Silicon

Category: metalloid Atomic number: 14 Colour: metallic, slightly blue Melting point: 1,414°C (2,577°F) Boiling point: 3,265°C (5,909°F) First identified: 1824

If your first thought on seeing the name silicon is the miniaturized chips that computers use, you might be surprised that, by mass, silicon makes up 28 per cent of the Earth’s crust (where it’s the second most abundant element after oxygen). In nature, it is only found in compounds, so you would be more familiar with its various oxides – which include flint, sand, rock crystal, quartz, agate, amethyst and opal – and silicates, which include granite, asbestos, feldspar, mica and clay. The silicon in these compounds was originally formed by nuclear fusion inside dying stars before being ejected when the star collapsed into a supernova.

These compounds have all been used extensively throughout history. Some of humanity’s earliest weapons were made from flint. Granite and other rocks used in construction are complex silicates. Sand (silicon dioxide) and clay (aluminium silicate) are key ingredients of concrete, cement, ceramics and enamels. Opals, quartz and amethysts were all valued by ancient civilizations. Glass, in the form of obsidian, occurs naturally in some places. By the second century BC, humanity had learned to manufacture it, after seeing how small droplets of glass were formed as a by-product of metalwork, when sand was melted only to solidify in a different form. And asbestos, a group of naturally occurring silicates, has been used for its fire-resistant qualities for millennia (although we use it increasingly cautiously nowadays, due to its carcinogenic nature).

Maybe it was the sheer variety of silicon’s forms that led to it being mostly ignored by chemists until the nineteenth century. In 1824, the first relatively pure silicon powder was extracted from potassium fluorosilicate by the Swedish chemist Jöns Jacob Berzelius, but it wasn’t until 1854 that the French chemist Henri Deville produced crystalline silicon.

Since then, silicon has become ever more useful – it is, for instance, used in alloys with aluminium and iron in transformer plates, engine blocks and machine tools. Mixed with carbon it forms silicon carbide, a strong abrasive. Together with oxygen it forms a polymer called silicone, which is a bit like rubber and can be used to seal bathrooms as well as, more controversially, in breast implants.

The name of Silicon Valley, the heartland of the digital world, is testament to the huge importance of silicon chips. These rely on silicon’s status as a ‘semiconductor’, meaning that it will conduct electricity under certain circumstances, but not under others. The material used in chips is actually ‘doped’ silicon (meaning slightly adulterated with other elements to make it function like a kind of miniaturized transistor).

Science-fiction writers (and some scientists) have suggested there could be alien lifeforms based on silicon rather than carbon – the two are neighbours in the periodic table and, as with carbon, a silicon atom can bond with up to four other atoms at once. However, silicon-based life would probably require a very different kind of planet, with ultra-low temperatures and abundant ammonia.

Nonetheless, silicon does play some intriguing roles in terrestrial lifeforms – we don’t really understand the role that phytoliths (tiny pieces of silica that form within plants) play, but they don’t rot, so they survive in fossils, which makes them extremely useful to scientists. When you are stung by a nettle, it is through tiny silicate shards that are on the surface of the plant; and complex silicate structures are found inside one of the smallest photosynthesizing species, diatoms – tiny algae that produce huge quantities of oxygen on our planet.

So, who knows, maybe silicon-based alien life is not as far-fetched as it sounds!