Hydrogen Bonding
Simple Explanation
Hydrogen bonding is an unusually strong intermolecular attraction that occurs when a hydrogen atom bonded to a highly electronegative atom (nitrogen, oxygen, or fluorine) is attracted to a lone pair on a nearby electronegative atom in another molecule.
Why Do We Need It?
Hydrogen bonding is responsible for many of water's most important, life-sustaining properties — its unusually high boiling point, the fact that ice floats, and its strong surface tension — none of which would be true if water only had ordinary van der Waals forces.
Why Does This Work?
Hydrogen is a very small atom, so when it's bonded to a strongly electronegative atom, its lone proton (with almost no electron cloud shielding it, since it started with only one electron) becomes strongly attracted to a lone pair of electrons on a nearby electronegative atom — much more strongly than an ordinary dipole-dipole attraction, because of how exposed and positively charged that hydrogen nucleus is.
Real-Life Example
Why ice floats on liquid water
Ice cubes float in a glass of water, even though ice is the solid form of the same substance.
In ice, hydrogen bonds lock water molecules into a rigid, open hexagonal lattice that is actually less dense than liquid water, where molecules can pack more closely — this is unusual (most solids are denser than their liquid form) and happens specifically because of how hydrogen bonds arrange the molecules.
Practice
Which pair of atoms directly bonded to hydrogen can lead to hydrogen bonding between molecules?
MediumCommon mistake
Calling hydrogen bonding a type of covalent bond — it is an intermolecular force (an attraction BETWEEN separate molecules), not a bond within a molecule, even though it is much stronger than typical intermolecular forces like van der Waals attractions.
Quick Review
- Hydrogen bonding needs H bonded to N, O, or F, attracted to a lone pair on another molecule.
- It is the strongest common type of intermolecular force.
- It explains water's high boiling point and why ice floats.