Silicon

Introduction

Silicon is a hard, brittle crystalline solid appears bluish-grey with a metallic lustre. It is a tetravalent metalloid and a semiconductor, a  member of group 14 in the periodic table. Carbon is located above it while the germanium, tin, and lead are below it. Silicon is the second abundant element in earth’s crust comprising of 27.7% of the crust’s entire mass. While silicon does not occur in its pure form but is available in the combined state as silicon dioxide in quartz which is an essential component of sand. It also occurs as silicates in the form of rocks, soils and clays and the various silicate forms are the asbestos, hornblende, feldspar, clay, granite, and mica. Furthermore, the oxide form of silicon includes sand, quartz, rock crystal, amethyst, agate, flint and opal. The large scale commercial production of silica is carried out by the reduction of sand with the carbon in an electric furnace while the high-purity silicon is prepared by the thermal decomposition of ultra-pure trichlorosilane, followed by recrystallization.

 

 

Discovery of silicon

The name silicon  is derived from the Latin word silex or silicis, meaning “flint” or “hard stone.” The amorphous form of elemental silicon was first isolated and described in the year 1824 by Jöns Jacob Berzelius, a Swedish chemist but the impure silicon had already been obtained in 1811.

 

 

A fact sheet about silicon

Silicon Element -Properties

Origin of the name                       

The name is derived from the Latin 'silex' or 'silicis', meaning flint.

Discovered by       

Jöns Jacob Berzelius

Atomic number

14

Atomic weight

28.086

Melting point

1,410 °C (2,570 °F)

Boiling point

2,355 °C (4,270 °F)

Density

2.33 grams/cm3

Oxidation state

−4, (+2), +4

Electron configuration

1s22s22p63s23p2

Key isotopes

28Si, 30Si

Density (g cm−3)  

2.3296

 

 

Biological role

Silicon is a non-toxic substance but some silicates, such as asbestos, can increase the risk of carcinogenesis. It is inevitable for plants and animals as it is significantly useful for the better functioning of cartilage, connective tissue and organs. It is present in traces in the walls of the aorta, trachea, tendons and ligaments as it helps in cross-linking the collagen (a protein that gives strength and elasticity of the tissues and bones). Phytoliths are the silica particles formed in some plants and they remain in fossil forms for a very long time, hence they serve as useful tools of evolutionary evidence.

 

 

Bond enthalpies

Preparation of silicon

Carbon can be used to reduce silica to produce silicon. The reaction is carried out in an electric furnace. i,e 

2C(s) + SiO2(s)  Si(s) + 2CO2(g)

 

 

 

Properties of Silicon

Just like carbon, silicon is relatively inactive at room temperatures, nevertheless, when heated it reacts vigorously with the halogens such as fluorine, chlorinebromine, and iodine resulting in the formation of halides as shown in the reaction below.

  1. 2Cl2(g) +Si(s)   SiCl4(l)

  2. H2O(l)+2OH-(aq) +Si(s)   SiO32-(aq) + 2H2(g)

 

The bonds in elemental silicon are strong enough. To break them, it requires high energies. When silicon and carbon are combined at electric furnace temperatures (2,000–2,600 °C [3,600–4,700 °F]), they form silicon carbide. Furthermore, with the hydrogen, silicon forms a series of hydrides, the silanes. When combined with hydrocarbon groups, silicon forms a series of organic silicon compounds. The 3 stable isotopes of silicon are silicon-28, silicon-29, and silicon-30, 3.0%t. Human tissue often contains about 6 to 90 milligrams of silica (SiO2) per 100 grams dry weight however, inhalation of silicon dust containing alpha SiO results in silicosis, a lung disorder. It is an occupational disorder seen in those who work as miners, stonecutters, and ceramic worker. Silicon has a high melting-point and it only reacts with hydrochloric acid to form hexafluorosilicic acid and is written as Si(s) + 6HF(g)   H2SiF6(aq) + 2H2(g)

Nevertheless, it also reacts with magnesium to yield magnesium silicate indicated as 2Mg(s) + Si(s)    Mg2Si(s)

 

 

Some important compounds of Silicon

 

Silicates

Silicates refer to the binary compounds of silicon with oxygen.  The Si-O bond is partly ionic and partly covalent since the degree of negativity between oxygen and silicon is approximately 1.7. The ratio of the radius of silicon ion to the ratio of oxide ion is about 0.29. The structure of the silicate is based upon SiO44- because silicon is sp3 hybridized and it`s co-ordination number is four.

 

Silicates can be divided into six groups

  1. Chain silicates

Chain silicates are classified into simple and double chain substances. The two corners of each tetrahedron simple chain are shared and forms along the chain of a tetrahedron with a general formula of (SiO3) n2n

 

  1. Orthosilicates

Orthosilicates are compounds which contain discrete [SiO4]- units.

 

  1. Pyrosilicates

In pyrosilicates, two tetrahedral unit`s are combined by sharing  oxygen at one corner yielding [Si2O7]6- units.

 

  1. Cyclic silicates

Cyclic silicates are those in which two oxygen atoms per tetrahedron are shared to produce closed rings whose structure is of the formula (SiO32-) n

 

  1. Two and three-dimensional sheet silicates

In two-dimensional sheets, three oxygen atoms of each tetrahedral are shared with adjacent SiO44- tetrahedral while in three-dimensional silicates, all the four oxygen atoms are shared with adjacent SiO44-  tetrahedral.

 

 

Zeolites

Zeolites are alumino-silicate of metals. Potassium ion, sodium ion and calcium ion are major positive ions that contribute to the formation of zeolites.  The zeolites are mainly used to remove the permanent hardness of water in water plants.

 

Silicones

Silicones refer to synthetic organosilicon substances containing repeated R2SiO unit`s held together by Si-O linkages.  The silicones have the general formula (R2SiO) n where R is an alkyl or aryl group. Silicones are prepared by hydrolysis of alkyl or aryl substituted chlorosilanes and their subsequent polymerization. After fractional distillation, the silane derivatives are hydrolyzed and the hydroxides immediately condense by intermolecular elimination of water. The final product depends upon the number of hydroxyl groups originally bounded to the silicon atoms.

 

 Uses of silicon and it`s compounds

Silicon varnishes are used as insulators in motor vehicle industries. They are also used as raw materials for the production of resin, rubber and oils. Aeroplane manufacturers use the silicon because of its thermal resistance abilities and viscosity.

 



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