If you were to be asked to rank the elements in order of importance, which elements would be at the top of the list? Of course, there are lots of elements that are important for everyday life but let’s limit the list to just five so things don’t get out of hand. The following is my list.
The whole of the living world is based upon compounds of carbon, and so from a self-centred point of view, carbon has to be at the top of the list. Carbon atoms have a normal valence of four and can form covalent bonds to other carbon atoms and to atoms of many other elements. Carbon has an electronegativity (look it up in the index of your textbook!) of 2.5, which is intermediate between the metals (with electronegativities around 1.0 to 1.5) and the non-metals in the upper right hand corner of the periodic table (with electronegativities ranging from 3.0 to 4.0). Because of this combination of properties, there are more known compounds of carbon than of any other element, and even now only a small fraction of the possible compounds has been investigated. Because so many carbon compounds are found in living organisms, the chemistry of compounds of carbon is called organic chemistry.
Hydrogen is the lightest element and has the simplest atoms, so it plays a very important role in chemistry. A large fraction of organic compounds contain hydrogen as well as carbon. Water, upon which life depends, contains hydrogen. Reactions between acids and bases dissolved in water consist of the transfer of hydrogen ions. Hydrogen makes up a large fraction of the sun and other stars, and is also found in interstellar space. Nuclear fusion reactions involving hydrogen are the source of energy in the sun, which keeps us warm.
Oxygen is the most abundant element in the earth’s crust (see CHEMBOOK page 84) and makes up about 20 per cent of the air we breath. Oxygen is essential to life, and without it we would suffocate. Oxygen oxidizes the sugars in the body to give carbon dioxide and water, and in the process produces the energy needed for life. Water contains oxygen as the second element, in addition to hydrogen. Oxygen is a highly electronegative element, and is a strong oxidizing agent (no surprise there!), and so makes up a large fraction of many minerals and rocks.
Nitrogen makes up about 80 per cent of the air we breath, but despite this the body does not absorb nitrogen directly from the air. Nitrogen is not one of the most abundant elements. It is nevertheless important to life because of its role in the chemical structure of proteins and deoxyribonucleic acid, or DNA. Plants absorb compounds of nitrogen (including inorganic compounds), and the nitrogen becomes part of large organic molecules. Agricultural fertilizers always contain nitrogen compounds. Ammonia and related compounds are bases, and this is an important factor in the biological activity of many drugs. On a more sombre note, some nitrogen-containing compounds decompose explosively and are the main tool of modern warfare, which has been so destructive in recent history.
The fifth place element in our list is a problem. Should it be iron, which for thousands of years has been used as a structural material? Or should it be silicon, which is the second most abundant element in the earth’s crust and is the basis for so much developing technology, in the form of both semiconductors and glass optical fibres for communication? Or is there yet another more important element? Can you decide? To go back to the title of this article, what do you think the word “important” means?