A silicon atom has 14 electrons, with 2 in the K shell, 8 in the L shell and 4 in the M shell. It is an element of group 14, like carbon, and it forms 4 covalent bonds.
Silicon is the most abundant element in the Earth's crust after oxygen. However, it does not occur in nature as the element, and it is found in rock and sand as silicon dioxide and silicates.
The element silicon forms a crystal in which, as in diamond, each atom is covalently bonded to 4 other atoms. It is grey with a metallic lustre, and it is hard and has a high melting point.
It conducts electricity a little, though not as well as a metal, and it conducts better as the temperature rises. A substance like this is called a semiconductor, and silicon is used in computer components and solar cells.
In silicon dioxide, \mathrm{SiO_2}, each silicon atom is bonded to 4 oxygen atoms and each oxygen atom to 2 silicon atoms, forming a three-dimensional network.
The crystal in which the atoms are arranged regularly is called quartz, and large, transparent crystals of quartz are called rock crystal. When silicon dioxide is melted and cooled, it becomes solid with its atoms still arranged irregularly, forming glass.
Silicon is made by strongly heating silicon dioxide with carbon in an electric furnace.
\mathrm{SiO_2} + 2\mathrm{C} \longrightarrow \mathrm{Si} + 2\mathrm{CO}
Silicon for semiconductors is purified to about 99.999999999 %. Then a small seed crystal is dipped into molten silicon and slowly pulled up while being rotated, to make a large single crystal, in which the whole piece is one crystal.
When a small number of atoms with 5 valence electrons, such as phosphorus atoms, are mixed into silicon, the extra electrons move and carry the current, and the silicon becomes an n-type semiconductor. When atoms with 3 valence electrons, such as boron atoms, are mixed in, places where an electron is missing carry the current, and the silicon becomes a p-type semiconductor. A place where an electron is missing is called a hole.
When light falls on the boundary where n-type and p-type silicon are joined, electrons and holes are produced, and the electrons move to the n-type side and the holes to the p-type side. So when the two sides are connected with a wire, a current flows. This is how a solar cell works.