A hydrogen bond is an attractive interaction involving a hydrogen atom covalently bonded to a strongly electronegative atom and an electron-rich site on another part of the same or a different molecule.

Common donors contain bonds such as

O—H
N—H

while common acceptors are electronegative atoms with available electron density.

Directionality

Hydrogen bonds are not determined by distance alone. Their strength depends strongly on geometry, especially the alignment of donor, hydrogen and acceptor.

This directionality makes them useful in organizing molecular structures.

Weaker than covalent bonds

A hydrogen bond is normally much weaker than the covalent bond holding the hydrogen to its donor atom. It should therefore not be drawn or interpreted as an additional ordinary covalent bond.

Competition with the environment

In water, potential hydrogen-bonding sites also interact strongly with surrounding water molecules. The stability of a molecular structure therefore depends on the balance among solute-solute, solute-solvent and solvent-solvent interactions, not simply on counting hydrogen bonds.

Biological importance

Hydrogen bonding contributes to protein secondary structures, molecular recognition and complementary DNA base pairing. Its selectivity comes from both chemical donor/acceptor patterns and geometry.