Silicon and Compounds

SILICON AND COMPOUNDS

Silicon Dioxide

·        The earth's crust is primarily composed of silica and silicates, which make up 95% of its composition.

·        Silicon dioxide is the chemical name for silica, which occurs in several crystallographic forms, including quartz, cristobalite, and tridymite.

·        These crystalline forms of silica are interconvertible at suitable temperatures.

·        Silicon dioxide is a covalent, three-dimensional network solid in which each silicon atom is covalently bonded in a tetrahedral manner to four oxygen atoms.

·        Each oxygen atom, in turn, is covalently bonded to another silicon atom, resulting in a network of tetrahedra.

·        The entire crystal structure can be considered as a giant molecule in which eight-membered rings are formed with alternate silicon and oxygen atoms.

 

 

Silica in its normal form is almost non-reactive because of very high Si—O bond enthalpy. It resists the attack by halogens, dihydrogen and most of the acids and metals even at elevated temperatures. However, it is attacked by HF and NaOH.

SiO2 + 2NaOH → Na2SiO3 + H2O

SiO2 + 4HF → SiF4 + 2H2O

  • Quartz is a piezoelectric material widely used in various technologies, including accurate clocks, radio and TV broadcasting, and mobile communications.
  • Silica gel, a form of silica, is used as a drying agent, support for chromatography materials, and catalysts.
  • Kieselghur, an amorphous form of silica, is used in filtration plants.

 

Silicones

The polymeric compounds containing   units, linear cyclic or cross linked are known as silicones. They are manufactured from alkyl substituted chlorosilanes

 

 

Silicones are chemically inert, water repellent, heat resistant, good electrical insulators. These are used as lubricants, insulators etc.

 

Silicates

Silicates are metal derivatives of silicic acid and can be obtained by fusing metal oxides or metal carbonates with sand e.g.

 

TYPE OF SILICATES

Silicates contain tetrahedral formed by hybridization, depending upon the number of  O-atoms shared between tetrahedra and fashion, Silicates have been classified into following groups

Orthosilicates - They contain discrete tetrahedra, Examples are phenacite , willimite forsterite .

Pyrosilicates - Here two tetrahedra units are
joined by one oxygen atom forming a large discrete
. Examples are thortveitite , hemimorphite

Chain silicates - Here two oxygen atoms per SiO4 tetrahedra are shared giving polymeric anion chains. Discrete unit is . Examples: synthetic sodium silicate,   lithium silicate ,  natural spodumene , jadeite , enstatite and diopside .

Double chains - Here two simple  chains are held together by shared oxygen atoms. The discrete unit is . Example mineral tremolde

Double chains silicates is also called amphibole

Cyclic silicates - Here two oxygen atoms per tetrahedra are shared giving discrete unit   and . Example Beryl

Sheet-silicates - Here three oxygen atoms per tetrahedra are shared giving two dimensional sheet having discrete unit . Example

Talc Kaolin.

Framework silicates - Here all four oxygen atoms of each tetrahedra are shared.  Example are quartz, zeolites, tridymite and cristobalite.