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Element Silicon, Si, Metalloid

About Silicon

Silicon stands to carbon in the same relation as sulphur to oxygen. The two elements are similar in many respects, but differ more from one another than, for example, chlorine, bromine, and iodine do.

Silicon, like carbon, occurs in several forms, of which an amorphous and a crystalline form are accurately known. Amorphous silicon is obtained by conducting its chlorine or fluorine compound over heated potassium; the metal unites with the halogen, and the silicon is set free. The soluble potassium salt formed is removed by washing with water, and the silicon is left behind as a greenish-brown powder. This is amorphous, and has the tendency to pass into the colloidal state; if, therefore, the washing is continued to a certain point, the silicon goes into a sludge and begins to pass through the filter.

Amorphous silicon is more easily obtained by heating its oxygen compound, finely powdered quartz, with magnesium powder. The magnesium combines with the oxygen of the silicon dioxide, forming magnesium oxide, and the silicon is liberated. The latter can be obtained pure by extracting the product with dilute acid, in which the magnesium oxide but not the silicon dissolves.

At a high temperature, silicon melts; and on solidifying, crystalline silicon is formed as a grey mass with a metallic lustre. The crystallisation is greatly facilitated by the addition of a metal such as zinc; the zinc can be removed by treating the product with dilute acids.

Amorphous silicon can be set on fire in the air, but its combustion is very incomplete, because the non-volatile silicon dioxide formed prevents further combustion. Crystalline silicon does not noticeably change in the air even at a red heat. Silicon is soluble in caustic soda on heating, the silicon thereby taking up oxygen from the water and passing into an acid, silicic acid, or rather into its sodium salt. The hydrogen of the water escapes as a gas.

The combining weight of silicon has been determined by the analysis of its halogen compounds, and amounts to Si = 28.4.

Occurrence of Silicon

Although not occurring naturally in the free state upon the earth, silicon is, next to oxygen, the most plentiful of the elements. It is always found combined with oxygen as silica, which exists in various forms either in the free state, or united with water or with metallic oxides, forming the numerous and frequently complex silicates of which the primitive rocks of the earth's crust so largely consist. Through the medium of water, silica is assimilated by certain plants and animal organisms; its presence has been detected in the sun and many stars, as well as in meteorites, as, for instance, that of the Diablo Canon.

History of Silicon

The history of silicon and its compounds commences with the observation of Becher that siliceous minerals are suitable for glass-making, and contain an "earth" which he named terra vitrescibilis. In 1660 Tachenius found that this "earth" combines with alkalis, and therefore possesses an acid nature. Further knowledge of the element was gained by Scheele, who in 1771 prepared hydrofluoric acid, and thence silicon tetrafluoride, which with water yielded silicic and hydrofluosilicic acids. The decomposition of the alkalis and alkaline earths in 1808 by Davy led to the belief that silica is also a compound; and in the same year Berzelius obtained what he believed to be silicon ("kiesels") by heating iron, carbon, and silica together.

Gay Lussac and Thenard were probably the first,- however, to obtain the element. This they did in 1811, by passing silicon tetrafluoride over heated potassium; whilst Berzelius, in 1823, decomposed potassium silicifluoride with potassium, and obtained what Vigouroux has shown to be impure silicon. Our knowledge of amorphous and crystallised silicon is due principally to the researches of Wohler and St. Claire-Deville, Moissan and Smiles, and Vigouroux.


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