in molecules of relatively small mass. This peculiarity creates the unique properties associated with substances with hydrogen bonding, especially in H20. In most compounds, the polarity of its elements has a negligible effect on the dipole forces which connect them. However, when hydrogen bonds to a small and highly electronegative atom, such as N, O, or F, the difference in electronegativity between the atoms causes the shared electrons to associate less with the hydrogen. Electrons, which normally orbit about both bonded atoms, are more attracted to the nucleus of the N, O, or F, than to the proton of the hydrogen atom. As a result, the hydrogen atom essentially loses its electron cloud and assumes the properties of a single proton. This allows it to pick up a pair of unshared electrons from another nearby atom of N, O, or F. Hydrogen bonding can only occur with the three mentioned atoms because their electronegativity is great enough to attract the hydrogen electron. More importantly, the small hydrogen atom and small atomic radii of the others allow the unshared electrons to come very close to the hydrogen atom. In larger atoms, as the unshared electrons attempted to bond with the hydrogen, they would be repelled by the electrons of the atom on the other side of the hydrogen.
As a result of the disassociation of its electron and ability to bond with two very electronegative atoms, hydrogen bonds are one of the strongest dipole forces, (though not nearly as strong as covalent bonds). Because of the strong bonds, the boiling points of hydrogen-bonded substances are much higher than the expected boiling points determined through molar mass trends. More energy is needed to break a hydrogen bond, raising the heat of vaporization.
In water, the hydrogen bond contributes to many of its unique properties. The high heat of vaporization causes water to be a in its liquid or solid state under most conditions. Since life on earth started in liquid water, it was essential for water to remain in this state for the majority of the water cycle. Because of this difficulty in breaking bonds connecting H20 molecules, water also has a very high specific heat, 4.184 J/g C . This allows water to not be affected by slight temperature fluctuations, and causes it to moderate the climate, cooling during the hot days, and warming the air during the cold night. It is for this reason that deserts experience such dramatic temperature extremes, and islands maintain a steady year-round climate. Also important, is the effect of hydrogen bonds on ice. When water freezes, the O atoms are bonded to four hydrogens: two covalent bonds and two hydrogen bonds to other H20 molecules. The covalent bonds are shorter than the hydrogen bond, causing the molecules to bond in hexagonal crystals with empty space on the inside. This causes the peculiarity of water in which its frozen state is less dense than its solid state. Such a property makes lakes and rivers freeze from the top down, an important consideration in the evolution of life. When lakes froze, primordial organisms could still dwell on the floor, allowing them to survive through the winters.