Introduction
By now you know that molecules can attract each other through intermolecular forces like van der Waals' forces and dipole-dipole interactions. But there's a special, extra-strong type of dipole-dipole attraction called the hydrogen bond, and it's responsible for many of water's unusual properties — from ice floating to the way DNA holds its double-helix shape.
What is a hydrogen bond?
A hydrogen bond is an intermolecular force that occurs when a hydrogen atom is bonded directly to a nitrogen (N), oxygen (O), or fluorine (F) atom, and that hydrogen is attracted to a lone pair of electrons on an N, O, or F atom of a neighbouring molecule (Talbot, p. 171).
Why only N, O, and F?
- These three elements are small and highly electronegative.
- This creates a very polar N–H, O–H, or F–H bond, giving the hydrogen a strong partial positive charge (δ+).
- Because N, O and F are small, the lone pair on a neighbouring molecule can get very close to that δ+ hydrogen, making the attraction unusually strong (Talbot, p. 171).
You can think of hydrogen bonding as an especially strong version of a permanent dipole-dipole force — strong enough that it's treated as its own category, even though it is still an intermolecular (not covalent) interaction.
Common mistake to avoid
It's tempting to think intermolecular forces are just about "how polar" a molecule is overall. But hydrogen bonding specifically requires H directly attached to N, O, or F — polarity alone (e.g. in a C–H bond) is not enough to create this type of interaction (Talbot, p. 171).
Worked Example 1: Identifying hydrogen bonding
Question: Which of these molecules can form hydrogen bonds with themselves: CH₄, NH₃, HF, CH₃OCH₃?
Answer:
- CH₄ — no H bonded to N/O/F → no hydrogen bonding
- NH₃ — H bonded to N → yes
- HF — H bonded to F → yes
- CH₃OCH₃ (dimethyl ether) — has O, but no H directly bonded to O (all H's are on carbon) → no hydrogen bonding between its own molecules
Worked Example 2: Comparing strength — NH₃ vs HF vs H₂O
Question: Rank NH₃, HF, and H₂O by the strength of hydrogen bonding, and explain why.
Answer:
- HF has a very