The First “Theory”:

The first theory developed in the 1950's is that water is a collection of "flickering clusters" of varying sizes (see diagram at the right). This theory has gradually been abandoned because it is unable to account for many of the observed properties of the water. The current belief, influenced greatly by molecular modeling simulations beginning in the 1980s, is that on a very short time scale (less than a picosecond), water is more like a "gel" consisting of a single, huge hydrogen-bonded cluster. On a 10-12-10-9 second time scale, rotations and other thermal motions cause individual hydrogen bonds to break and re-form into new configurations, inducing ever-changing local discontinuities whose extent and influence depends on the temperature and pressure.

It is quite likely that when water is in very small volumes, localized (H₂O)n, (where “n” represents any specified number of molecules,) polymeric clusters may have a fleeting existence, and many theoretical calculations have been made showing that some combinations may be more stable than others. It does not appear that these “more stable” clusters remain intact long enough to actually be detected directly as observable entities in ordinary water at normal pressures.
 
Think of liquid water of as a seething mass of H₂O molecules in which hydrogen-bonded clusters are continually forming, breaking apart, and re-forming.

Theoretical models suggest that an average cluster may encompass as many as 90 H₂O molecules at 0°C, so that very cold water can be thought of as a collection of ever-changing ice-like structures. At 70° C, the average cluster size is probably no greater than about 25.

Prof. Martin Chapin of the London South Bank University has reviewed much of the existing literature on water clustering, and has recently proposed an icosohedral clustering model in which twenty 14-molecule tetrahedral units form an icosohedron containing a total of 280 H₂O units. This model is consistent with X-ray diffraction data and is able to explain all of the unusual properties of water.

Why study water molecules?

Water clusters are of considerable interest as models for the study of water and water surfaces, and many articles on them are published every year. Some notable work, reported in 2004, extended our view of water to the femtosecond time scale.

• A femtosecond is a million times shorter than a nanosecond. Yes, that's very fast. It's "ultra fast".
• In the mathematician's lexicon, a femtosecond is 1x10-15 seconds.
• In words, it's a quadrillionth of a second.
• If one assumes the universe is 12 billion years old, a femtosecond compares to a second as 10 minutes compares to the life of the universe.
• A femtosecond is one-millionth part of one billionth of a second.

The principal finding was that 80 percent of the water molecules are bound in chain-like fashion to only two other molecules at room temperature, thus supporting the prevailing view of a dynamically-changing, disordered water structure.

Liquid and solid water

Ice, like all solids, has a well-defined structure. Four neighboring H2O molecules surround each water molecule. Two of these are hydrogen-bonded to the oxygen atom on the central H₂O molecule, and each of the two hydrogen atoms is similarly bonded to another neighboring H₂O.
 
The dashed lines in this 2-dimensional schematic diagram on the left represent the hydrogen bonds. In reality, the four bonds from each O atom point toward the four corners of a tetrahedron centered on the O atom. This basic assembly repeats itself in three dimensions to build the ice crystal.

When ice melts to form liquid water, the uniform three-dimensional tetrahedral organization of the solid breaks down as thermal motions disrupt, distort, and occasionally break hydrogen bonds. The methods used to determine the positions of molecules in a solid do not work with liquids, so there is no unambiguous way of determining the detailed structure of water. The illustration to the right is probably typical of the arrangement of neighboring water molecules surrounding any particular H₂O molecule, but very little is known about the extent to which an arrangement like this is replicated to more distant molecules.

Below are three-dimensional views of a typical structure of liquid water (right) and of ice (left). Notice the greater openness of the ice structure that is necessary to ensure the strongest degree of hydrogen bonding in a uniform, extended crystal lattice. The more crowded and jumbled arrangement in liquid water can be sustained only by the greater amount thermal energy available above the freezing point.