Structures+-+Audrey

Describe the structure of and bonding in silicon and silicon dioxide
__**Silicon:**__ Silicon is the eighth most common element on earth by mass and has the symbol **Si** with an atomic number of 14. It has the electronic configuration of [Ne] 3s2 3p2 Silicon has a giant covalent structure, held together by strong molecular bonds. It is commonly found in it's solid state and is used as a semi-conductor in micro controllers, resistors, diodes... As it is a semi conductor, it readily shares and gives away it's four valence electrons, hence having many different forms of chemical bonding. This property hence also allows it to be made into Glass, Abrasives, Pottery and even Silly Putty. It is an amphoteric oxide with an oxidation state of 1,2,3,4 (-1,-2,-3,-4). Silicon crystalizes in to a diamond cubic crystal structure as shown below: Silicon dioxide has a chemical formula of **SiO2** and is most commonly known as silica. Silica is most commonly found in nature as sand or quartz and is mainly used to produce glass. Silicates often are found with a tetrahedral coordination, with 4 oxygens surrounding a silicon atom. That means in the case of silicon dioxide, it will share 2 of its electrons with another silicon dioxide molecule, just like in aluminum chloride. Silicon Dioxide hence has many crystalline forms. The table below from wikipedia lists them all: [|Pearson symbol], group No. ||~ Notes ||~ Structure || hP9, P3121 No.152[|[][|12][|]] || Helical chains making individual single crystals optically active; α-quartz converts to β-quartz at 846 K || || hP18, P6222, No.180[|[][|13][|]] || closely related to α-quartz (with an Si-O-Si angle of 155°) and optically active; β-quartz converts to β-tridymite at 1140 K || || oS24, C2221, No.20[|[][|14][|]] || metastable form under normal pressure || || hP12, P63/mmc, No. 194[|[][|14][|]] || closely related to α-tridymite; β-tridymite converts to β-cristobalite at 2010 K || || tP12, P41212, No. 92[|[][|15][|]] || metastable form under normal pressure || || cF104, Fd__ 3 __m, No.227[|[][|16][|]] || closely related to α-cristobalite; melts at 1978 K || || cF576, Fd__ 3 __m, No.227[|[][|17][|]] || [|sodalite] cages connected by hexagonal prisms; 12-membered ring pore opening; faujasite structure.[|[][|9][|]] || || tP36, P41212, No. 92[|[][|20][|]] || Si5O10, Si4O14, Si8O16 rings; synthesised from glassy silica and alkali at 600–900K and 40–400 MPa || || mS46, C2/c, No.15[|[][|21][|]] || Si4O8 and Si6O12 rings || || mS48, C2/c, No.15[|[][|22][|]] || Si4O8 and Si8O16 rings; 900 K and 3–3.5 GPa || || tP6, P42/mnm, No.136[|[][|23][|]] || One of the densest (together with seifertite) polymorphs of silica; rutile-like with 6-fold coordinated Si; 7.5–8.5 GPa || || oP12, Pnc2, No.30[|[][|24][|]] ||  ||  || oI12, Ibam, No.72[|[][|25][|]] || like SiS2 consisting of edge sharing chains || || oP, Pbcn[|[][|26][|]] || One of the densest (together with stishovite) polymorphs of silica; is produced at pressures above 40 GPa.[|[][|27][|]] || ||
 * __Silicon Dioxide:__**
 * ~ Form ||~ Crystal symmetry
 * α-quartz || [|rhombohedral] (trigonal)
 * β-quartz || [|hexagonal]
 * [|α-tridymite] || [|orthorhombic]
 * β-tridymite || hexagonal
 * [|α-cristobalite] || [|tetragonal]
 * β-cristobalite || [|cubic]
 * [|faujasite] || cubic
 * [|melanophlogite] || cubic (cP*, P4232, No.208)[|[][|4][|]] or tetragonal (P42/nbc)[|[][|18][|]] || Si5O10, Si6O12 rings; mineral always found with hydrocarbons in interstitial spaces-a clathrasil[|[][|19][|]] || [[image:http://upload.wikimedia.org/wikipedia/en/thumb/a/ac/MelanophlogiteStucture.png/100px-MelanophlogiteStucture.png width="100" height="100" caption="MelanophlogiteStucture.png" link="http://en.wikipedia.org/wiki/File:MelanophlogiteStucture.png"]] ||
 * [|keatite] || tetragonal
 * [|moganite] || [|monoclinic]
 * coesite || monoclinic
 * stishovite || Tetragonal
 * [|poststishovite] || orthorhombic
 * fibrous || orthorhombic
 * seifertite || orthorhombic