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Element Fact
Cesium is used in atomic clock. As in 1999, the best clock is accurate to a second in 20 million years.

Silicon, Si

silicon sample

Atomic number: 14
Atomic mass: 28.0855(3)
Natural abundance: 272000 ppm
Isotopes:
28Si (27.9769265) - 92.23%
29Si (28.9764947) - 4.67%
30Si (29.9737702) - 3.10%

History and Production

Derived from Latin silex, silicis, meaning flint. Amorphous silicon was first prepared by J.J. Berzelius in 1824 by reducing K2SiF6 with molten potassium. The crystalline form, which the second allotrope form of the element, was first prepared by Deville in 1854. It was T. Thomson who named the element in 1831. He added the ending on in order to emphasise the analogy with boron and carbon. Silicon is one the most useful element to man kind. It can be prepared commercially (96-99% pure) by heating silica (SiO2) and pure coke in an electric arc furnace. Very pure silicon, which is used for semiconductor application can be obtained from silicon tetrachloride by reduction action with pure zinc or magnesium. It is then melted and grown into cylindrical single crystals, then purified by zone refining. Other methods, such as reduction of Na2SiF6 by sodium metal for producing solar cells. The crystalline product can also be doped with boron, gallium, arsenic etc., for use in transistors, rectifiers and other solid-state devices. In the form of sand and clay it is used to make concrete, refractory material, glass making etc. Silicon carbide, is impotant as an abrasive.

Physical Data

quartz mineral
Crystalline silicon has a metellic luster and grayish in color while amorphous silicon is a brown powder. It is the most abundant (27% by weight) element in the earth's crust, after oxygen. However, in universe, it is only seventh order of abundance. It never occurs in nature freely but found in a variety of minerals: sand, quartz (pictured on left), agate, jasper etc. It is usually combine with oxygen in nature.

Interatomic distance: 235.2 pm
Melting point: 1414C
Boiling point: 3265C
Thermal conductivity/Wm-1K-1: 148 (27C)
Density/kgm-3: 2329 (20C), 2525 (m.p.)

Standard Thermodynamic Data (atomic gas)

Enthalpy of formation: 450 kJ/mol
Gibbs free energy of formation: 405.5 kJ/mol
Entropy: 168 J/mol K
Heat capacity: 22.3 J/mol K

Electronic data

Electronic configuration: [Ne] 3s2 3p2
Term symbol: 3P0
Electron affinity: 134.0685 kJ/mol   Electronegativity (Pauline): 1.90
Ionization energy (first, second, third): 786.519, 1577.14, 3231.59 kJ/mol

Chemical properties

Large crystalline silicon is generally unreactive at room temperature. Oxygen, water and steam do not react with silicon, probably due to the protective layer of SiO2 on the surface. However, it reacts with water from 900C and at 1400C with nitrogen to give Si3N4 and SiN.

In contrast to solid states, molten silicon is very reactive. It forms silicides with most metals. Most of silicon compounds are tetrahedrally coordinated. The usual {SiO4} unit may exist as an individual group or make up of chains, rings or even complex 3-dimensional frameworks.


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