Read the clear and complete summary on hydrogen bonding . The hydrogen bond is a type of bond that can only be formed with the involvement of a hydrogen atom : this uniqueness is a consequence of the fact that the H atom is the smallest atomic species, and is the only one to possess only one electron. If a hydrogen atom is covalently bonded to a strongly electronegative atom X (e.g. F, O, N), a dipole δ-X-Hδ+ is generated with a high dipole moment, because the X atom is strongly electronegative, and because the H atom, very small and without shielding electrons, generates a high electric field.
How hydrogen bond is formed
The hydrogen atom is arranged in a “bridge” by connecting in a straight line the atom to which it is bonded with the electronegative atom of the neighboring molecule. The hydrogen bond is usually indicated with a series of dots:
XH ••••••
And it leads to the formation of chains, as in hydrofluoric acid, or more complex structures, such as the hexagonal one of water.
The hydrogen bond can be interpreted as a mixed electrostatic-covalent bond : the electrostatic part due to the dipolar bond, and the covalent part to a dative bond between the very small protonated (i.e. positivized) hydrogen atom that behaves as an acceptor of the electrons supplied by the X atom (donor), which, like all strongly electronegative atoms, has at least one lone pair.
Hydrogen bond: types
Molecules containing HF, HO, HN groups can form hydrogen bonds in the gaseous state , in the liquid state , in the solid state : thus hydrogen bonds exist between molecules of hydrogen fluoride, ammonia, water and a large number of organic compounds, especially alcohols (molecules containing OH groups) and acids (molecules containing COOH groups).
In liquid water, hydrogen bonds are not localized, but are temporarily established between two molecules, then broken and rebuilt elsewhere. When water solidifies, the molecules retain both the hydrogen bonds and their bond angles, thus forming the hexagonal structure found in snow crystals. It is because of this internally empty structure that ice has a lower density than liquid water because during solidification, its volume increases.
