Hydrogen Bond
Definition and meaning of Hydrogen Bond in chemistry.
A hydrogen bond is a pulling force between a hydrogen atom and a nearby atom with extra electrons to share. It happens when hydrogen is already attached to nitrogen, oxygen, or fluorine. That hydrogen atom then gets pulled toward a lone pair of electrons on a close nitrogen, oxygen, or fluorine atom.
In more detail
Nitrogen, oxygen, and fluorine pull electrons strongly toward themselves. This leaves the attached hydrogen atom with a small positive charge. That charged hydrogen is then drawn to a lone pair of electrons on a nearby atom, like a tiny magnet.
Hydrogen bonds are weaker than normal chemical bonds, but stronger than most other attractions between molecules. Each one is usually worth about 5 to 30 kilojoules per mole of energy. Hydrogen bonds explain many everyday facts about water.
They give water a surprisingly high boiling point for such a small molecule. They also create water's strong surface tension, which lets some insects walk on top of it. In ice, hydrogen bonds lock water molecules into a spread-out honeycomb pattern, which is why ice floats instead of sinking.
Hydrogen bonds also hold the two strands of DNA together and help fold proteins into their working shapes. Hydrogen bonds are not unique to water. Ammonia (NH3) and hydrogen fluoride (HF) also form hydrogen bonds, though usually not as many, since water has two hydrogens and two lone pairs per molecule that can all take part at once.
Key facts
| Field | General Chemistry |
|---|---|
| Typical strength | 5 to 30 kJ/mol |
| Common donor/acceptor atoms | Nitrogen, oxygen, fluorine |
| Classic example | H2O (water) |
| Effect on water | Raises boiling point and surface tension, makes ice float |
| Biological role | Holds DNA strands together and folds proteins |
In liquid water, the hydrogen atom on one water molecule is drawn to a lone pair of electrons on the oxygen atom of a neighboring water molecule. This pull is written as O-H...O. Millions of these bonds form and break every second in a glass of water.
Frequently asked questions
Is a hydrogen bond a true chemical bond?
It is usually classed as an intermolecular force rather than a true bond, since it is much weaker than a covalent or ionic bond.
Why does ice float on liquid water?
In ice, hydrogen bonds lock water molecules into an open honeycomb-shaped lattice. This open structure takes up more space than the more crowded arrangement in liquid water, so ice is less dense.
Can hydrogen bonds form without nitrogen, oxygen, or fluorine?
No, not a true hydrogen bond. The hydrogen must be attached to one of these three highly electronegative atoms for the bond to form and count as a hydrogen bond.