If you think chemistry is what happens between men and women, then you should read this section. It provides the chemistry ideas necessary to understand about fats and fatty acids.
The idea that everything is made from atoms is as old as the Greeks, although it was only scientifically established in 1803 (by Dalton at Manchester). We cannot see atoms, because they are too small for the eye to resolve, but recent instruments like the atomic force microscope enable atoms to be "seen" as video images [picture] . In fact, atoms are incredibly small. In a millilitre (cubic centimetre) of matter, there will be more than 10**22 (10,000 million million million) atoms.
There are some 90+ different types of atoms of increasing size and weight from hydrogen, which has an "atomic weight" of 1 to uranium, which has an atomic weight of 238 (ie, weighs the same as 238 atoms of hydrogen). These are the "elements". Even larger elements are possible, but they are increasingly unstable and quickly break down into smaller atoms. Atoms themselves are made of much smaller components like protons and electrons, but this is not important to us, because the energy necessary to break down an atom is enormously greater than anything available in our normal daily lives, so that the atoms can effectively be regarded as the building blocks of everything around us.
The most important property of atoms is that they bind together in clumps, to form molecules, and chemistry can be regarded as the study of all the different types of molecules (chemicals) that are possible. There are a number of slightly different ways in which the atoms bind together, but it is possible to simplify this by saying that each atom has a "valency", which is the number of links that it can make to other atoms. Thus, an atom of hydrogen has a valency of one, an atom of oxygen has a valency of two, an atom of nitrogen has a valency of three and an atom of carbon has a valency of four - which, for most purposes, is that largest valency that is possible.
A molecule is just a clump of atoms in which all the valency links are paired together, so that no free links remain. Thus, an oxygen atom (valency 2) can link to two hydrogen atoms (each valency 1), to form a molecule of water (described by the chemical formula H20) . A carbon atom (valency 4) can link with four hydrogen atoms (each valency 1) to form a molecule of methane (CH4, marsh gas) or with two carbon atoms (each valency 2) to form a molecule of carbon dioxide (CO2); this happens because each oxygen atom uses its two valency links to match two of the four valency links of the carbon.
The important thing to remember is that each molecule is three dimensional and has a shape. Thus, in methane, the four atoms of hydrogen are distributed symmetrically about the atom of carbon, so that we can visualise the molecule as a tetrahedron, with the carbon atom in the centre and the hydrogen atoms at each of the four corners [picture]. Indeed, if we think of the atoms as spheres of different sizes that increase with atomic weight, and the valency links as springs connecting the spheres, then the shape of a particular molecule is more or less the shape of its mechanical model.
Shape is the essence of chemistry; virtually all the properties of a molecule, such as its melting point and the way it interacts with other molecules are determined by its shape. Almost any set of atoms in which the valency bonds match can form a real molecule, remembering that the smaller atoms are more reactive, that there is more difficulty in fitting larger atoms into a particular space, and that some configurations are not possible because, it would be necessary for two atoms to occupy the same space. Using computers, it is now possible to calculate the precise shape that a particular combination of atoms will assume, and thus to predict its chemical properties.
Armed with these simple ideas, it is now possible to understand even the most complicated metabolic chemistry.
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(update 2.1: 18 July 2002)
(issue 2: 24 December 1997)