What silicon builds

Section 4 of 5

Silicon's four hands and its strong bonds to oxygen build three of the most useful materials there are: glass, computer chips and silicones.

Glass

Window and bottle glass is made by melting sand with sodium carbonate and limestone at about 1,500 °C. About 70% of it is silica. As the melt cools, its silicon and oxygen atoms freeze in place in a jumbled network before they can line up into crystals, so glass is clear and smooth all the way through, with no grains.

Chips

To make pure silicon, quartz is heated with carbon to about 2,000 °C. Carbon takes the oxygen and leaves as carbon monoxide: SiO₂ + 2 C → Si + 2 CO. The silicon is then purified to more than 99.9999999% and grown into large single crystals, which are sliced into thin wafers. In the crystal, every silicon atom holds four others, in the same 3D network as diamond.

Silicon is a semiconductor: it carries a small electric current, and a few added atoms of boron or phosphorus change how well it conducts. A chip uses this to build billions of tiny switches, called transistors, into a small square of silicon cut from a wafer.

Silicones

A silicone is a long chain of silicon and oxygen atoms in turn, -⁠Si-⁠O-⁠Si-⁠O-⁠, with carbon groups on each silicon, most often two methyl groups, -⁠CH₃. The backbone of strong Si–⁠O bonds stands up to heat, and the methyl groups on the outside make the chain repel water.

Hexamethyldisiloxane, (CH₃)₃SiOSi(CH₃)₃Two silicon atoms joined through an oxygen atom, Si–O–Si, with three methyl groups on each silicon. It is a silicone chain at its shortest, just the two end pieces.SiSiO
Hexamethyldisiloxane: two silicons joined through one oxygen, each holding three methyl groups. It is a silicone chain at its shortest, just the two end pieces, and a runny liquid that boils at about 100 °C.
A short length of a silicone chain A chain of atoms across the page: oxygen, silicon, oxygen, silicon, oxygen, with dotted bonds at each end where the chain carries on. Each silicon atom also holds two methyl groups, CH₃, one above and one below. O CH₃ CH₃ Si O CH₃ CH₃ Si O
A short length of a silicone chain. Silicon and oxygen take turns along the backbone, and each silicon holds two methyl groups. Real chains run to thousands of atoms.

Silicones seal baths and showers, make bendy baking moulds and soft contact lenses, and smooth hair in conditioner. Silly Putty, which bounces like a ball and slowly flows like a liquid, is a silicone too.

Quick check

What makes up the backbone of a silicone chain?