Turn on the shower and you bombard yourself with millions upon millions of water molecules. Of course we don’t perceive the individual molecules, we experience the bulk effects of the liquid water. It is, however, the anatomy of the individual molecule and its interactions, that gives water its familiar feel and properties.
A water molecule is made up of just three atoms: two atoms of hydrogen and one atom of oxygen: hence that most familiar of chemical formulae, H2O. The formula tells us what the molecule is made of, but it gives no information as to the shape of the molecule. The water molecule has a distinct three dimensional structure. It is this microscopic structure, which affords water many of its unique physical properties on the macroscopic scale.
First, each hydrogen atom is bonded individually to the oxygen atom. A better way to write the formula for water would be HOH for this gives us an idea of how the atoms are connected. It still gives no clue as to the three dimensional structure, however, for the atoms are not arranged in a straight line. The three atoms form a shallow ‘V’ shape, the angle of the ‘V’ being approximately 104.5° . The oxygen atom forms the apex of the ‘V’ and the hydrogen atoms are at the extremities of the arms. The molecule is not twisted, all the atoms lie in the same plane.
The water molecule does not actually look like a shallow ‘V’, this is just an analogy and gives a way of defining the basic geometry of the molecule. In the liquid state, the average distance between the oxygen atom and either one of the hydrogen atoms is 0.9584 angstroms. The angstrom is a unit of measurement which chemists and physicists find convenient to use when describing the size of atoms and molecules. 1 angstrom is equivalent to 1/10,000,000,000 of a metre, a very small measurement indeed.
The bonds between the atoms in water are known as covalent bonds and are each made up from a shared pair of electrons. The actual mechanism of bonding is beyond the scope of this article. Sufficient to say that a lone hydrogen atom has only one electron, whereas a lone oxygen atom has 8 electrons. Thus each hydrogen atom donates its electron to the bond in a water molecule and the oxygen atom must donate 2 electrons, one to each bond. All the electrons present , bonding and non bonding, are in constant movement around the atomic nuclei of the oxygen and hydrogen and overall the shape of water molecule is approximately spherical with a molecular diameter of 2.75 angstroms.
All measurements are given as approximations, because the water molecule is in a constant state of motion and flux. The bonds vibrate and the angle between the atoms also changes slightly. In the liquid state oxygen-hydrogen bonds are constantly breaking and reforming. The average water molecule only remains stable for a millisecond before bonds are broken and new bonds formed. Non of this is apparent on the macroscopic scale of course, but is something to ponder upon the next time you are having your shower.