What type of bond holds hydrogen fluoride molecules together when forming larger aggregate?

1. Analyze the question The question asks about the force that holds separate HF molecules to one another, so that they form larger aggregates. This is an intermolecular force, not the bond inside each HF molecule.

2. Explain the concept

• Inside an HF molecule, hydrogen and fluorine share electrons through a polar covalent bond. This is an intramolecular bond.
• Fluorine is very electronegative, so it pulls the shared electrons strongly. The H atom becomes partially positive (δ+) and the F atom partially negative (δ−).
• The δ+ hydrogen of one molecule is attracted to the lone pair on the δ− fluorine of a neighbouring molecule. This attraction is called a hydrogen bond.
• Hydrogen bonding occurs when H is bonded to a small, highly electronegative atom (F, O or N).

3. Solution

• Covalent (A): this holds H and F together within one molecule, not between molecules.
• Dative (B): this is a shared pair donated by one atom. It is not involved here.
• Hydrogen (C): this is the attraction between H(δ+) and F(δ−) of different molecules, which forms chains such as (HF)ₙ. ✔
• Ionic (D): this needs full transfer of electrons, which does not happen in HF.
• Metallic (E): this occurs only in metals.

Hydrogen bonding explains why HF has an unusually high boiling point (about 19.5 °C) compared with HCl, HBr and HI. It also explains why HF exists as associated molecules (HF)ₙ.

4. Answer The correct option is C. Hydrogen.

5. Exam tip Remember "FON": Fluorine, Oxygen and Nitrogen. If H is attached to F, O or N and the question asks about attraction between molecules, think hydrogen bond. Examples are HF, H₂O and NH₃. Always check whether the question asks about bonds within a molecule (covalent) or between molecules (intermolecular forces).