutral molecule.