The properties of glass explained by the bonding of Silicon and Oxygen.

Essay by crispiesHigh School, 11th grade May 2003

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The properties of glass lie within the bonding structure of silicon. Silicon cannot form strong ð bonds with oxygen by using its valency 3p orbitals. This is because of the large nuclear radius of the silicon atom and its orbitals which results in a less effective overlap with the smaller oxygen orbitals. The silicon atoms then form four single bonds to satisfy the octet rule. Therefore, instead of forming a SiO2 covalent molecule, it will form a SiO4 covalent network (although it is still represented in its molecular state as SiO2). The silicon atom is based at the centre of a tetrahedron, with shared oxygen atoms on the outside.

Glass is generally classified as being chemically inert making it a highly suitable material for windows and containers. This is because glass reacts to very few chemicals. However, glass does react with hot, concentrated alkalis and acids. Alkalis work by attacking the silica directly and dissolving the glass, and acids work by attacking the alkalis in the glass.

The corrosion of glass by acids is slowed down because a porous surface of silica is left with holes where the alkalis had been. This porous surface decreases the surface area of the reacting acid, consequently slowing it down. Water corrosion is similar to the acids.

Glass differs from quartz (sand) in that quartz is a solid and glass is a supercooled liquid. Quartz has a finite structure, whereas glass does not. In quartz, many covalent bonds must be broken for the network to melt. This is exemplified by the fact that it must be heated to 1500°C to melt. When quartz is melted and in the process of cooling, the atoms form in irregular positions and are incapable of reaching a stable finite arrangement and give glass properties of a supercooled liquid.