Carbon

Category: non-metal Atomic number: 6 Colour: clear (diamond), black (graphite) Melting point: n/a (turns to vapour before it melts – a process of sublimation) Sublimation point: 3,642°C (6,588°F) First identified: circa 3750 BC

All life on Earth is carbon-based. Indeed, it is not known for certain that any other kind of life is even feasible. Carbon is a tetravalent atom, meaning it can bond to four different atoms at once, and this allows it to form as many as 20 million different compounds, and to form chains of different lengths.

Until the early nineteenth century it was thought that living matter as well as chemicals such as proteins or carbohydrates contained a ‘spark of life’ that made them completely different to inorganic matter. Then, in 1828, it was discovered that urea crystals, which are found in animal urine, could be synthesized in a laboratory, proving that there was no essential difference between organic and inorganic matter.

In the carbon cycle, photosynthesis by plants and plankton allows them to derive carbon from carbon dioxide, releasing oxygen as a by-product. At the same time, hydrogen combines with carbon to form carbohydrates. These combine with nitrogen, phosphorus and other elements to form the molecules required for life, including bases and sugars for DNA and amino acids. Species such as humans, that don’t photosynthesize, have to consume other plants or animals to get the carbon they need for their own cellular structures. And carbon returns to the start of the cycle either through being exhaled as carbon dioxide or through the decay of living matter after the cells die.

So, carbon is extremely important, even before you consider the many physical uses we have for its different forms. One of the most fascinating things about pure carbon is the huge difference between its allotropes. It occurs naturally in three allotropes, all of which have been known about since the time of the ancient Egyptians sixty centuries ago: diamonds, anthracite (a type of coal) and graphite. The only fundamental difference between these is in the atomic structure. However, diamonds are transparent and extremely hard, whereas graphite is black and soft: so how can they possibly be the same substance?

The answer is that many solids contain a ‘crystal lattice’ in their structure – the atoms are arrayed in a repetitive three-dimensional structure defined by the way the bonds hold them together. Diamonds have their atoms arrayed in a tight arrangement of three-dimensional tetrahedrons (pyramids with four triangular faces). By contrast, graphite’s atoms are bonded equally tightly, but in two-dimensional layers that are only weakly connected to layers above and below them, which is why it seems soft. These atomic differences account for the very different appearances of the allotropes.

The connection between diamonds and other types of carbon was not understood for many millennia. In the seventeenth century, two Florentine scientists (Giuseppe Averani and Cipriano Targioni) discovered that it was possible to destroy a diamond by using a large magnifying glass to focus the heat of the sun on it. In 1796, Smithson Tennant, an English chemist, astonished the world by proving that diamond was merely a different form of carbon – he did this by demonstrating that when a diamond burned, carbon dioxide was the only product.

Carbon combines with hydrogen in strong, bonded chains to form hydrocarbons, which are extracted from the earth as fossil fuels, and also form the basis of plastics, polymers, and many fibres, solvents and paints. Global warming has been caused by the increased release of carbon dioxide from the burning of fossil fuels, and this will continue to be a problem until alternative, sustainable energy sources are available on a much larger scale.

Carbon is also crucial in many manufacturing processes – charcoal or coke is used to turn iron into steel; graphite is used in pencils (though wrongly called ‘lead’), in electric motor brushes and furnace linings; diamond is used for cutting through rocks and drilling; carbon fibre is a particularly strong, light material used in fishing rods and tennis rackets as well as in moving parts in aeroplanes and rockets.

In recent years, scientists have even discovered ways to form carbon into exotic new allotropes with astonishing properties: fullerenes, which were identified in 1985, are hollow cages made of carbon atoms: the ‘Buckyball’, a type of fullerene, is shaped like a hollow ball of sixty carbon atoms; and nanotubes, which were discovered in 1991, are thin tubes only a nanometer (0.000001 mm) in diameter, formed from curled-up sheets of carbon atoms.

Perhaps the most amazing allotrope is graphene, which has been widely hailed as a miracle material of the future. While graphite is soft at a macro-scale, the individual sheets that make it up are incredibly hard. Scientists had theorized since the 1960s that it might therefore be possible to create a carbon material that was effectively two-dimensional and very light but extremely strong and flexible. In 2004, this extraordinary material, known as graphene, was finally produced – its possible uses include electric circuits, highly efficient solar cells, ‘intelligent shoes’, lightweight aeroplanes and even a new kind of neural implant that functions as a brain-to-computer interface.

The story of carbon is ongoing and about to become even more astonishing!