INTRODUCTION
There are just over 90 different elements that occur in nature and quite a few more that have been made in nuclear reactors or particle accelerators. (Most of these 'artificial' elements are radioactive and decay with rather short lifetimes).
These elements are shown in the "periodic table" below. This is only an outline table showing the elements arranged according to their 'atomic number', which is the number of protons in the nucleus of the atom, and thus the number of electrons in the neutral atom.
The table starts with hydrogen on the top left which has one proton, skips over to the top right where we find helium with two protons, then back to the second row at the left with the third element lithium, and so on. Much more attractive and colourful periodic tables can be found on the web with detail about each element within the boxes.
However, within these elements there is one very special atom which makes the element carbon. An element which occurs in most of the molecules of your body - with the exception of water.
WHAT MAKES CARBON SPECIAL?
Carbon is the element that divides chemistry into two major branches: organic chemistry and inorganic chemistry.
Organic chemistry is the chemistry of carbon - of molecules and compounds that contain at least one carbon atom and often many more. There are almost 20 million known types of carbon molecules.
On the other hand inorganic chemistry is the chemistry of everything else but carbon. That is, the chemistry of molecules and compounds that do not have carbon atoms in their makeup. There are only around 500,000 or half a million known inorganic molecules.
What then makes the atom of carbon so special the it can form forty times more molecules than all the other 90+ atoms combined? The reason lies in the construction of the carbon atom.
In forming elements it is the electrons in the outermost shell that are the important ones. These are called the called the valence electrons of the atom. Carbon is tetravalent, which meads it has 4 electrons in the outermost shell out of a possible eight electrons that can exist in this shell. A shell that has a full complement of electrons in this shell is a very stable atom. Witness the elements to the very right of the periodic table which have full outer shells. These are called the noble elements and most of them are gases. These noble gases are so stable that they react and form compounds with almost no other elements.
However, there is another way for an atom to have a stable outer shell and that it to share electrons with other atoms to produce a full shell. Carbon can share 4 electrons with other atoms, the most it is possible to share. This is the secret to the ability of carbon to form so many diverse molecules.
It could be questioned why silicon or germanium, underneath carbon in the periodic table, and which are also tetravalent, do not form as many difference molecules as does carbon. The answer again lies in the electronic structure of these atoms.
We can see that silicon does indeed have 4 electrons in the outer valence electron shell and this again is out of a possible 8 electrons from 4 electrons shared with other atoms.
However, these 4 electrons are shielded from the positive electric charge (from 14 protons) in the nucleus by 12 electrons. Contrast this with carbon in which the outer 4 electrons are shielded from the positive charge of 6 protons by only 2 electrons. This means that the outer electrons in a carbon atom are bound much more strongly in the atom. Subsequently the bonds carbon forms with other atoms are much stronger than the bonds silicon can form. Hence the much greater diversity of molecules.
ORGANIC COMPOUNDS
Organic compounds can be divided into major groups as shown below.
The most important additional atoms in organic chemistry are hydrogen, oxygen, sulphur and phosphorus.
BIOCHEMICAL COMPOUNDS
Not all organic compounds are found in living cells. So biochemistry might be regarded as a sub-branch of organic chemistry. One major difference is that living cells also contain a lot of water molecules. Again there are major divisions of biochemical compounds.
LIFE
Norman Horowitz, head of the NASA Mariner and Viking missions to Mars (1965–1976), considered that the unique characteristics of carbon made it unlikely that any other element could replace it, on other planets, to generate the biochemistry necessary for life. The same could be said for water. Not everyone agrees with this, but the only places NASA believes life may exist is places where carbon and liquid water may exist. Water is in fact the more constraining element than is carbon, as carbon can be found in places where water cannot.
Australian Space Academy