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why do water molecules form a lattice structure

Below 4`C, the hydrogen bonds between water molecules become stronger and cause the matter to expand. Because the bonds between water molecules are stronger, they are less likely to break and re-form as they do in the liquid state. As a liquid, water molecules are constantly moving (forming and breaking hydrogen bonds) resulting in less expansion. When frozen, water molecules take a more defined shape and array themselves in six-sided crystalline structures. The crystalline arrangement is lease dense than that of the molecules in liquid form which makes the ice less dense than the liquid water. When water freezes, its volume expands by approximately 9%. When water molecules are in the liquid state, hydrogen bonds are continuously being formed and reformed in a disordered fashion.

The average amount of H-bonds a water molecule has at 25`C is 3. 4 bonds. During freezing, water molecules lose energy and do not vibrate or move around as vigorously. This allows more stable hydrogen-bonds to form between water molecules, as there is less energy to break the bonds. Because of the crystal lattice structure imposed on water molecules once the freezing point is reached, the average H-bonds per water molecule is much closer to its maximum, 4 bonds. The orderly, crystalline way in which the hydrogen-bonds form causes density to decrease because each water molecule is held away from its neighbors at a distance equal to the length of the hydrogen bonds. Thus water expands as it freezes, and ice floats atop water. This property is crucial to life as we know it.

Ice sheets on the surface of the ocean insulate the water beneath and allow organisms to live in polar regions of the world, and also prevents oceans from freezing solid.
The structure of water varies considerably, depending on its physical state. In all forms, water is a polar molecule with electron-poor hydrogen atoms and an electron-rich oxygen. It is this that leads to the hydrogen bonding interaction between water molecules. O. Each molecule is bent with a bond angle of 105 deg. The negative charge is concentrated around the oxygen atom. The protons have a partial positive charge. The electron density map (above right) shows that the electron density is approximately 10 times greater around oxygen than around the hydrogen atoms.

Unlike water vapor, in the solid phase the oxygen atoms in water are in a tetrahedron of hydrogen atoms. On the right is a diagram showing the position of the oxygen atoms of water in a cubic close packed lattice. In the center of the lines connecting each oxygen atom to another is a hydrogen atom. The bonds are rigid within the ice structure. But despite its prevalence and importance, liquid water is not as well understood in the other phases. In the recent study, Nilsson and colleagues probed the structure of liquid water using X-ray Emission Spectroscopy and X-ray Absorption Spectroscopy. These techniques use X-rays, generated by a synchrotron light source, to excite electrons within a water molecule's single oxygen atom. Tuning the X-rays to a specific range of energies can reveal with precision the location and arrangement of the water molecules.

The researchers found that water is mostly made up of tetrahedral groups, as in ice, but there is also a less defined structure that seems to be like a distorted, hydrogen-bonded form of water vapor. The oxygen atoms in distorted water molecules have 2 strong bonds to hydrogen and 2 weak ones. The oxygens in tetrahedral, ice-like water have 4 equivalent bonds to hydrogen. Even in its tetrahedral form, liquid water is different from ice because the bonds are constantly breaking, the molecules of water moving, and more bonds forming. Water is more dense than ice because of water molecules held within holes of the cubic close packed lattice.

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