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Silicones 1. Silicate Structures

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Heat and Chemical Resistant Silicone Rubber  

This is the story of chemical transition from the common silicate minerals and rocks of the earth to the uncommon chemical compounds of silicon and carbon.


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Figure 1. used with permission by author: Benjah-b7

We begin with silicates, the form in which we find almost all silicon, the second-most prevalent element in the earth's crust.

In General Chemistry we study the group 14 elements, carbon. silicon, germanium, tin and lead. We learn that the fundamental structural unit of all the silicate minerals is a single silicon atom at the center of a tetrahedral array bonded to four oxygen atoms (left). We remember also, that we find thousands of silicate minerals on and under the surface of the earth. If we look at neighboring tetrahedra in these minerals, some neighbors share a single oxygen with their neighbor, others share two oxygens with the neighboring tetrahedron.

Should we draw a silicate structure composed solely of a sequence of tetrahedra in which single rather than pairs of oxygen are shared with the neighboring tetrahedra, we would sketch a linear structure (right). The oxygen atoms not associated with the Si-O-Si-O sequence bear a negative charge, balanced in minerals by metal and nonmetal cations. One message we might get from the prevalence of alternating silicon and oxygen atoms is that silicon and oxygen strongly attract each other.

In the nineteenth century chemists focused on making new materials. They began to develop new chemical tools, an understanding that materials could be converted one to the other, and an inventory of chemical techniques to engineer these chemical changes.


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Figure 2: Silicate chain with 3lyhedtra. Used with permission.

When these chemists began to make new silicon containing chemical species, they learned to form bonds of silicon with hydrogen: Si-H, and with the halogens: Si-F, Si-Cl, Si-Br, Si-I. Silicon compounds containing these bonds were remarkably different from the rocky mineral silicates. SiH4 was a gas, as was SiF4, SiCl4 was a liquid with a low boiling point. And silicon hydrides and silicon halides were not stable out in the humid air in the room. They reacted immediately with water, with moisture in the air, to form compounds with the favored Si-O-Si-O- sequence of atoms.

And in Germany, Russia, Sweden and France, other chemists began to make new chemical compounds with silicon directly bound to carbon. They began to build an inventory of new, uncommon materials. In contrast to the Si-H and Si-X (X=F, Cl, Br, I) that reacted so rapidly with water, compounds with Si-C bonds were quite stable.