Life on earth depends on the existence of large molecules built around chains of carbon atoms. Chains and rings are fundamental to the way life forms build themselves which puts carbon in a class of its own. With carbon having the largest range of subtle bonding capabilities, 95% of everything that exists in the universe is made up of carbon atoms that are stuck together.
Carbon is in the food we eat, the clothes we wear, the shampoo we use and the petrol that fuels our cars. It is an extraordinary element for many reasons not least because the carbon-nitrogen cycle provides some of the energy produced by the sun and the stars. It has the highest melting point of all the elements, and its different forms include two extremes: one of the softest and one of the hardest substances known to man.
It is very hard to overstate the importance of carbon because of its unique capacity for forming multiple bonds and chains at low energies. In fact, most of the compounds known to science are carbon compounds, often called organic compounds. This branch of chemistry is thus an important one and it has its own name – organic chemistry – so called because it deals mainly with the the chemistry of life. It plays an important role in economic activity, particularly in the manufacture of industrial chemicals and pharmaceuticals, all branches of medicine and in food production, both on the farm and in the factory
The most important criteria for life, and most fundamental, is the ability to organise into repetitive structures at different orders of complexity – molecular and cellular, for instance. Living organisms are mostly water (oxygen and hydrogen). Carbon and these two elements, along with nitrogen, phosphorus, calcium and sulfur, can form the bulk of the compounds found in living things. With carbon, molecular chains, known as Linear Polymeric Molecules (LPM), can form and these are the key to replication and the conservation of the life creation instructions, better known as DNA, RNA and peptides. Hence carbon plays an essential part in human growth and repetition of cells.
What makes carbon so special are its bonding role in DNA replication and growth processes, its base in fuels and its formation of graphite and diamond. In fact, the carbon chain is able to link to a wide range of chemical elements and molecules that constitute metabolism in its simplest form. Thousands of molecules are possible using just carbon and hydrogen. When oxygen, nitrogen, calcium, phosphorus and sulfur are included, this rises to tens of millions.
Another ubiquitous aspect of carbon is fuel. The world needs fuel for food, power and mobility and fuel is also based on one kind of carbon-based chain or another; everything from natural gas through petrol and alcohol to oil, wax and plastic, is composed of hydrocarbon chains of various lengths.
Besides its millions of compounds, carbon also bonds with itself in different ways to form graphite, diamond, fullerenes and amorphous solids. In graphite, it is probably best known for its use in pencil ‘lead’. It is also used as a lubricant, in clay, and as an electrical conductor – one of the few solid, non-metallic conductors. By contrast, diamond is the hardest substance in the world; it has a dense, extremely stable crystalline structure, and conducts heat extraordinarily well.
It seems that the life of carbon can be traced to some time after the Big Bang, when the universe was very young. The only elements present then were hydrogen, helium, and a trace amount of lithium. As stars formed and nuclear fusion ignited within their cores, other elements were created. These were all lighter than iron, and included carbon, oxygen, and nitrogen. As low-mass stars neared the ends of their lives, they lost their outer layers into space where the material became the interstellar medium – the gas and dust between stars. It is thought that the majority of the carbon in the universe comes from this phase of stellar evolution.