SILICON

Atomic Number: 14
Mass Number: 28.0855
Electron Configuration: 1s²2s²2p⁶3s²3p²
Melting Point: 1420^oC
First Ionization Energy: 782 kJ/mol
Second Ionization Energy: 1570 kJ/mol
Third Ionization Energy: 3230 kJ/mol
Fourth Ionization Energy: 4350 kJ/mol
Electronegativity: 1.9
Atomic Radius: 117 pm

While silicon constitutes approximately 28% of the earth's crust, it does not appear in elemental form. Instead, silicon appears as silicon dioxide and in a wide range of silicate minerals. Pure silicon is lustrous gray solid and has a crystal structure similar to diamond. Since silicon lies immediately below carbon on the periodic table, one might expect it to have similar chemical properties. The properties of silicon and carbon are, however, quite different. Silicon differs from carbon in that silicon does not have the same tendency to catenate. This is due to the fact that the silicon-silicon bond is not as strong as the carbon-carbon bond. Therefore, while carbon forms an extensive series of compounds with hydrogen, the list of silicon analogs is limited.  Silicon also differs from carbon in that it does not form pi bonds as readily as carbon does. Therefore silicon analogs of carbon dioxide and carbon monoxode, which contain double and triple bonds, are not observed.

Compounds between silicon and hydrogen are called hydrides and can be viewed as analogs of alkanes. All are colorless liquids or gases at room temperature. There is a steady increase in the boiling point with increasing molecular weight, and trisilane is the first member of the series that is a liquid at room temperature. Instability increases with increasing number of Si-Si bonds, and only silane and disilane are indefinitely stable. Silane analogs of alkanes, alkenes, and aromatic hydrocarbons are unknown. All silanes react explosively with oxygen to produce silicon dioxide and water. The electronegativity of hydrogen is greater than that of hydrogen, and silanes behave as if they have hydridic hydrogens. For example, when traces of base are present, silanes react with water to form hydrated silicon dioxide and hydrogen gas.

All four of the silicion tetrahalides are known, and their properties are listed in the table below. Unlike the carbon tetrahalides, the silicon tetrahalides are completely hydrolized in water. The only exception is silicon tetrafluoride, which produces the hexafluorosilicate ion. Silicon tetrachloride us a fuming liquid used in the manufacture of elemental silicon.

Whereas carbon combines with oxygen to form molecular compounds such as carbon monoxide and carbon dioxide, silicon forms a complex series of compounds called silicates. The building block of the silicates is the orthosilicate ion, in which silicon is tetrahedrally bonded to four oxygen atoms. The simplest silicate minerals, the olivines, consist of discrete orthosilicate anions. 

Discrete Tetrahedra

Formula	Example	Illustration	Model
SiO44-	Olivines Mg2SiO4 Fe2SiO4 Ca2SiO4

Linked Tetrahedra

If two orthosilicate tetrahedra share an oxygen atom, the result is the pyrosilicate or disilicate ion. Thorvetite, a mineral containing scandium, is an example of a pyrosilicate.

Formula	Example	Illustration	Model
Si2O76-	Sc2Si2O7 (Thorvetite)

Cylic Structrues

Orthosilicate tetrahedra may also be joined together into ring structures. Two such structure are known, one containing three tetrahedra and one containing six. Beryl, an important beryllium mineral, is an example of the latter.

Formula	Example	Illustration	Model
Si3O96-	Ca3Si3O9

Formula	Example	Illustration	Model
Si6O1812-	Be3Al2Si6O18 (Berryl)		no model available

Beryl is an important mineral of beryllium. It exhibits various color due to impurities in the crystal structure. When blue-green in color, it is known as aquamarine. It may also be in yellow in color, as shown in the photo below. Emerald is beryl with traces of chromium (III).

Single Chains

A group of minerals known as pyroxenes, contain infinite chains of linked tetraedra such that each silicon atom shares two oxygen atoms. Examples of pyroxene minerals include enstatite, diopside, spodumene, and jadeite.

Formula	Example	Illustration	Model
(SiO3)2-	Pyroxenes MgSiO3 (enstatite) CaMg(SiO3)2 (diopside) LiAl(SiO3)2 (spodumene) Na3Al(SiO3)2 (Jadeite)

Double Chains

In the amphiboles, the tetrahedra are linked to form a double chain such that half of the silicon atom share two oxygen atoms, and the other half of the silicon atoms share three oxygen atoms. 

Formula	Example	Illustration	Model
(Si4O11)6-	Amhiboles		no model available

Infinite Sheets

When each silicon atom shares three oxygen atoms, the result is an infinite sheet. The sheets may be held together by the cations lying between them. Such substances cleave readily into sheets. Mica is an example.

Formula	Example	Illustration	Model
(Si2O5)2-	Micas		no model available

Quartz Quartz results from the complete sharing of all oxygen atoms and has a three-dimensional network structure. Quartz crystals are optically active and have left and right-handed forms. Quartz crystals are piezoelectric and will vibrate when an alternating electrical current is applied to them.

Amethyst Amethyst is composed of quartz, with the color arising from traces of manganese(II).

Another Quartz Sample The color of this sample of quartz is due to the traces of iron(III).