Hydrogen Bond
Definition and meaning of Hydrogen Bond in chemistry.
A hydrogen bond is an attractive force between a hydrogen atom covalently bonded to a highly electronegative atom (N, O, or F) and a lone pair of electrons on another electronegative atom nearby. It can occur between separate molecules (intermolecular) or between two parts of the same molecule (intramolecular).
In more detail
Because N, O, and F pull electron density away from hydrogen, the hydrogen atom carries a significant partial positive charge and is attracted electrostatically to a nearby lone pair, giving the interaction some directional, partly covalent character as well. Hydrogen bonds are much weaker than covalent or ionic bonds but stronger than ordinary dipole-dipole or dispersion forces, typically 5 to 30 kilojoules per mole. They explain water's unusually high boiling point and surface tension, ice's lower density than liquid water, and the folding of proteins and pairing of DNA bases.
Key facts
| Field | General Chemistry |
|---|---|
| Typical strength | 5-30 kJ/mol |
| Common donor/acceptor atoms | N, O, F |
| Classic example | H2O (water) |
In liquid water, the hydrogen atom of one H2O molecule (bonded to its own oxygen) is attracted to a lone pair on the oxygen atom of a neighboring H2O molecule, forming a hydrogen bond denoted O-H...O.
Frequently asked questions
Is a hydrogen bond a true chemical bond?
It is generally classified as an intermolecular force rather than a true bond, since it is much weaker than covalent or ionic bonds, though it does have some directional, partly covalent character.
Why does ice float on liquid water?
In ice, hydrogen bonds lock water molecules into an open hexagonal lattice that takes up more space than the more disordered arrangement in liquid water, making ice less dense.