What is a hydrogen bond?
A hydrogen bond is a special, relatively strong intermolecular force. It occurs when a hydrogen atom is covalently bonded to a very electronegative atom — fluorine (F), oxygen (O) or nitrogen (N) — and is attracted to a lone pair of electrons on an F, O or N atom in a neighbouring molecule.
Because F, O and N pull the shared electrons strongly, the H atom carries a small positive charge (δ+) and the F/O/N carries a small negative charge (δ−). The δ+ hydrogen is then attracted to the δ− atom next door.
Key idea / 要点
A hydrogen bond is not a covalent bond. It is much weaker than the covalent O–H bond inside a molecule, but much stronger than ordinary forces between molecules. It exists only where H is bonded to F, O or N.
Why it matters: water
In water, H2O molecules form extensive hydrogen bonds. Breaking these extra attractions needs more energy, so water has an unusually high boiling point (100 °C) compared with similar-sized molecules. Hydrogen bonding also explains why ice is less dense than liquid water and why water has a high surface tension.
Worked example / 例题
Compare H2O and H2S. Both are group-16 hydrides, but only water forms hydrogen bonds (O is electronegative enough; S is not). Extra hydrogen bonds in water raise its boiling point to 100 °C, while H2S boils at about −60 °C.
Remember / 记住
- Hydrogen bonds form only with H–F, H–O or H–N.
- They are intermolecular (between molecules), not intramolecular bonds.
- They raise melting and boiling points of substances like water, ammonia and hydrogen fluoride.