Molecule of Water

molecule of water

A chemist's view of the world is not as narrow as one might think! Yes, we start with the atom, and then go on to the rules governing the kinds of structural units that can be made from them. We are taught early on to predict the properties of bulk matter from these geometric arrangements.
And then we come to H2O, and are shocked to find that many of these predictions are way off, and that water (and by implication, life itself) should not even exist on our planet! But we soon learn that this tiny combination of three nuclei and ten electrons possesses special properties that make it unique among the more than 15 million chemical species we presently know. When we stop to ponder the consequences of this, chemistry moves from being an arcane science to a voyage of wonder and pleasure as we learn to relate the microscopic world of the atom to the greater world in which we all live.
The molecule of water
A molecule is an aggregation of atomic nuclei and electrons that is sufficiently stable to possess observable properties— and there are few molecules that are more stable and difficult to decompose than H2O. In water, each hydrogen nucleus is bound to the central oxygen atom by a pair of electrons that are shared between them; chemists call this shared electron pair a covalent chemical bond. In H2O, only two of the six outer-shell electrons of oxygen are used for this purpose, leaving four electrons which are organized into two non-bonding pairs. The four electron pairs surrounding the oxygen tend to arrange themselves as far from each other as possible in order to minimize repulsions between these clouds of negative charge. This would ordinarily result in a tetrahedral geometry in which the angle between electron pairs (and therefore the H-O-Hbond angle) is 109.5°. However, because the two non-bonding pairs remain closer to the oxygen atom, these exert a stronger repulsion against the two covalent bonding pairs, effectively pushing the two hydrogen atoms closer together. The result is a distorted tetrahedral arrangement in which the H—O—H angle is 104.5°.
Because molecules are smaller than light waves, they cannot be observed directly, and must be "visualized" by alternative means. This computer-generated image comes from calculations that model the electron distribution in the H2O molecule. The outer envelope shows the effective "surface" of the molecule as defined by the extent of the cloud of negative electric charge created by the eight electrons.