The physicist of antiquity called it one of nature's fundamental elements; third-graders know its chemical formula; and all known forms of life need it to exist. Yet what water really is - at least in its liquid form - is still, to a large extent, a mystery. A team led by scientists at Stanford Synchrotron Radiation Laboratory (SSRL) and Stockholm University now has achieved a breakthrough in understanding the structure of liquid water. They found that water molecules clump much more loosely than previously thought.

The findings appeared April 1 in Science magazine's advance publication website. ''The results overturn 20 years of research in the physical chemistry of water,'' says team leader Anders Nilsson, a chemical physicist at the Stanford Linear Accelerator Center (SLAC). ''It's going to be a big shock in the whole field.''

The SSRL is a division of SLAC, a U.S. Department of Energy (DOE) facility operated by Stanford University. The project was a collaboration between researchers at SSRL, Stockholm University, Linkping University (Sweden) and the University of Utrecht (Holland).

As its H2O formula suggests, each water molecule is made of two atoms of hydrogen and one of oxygen. In ice, water molecules are arranged in a crystal structure, with each molecule typically linked to four others through what chemists call hydrogen bonds. In a hydrogen bond, electrostatic forces stick together a hydrogen atom from one molecule with the oxygen atom from a different molecule. The oxygen can form two hydrogen bonds, so a molecule can link to as many as four others - with two links through its oxygen and one through each of its hydrogens.

Although they are 10 times weaker than the covalent bonds within the molecule itself, hydrogen bonds between molecules still take a lot of energy to break up - which is why ice melts so slowly. Even in liquid water, molecules spend most of their time clumped together by hydrogen bonds, though not in a stat
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Contact: Neil Calder
Neil.Calder@slac.stanford.edu
650-926-8707
Stanford University
2-Apr-2004

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