Silicon – the second most abundant element in the Earth’s crust; building block of civilisation; the substance of the electronic age; relied on by some to enhance their appearance. Silicon is a remarkable element but not one that receives much attention in GCSE or even A level chemistry courses.
We meet silicon as one of the first twenty elements, atomic number 14 and in the same group as carbon. It is one of the main elements formed by nuclear reactions in stars. Like carbon it forms four covalent bonds and forms a dioxide with the formula SiO2. Unlike carbon dioxide however, silicon dioxide, also known as silica, is a solid. Carbon dioxide is discrete molecules containing two C=O double bonds while silicon forms single bonds building up chains and rings of alternating silicon and oxygen atoms in a tetrahedral arrangement. This is where silicon chemistry becomes a little complicated – and versatile.
Pure silicon dioxide is found as quartz, where the silicon and oxygen atoms are arranged in a diamond-like pattern. Silica is an acidic oxide that forms silicate salts with aluminium, magnesium and other metals. These compounds make up most of the minerals in the Earth’s crust. Silicates provided us with our first tools chipped from lumps of flint, sandstone and granite for building materials and clay for our pottery and bricks. Silicon dioxide is the main component of sand which is used to make glass and concrete.
Silica and silicates have high melting points that make them difficult to decompose. It took until 1824 before silicon was isolated by the Swedish chemist Jons Jacob Berzelius and it was 1854 before crystals of silicon were obtained by Henry Deville. Crystalline silicon has a metallic appearance but has a giant covalent structure like diamond. About half of the silicon that is extracted is used to make alloys with steel and aluminium that have a variety of uses.
The semiconductor and photovoltaic properties of silicon have made it the foundation of the computer chip and solar energy businesses. For these purposes it has to be prepared in an extremely pure form and then tiny amounts of other elements are added to give the desired electronic properties.
Silicones have made headlines for their use in breast implants which are frequently and incorrectly referred to as “silicon implants”. Silicones are a range of polymers discovered by British chemist Fred Kipping in the 1920s. The molecules have a silicon-oxygen backbone with two methyl groups (CH3¬) attached to each silicon atom. Silicones are liquids with a viscosity that varies with the length of the chain. Silicones are stable to heat and are very useful as lubricants. Cross-linking the chains turns the silicone into a rubbery solid. Varying the length of chain and number of cross-links produces a huge variety of materials which makes them useful as sealants, hoses and much more. They are used in flexible kitchen utensils that withstand the heat of cooking. Silicones are “bio-inert”. This means they are not toxic and do not cause allergic reactions or rejection when used inside the body. Heart valves, joints and other surgical replacements, in addition to breast enhancements, are made from various silicone rubbers.
The full diversity of uses of silicon, silicones and silica has barely been touched on and we probably make use of the element’s fascinating properties every moment of our lives. It really is a superstuff.
Activities:
1 Find out the names of rocks and minerals that contain silicon and some of their uses.
2 Make a list or picture of all the silicon based materials that you use in a day. How important is silicon to our lives?
3 Explore the lives and work of the chemists mentioned in this article viz. Jons Jacob Berzelius, Henri Deville and Fred Kipping.
4 Nettle stings are delivered by a tiny needle of silica. Investigate the use of silicon by organisms.
5 There have been a number of scares about the use of silicones in breast implants but silicones are supposed to be non-toxic. Investigate the stories about the health effects of silicones. Do you think the use of silicones in cosmetic surgery is justified?
6 Find out why many clocks and watches contain a piece of quartz.
7 Find out about the structure and properties of different types of clay (such as kaolin and vermiculite).
8 Asbestos is a silica based mineral with tiny fibre-like crystals. Its properties made it a sought after mineral that was used for a hundred years but its use has now been banned. Find out the properties and uses of asbestos and the reasons why it is no longer used.
Peter Ellis
May 2012
