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Friday, 1 November 2019

Amorphous Silicon and Crystalline Silicon


Atomic Weight - 28.0855 u
Atomic Number - 14
Symbol - Si

History of Silicon:

The process of manufacturing glass from Silicates was known from ancient times. Silica was regarded by Becher as an earth which he called terra vitrescibilis owing to its use in glass-making. Berzelius prepared amorphous Silicon in 1823 ; and Deville prepared crystalline Silicon in 1854.
Occurrence —Silicon is not known to occur free in nature ; but next to Oxygen, it is the most abundantly distributed element known, and constitutes nearly one-fourth of the mass of the earth's crust.

(a) In combination with Oxygen, as Silicon dioxide or Silica (Si02), it occurs as flint, sand, quartz, rock-crystal, and chalcedony.

(b) In combination with Oxygen and the metals like Calcium, Magnesium, Aluminium, it occurs as Silicates in clay, in many rocks, and in the soil. Silicon, in the form of Silica, is also present in some plants and trees.

Allotropic Modifications of Silicon.

1. Silicon is found to exist in two different allotropic modifications 
2. Amorphous and crystalline.

Amorphous Silicon. Preparation.

(i) By beating Sodium or Potassium in a current of Silicon chloride vapour.
                        SiCl4 + 4K = 4KCl + Si.
(ii) By beating strongly an intimate mixture of quartz or sand and Magnesium powder.
                        2Mg + SiO2 =2MgO + Si

Properties of Amorphous Silicon :


It is a brown powder with a specific gravity of 2.35. It can be fused at 1550°C and it boils in the electric furnace. It is a non-conductor of electricity.


When heated in air, Silicon burns to Silicon dioxide. It is insoluble in water, but slowly decomposes steam at a red heat.

                         Si+ 2H2O = SiO2 + 2H2.

Silicon is insoluble in all acids except Hydro-fluoric acid (mixed with a little HNO3) in which it dissolves giving off Hydrogen and producing Hydrofluosilicic acid.

                        Si +6HF =2H2 +H2SiF6

A mixture of KC1O3 and HNO3 has no action on Silicon (Gf. Carbon), though hot solutions of KOH and NaOH dissolve it with the evolution of Hydrogen and formation of the Silicates.
                        Si + 2KOH+H2O = K2SiO3 + 2H2

Silicon combines directly with fluorine at the ordinary temp. with incandescence giving Silicon tetra fluoride ; while it burns in Chlorine giving Silicon tetrachloride. It combines with many elements at a high temp. forming silicides.

 Crystalline Silicon. Preparation

(i) By the action of the vapour of SiCl4 on excess of Al, melted in a current of Hydrogen.

Experiment - Melt down Aluminium in a current of Hydrogen and pass a slow stream of Silicon tetrachloride vapour over the melted metal. Aluminium reacts with Silicon tetrachloride and sets Silion free which dissolves in excess of melted Aluminium. Now cool the mass, when crystalline Silicon separates out. To free it from Aluminium, treat the mass with HCl. Aluminium dissolves and we get Silicon.
                         3SiC14+4Al = 3Si+ 4AlCl3.

(ii) By strongly heating Sodium silico-fluoride (Na2SiF6) with Zn and Na.
Heat Na2SiF6 with Na and Zn in a crucible. Na liberates Silicon from Na2SiF6. Silicon, thus liberated, dissolves in melted Zinc. Now cool the mass, when needle-shaped crystals of adamantine Silicon separate out. Dissolve away the excess of Zn by means of dil, HCl. We thus get crystalline Silicon.
                        Na2SiF6 + 4Na = Si + 6NaF

(iii) By melting down amorphous Silicon in a crucible and then cooling it, when crystalline, graphitoidal Silicon separates out.

(iv) By heating excess of quartz with coke in the electric furnace. Excess of Silica must be used; otherwise Carborundum is formed.

SiO2 + 2C = Si + 2CO

Properties of Crystalline Silicon:

Physical.— It is a dark-grey crystalline solid having a metallic lustre, and a specific gravity of    2.94. It is hard enough to scratch glass. Like the amorphous variety it fuses at 1550o C and boils in the electric furnace; but, unlike it, crystalline Silicon slightly conducts electricity. Hence it is sometimes called metallic Silicon.

Chemical — It is chemically less active than the amorphous variety. It does not burn in Oxygen even when strongly heated. It is insoluble in all acids (except Hydrofluoric acid mixed with a little HNO3). It, burns when heated in Chlorine, and spontaneously takes fire in Fluorine. Silicon combines with Nitrogen, Carbon, and also, many metals forming silicides.

Comparison of Amorphous Silicon and Crystalline Silicon

It is a brown powder
A dark grey crystalline solid
Sp. gr. = 2.35
Sp. gr. = 2.94
Non-conductor of electricity
Slightly conducts electricity.
Melts in the electric furnace
Melts in the electric furnace.
It is more active than the crystal-line variety
It is less active than the amorphous variety.
Air—when boated in air, it oxidizes superficially, and it takes fire in Oxygen at 400°C burning brilliantly to Si02.
Air—It does not burn in air free from CO2 or in Oxygen even when strongly heated.

Water —It does not react with water at the ordinary temp. but red-hot Silicon decomposes steam.
Si + 2H2O = 2H2 + SiO2
Water—It does not react with water at any temp.

Insoluble in all acids except HF (mixed with a little HNO3 ).

Insoluble in all acids except HF (mixed with a little HNO3).
 Si+6HF = 2H2 + H2SiF6

Alkalies —It dissolves in hot conc. NaOH or KOH sol.
Si + 2KOH + H2O = 2H2 + K2SiO3

Alkalies—It reacts with fused Na2C03,  giving Na2SiO3 and Carbon.

halogens--It combines readily with Fluorine with incandescence at the ordinary temperature giving SiF4 .
Halogens —It spontaneously takes fire in Fluorine giving SiF4.

Other  elements —It combines with N, 0 and other elements giving the silicides.

Other elements—It combines with N, 0 and other elements giving the silicides.

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