# Hydrogen bonding

A hydrogen bond is a strong attraction between a hydrogen atom bonded to N, O or F and a lone pair on another N, O or F. It is the strongest type of intermolecular force, but still far weaker than a covalent bond.

Chemistry (university) · Part 03: Intermolecular forces, gases and solutions · Card 2 of 9 · Last updated 2026-10-19

## Analogy

Like a small magnet on a fridge: the H atom is the magnet, and lone pairs on N, O or F are the metal.

## Example

**Group 16 hydrides**

H₂S boils at −60 °C.
H₂O boils at 100 °C.
Same shape, but only H₂O
has hydrogen bonds.

## In practice

Hydrogen bonds make water boil at 100 °C, not at about −80 °C, and make ice float.

## Practice

### Flashcards

- **What is needed for a hydrogen bond?** H bonded to N, O or F, and a lone pair on another N, O or F.

### Interview questions

**Explain why water has a much higher boiling point than hydrogen sulfide.**

- Water forms hydrogen bonds
- H₂S has weaker dipole-dipole and London forces
- More energy needed to separate water molecules
- Boiling point reflects intermolecular forces

## Questions people ask

### Why does ice float on water?

In ice, hydrogen bonds hold each water molecule in an open six-sided lattice with gaps. This takes up more space than the jumbled liquid, so ice is about 9 percent less dense than water. Lower density means ice floats, which protects fish by insulating lakes.

### Do hydrogen bonds exist inside DNA?

Yes. Hydrogen bonds hold the two strands of DNA together by linking base pairs: two bonds between A and T, and three between G and C. They are weak enough that enzymes can unzip the strands to copy DNA, yet strong enough, in large numbers, to keep the helix stable.

## Related concepts

- [Intermolecular forces](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-3/chemistry-university-part-3-concept-1)
- [Vapour pressure](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-3/chemistry-university-part-3-concept-3)

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