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Silicon and Group 4A Elements f

Skills to develop

Group 4A Elements C, Si, Ge, Sn, Pb

Group 4A elements play more important roles in our lives and our civilization than elements of any other group. Thus, every educated person should know something about them.

Chemistry of Carbon

Carbon exists as diamond, graphite, fullerenes, and charcoal. Their structures are interesting, so are their properties. You probably know a lot about diamond and graphite, but the fullerenes were discovered after 1970, and this discovery has opened a door for a lot of interesting research. Read about them in books, magazines and journals. You might find yourself working with them some day.

Among the fullerenes, one of the most common "molecule" has 60 C atoms, and is represented by C60, a diagram is shown here. If you connect the 60 carbon atoms with bonds, the structure looks like a cage with 5- and 6- member rings. Synthesis, bonding, symmetry and stability of the cagelike fullerenes have already attracted a lot of attention, and their properties are even more fascinating.

Regarding carbon compounds, you already know something about CO2 and CO including their roles in the environment. The hard carbides such as Fe3C, WC, and TiC are more interesting to material scientists and engineers for their application in cutting tools. The calcium carbide CaC2 produced by reducing CaO by carbon was a valuable commodity at one time due to its reaction with water to give acetylene gas:

CaC2 + 2 H2O ® Ca(OH)2 + C2H2 (g). Acetylene is an important industrial gas, for the manufacture of polymers.

Silicon

Do you know that:

Silicates

By weight, silicon is the most abundant element in the Earth's crust. It usually exists in the form of oxide, SiO2. This formula does not do justice to represent so many different materials we call silicates, but these substances are indeed SiO2. Some of the minerals contain impurities.

In pure form, SiO2 is quartz. Small particles of quartz are sand. They are hard. In the structure at the atomic level, every silicon atom is bonded to 4 oxygen atoms, and every oxygen is bonded to two Si atoms. The four Si-O bonds point towards the corner of a tetrahedron, as do the C-C bonds in the diamond structure. When impurity is present, the quartz may be colored. Due to various arrangement of the Si-O-Si bonds, the same substance appear in many forms.

A basic unit of silicate structures is SiO44-. The gemstone zircon has a formula ZrSiO4, and olivine has a chemical formula of (MgFe)2SiO4. Two SiO44- units combine to give the pyrosilicate unit Si2O76-, and it appears in akermanite, Ca2MgSi2O7. When the number of units increase, the tetrahedral units combine to form rings, chains, layers and 3-dimensional networks. Thus, the structure and classification of silicate is a major part of minerals. This site has some interesting pictures.

Silicon and Silane

Elemental silicon can be obtained from reduction of silicates. The reduction of sand, SiO2 by carbon at 3300 K in the reaction,

SiO2 + 2 C ® Si(l) + 2 CO     at 3300 K gives liquid silicon. The silicon so obtained is usually not pure, and for the computer industry, the element must be purified. Crystal growth and silicon fabrication dominate the industry in the 1980s and 1990s, and perhaps into the next century, and the production of the element is only the beginning of the process.

If a more reactive element, Mg, is used in the reduction, Mg2Si is formed,

SiO2 + 4 Mg ® Mg2Si + 2 MgO Mg2Si is a compound, and it reacts with water to form silane.

Silane, SiH4, can be produced by reacting Mg2Si with acids

Mg2Si + 4 H2O ® 2 Mg(OH)2 + SiH4 and SiH4 is ignited when it contacts air, much more reactive than methane, SiH4 + O2 ® SiO2 + H2O In a basic solution, SiH4 reacts with water to give SiO(OH)3-, SiH4 + OH- + 3H2O ® SiO(OH)3- + 4 H2

Silicon Halids

Silicon tetrafluoride is formed when glass (SiO2) is exposed to HF, and when Si reacts with F2, SiO2 + 4 HF(aq) ® 2 H2O + SiF4(g)
Si + 2 F2 ® SiF4(g)

When chlorine pass through hot sand (SiO2) and carbon, SiCl4 is produced,

SiO2 + Cl + 2 C2 ® 2CO + SiCl4(g) SiCl4 and SiF4 react with water to give silicic acid, SiCl4 + 4H2O ® 4 HCl + Si(OH)4(aq),
SiF4 + 4H2O ® 4 HF + Si(OH)4(aq).

Silicone Polymers

Silicones are polymers with general formula (R2SiO2)n or (RSiO3)n, (R = CH3, C2H5, C6H5, etc). The chain is held together by Si-O-Si bonds. A simple one is ((CH3)2SiO2)n,
     CH3    CH3    CH3
  -O-Si--O--Si--O--Si--O--
     CH3    CH3    CH3
Of course, the 4 bonds around the Si atoms points to the corners of a tetrahedron. These siloxane polymers are widely used as sealants, adhesives, additives, flame retardants, and lubricants. They have a wide application in industries. Depending on the organic group attached to silicon, the inorganic polymer Silicones has been an important class of materials.

Polymers are really interesting, but there are so many kinds of them that make you wonder if you ever become an expert on it.

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