Shared Bonds

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IMAGE: BELL LABS

PROPERTIES OF WATER result from two types of bonds. Strong sigma--or covalent--bonds bind two atoms of hydrogen (white balls) to an atom of oxygen (red balls) to form a molecule of H₂O. These electronic bonds are represented by the dark yellow clouds in the diagram.

A second and much weaker attraction, known as a hydrogen bond, glues groups of water molecules together. Hydrogen bonds are principally the electrical attractions between a positively charged hydrogen atom--which readily gives up its electron in water--and a negatively charged oxygen atom--which receives these electrons--in a neighboring molecule. These bonds are indicated by the lighter yellow clouds.

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The covalent bonds that hold water molecules together can only be described by the bizaare rules of quantum mechanics, the modern theory of matter and energy. In these bonds, each electron does not really belong to a single atom but is shared by both simultaneously.

Until recently, the accepted explanation for hydrogen bonds was much simpler. These electrostatic interactions could be explained by Coulomb's law, named after the French engineer Charles Coulomb, who formulated the law in the 18th century to describe the attraction and repulsion between charged particles separated from each other by a distance.

But the two theories did not account for all of water's curious properties. Now researchers have shown that hydrogen bonds also behave according to the laws of quantum mechanics. In a classic experiment, investigators zapped ice with x rays from a synchotron (blue arrow) and measured how the energy and direction of the x-ray photons were altered by interacting with electrons in the ice crystal (red arrow). The results confirmed the controversial prediction of Linus Pauling in the 1930s that hydrogen bonds between water molecules would also be affected by the sigma bonds within the water molecules and would partially assume the identity of those bonds.

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