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Network solids are giant molecules formed with strong covalent bonds. Due to their bonding nature, they generally are brittle, have very high boiling points, and do not conduct heat or electricity well. The two important network solids are carbon and silicon.  

Carbon

Common forms:

Diamond

  • Hardest naturally occurring substance
  • tetrahedral arrangement to form a huge molecule
  • Stabilized by covalent bonds formed by overlap of sp3 hybridized carbon orbitals
  • Large gap between filled and empty energy levels
  • Poor transfer of electrons à poor electrical conductor
  • Diamonds used in industrial cutting implements

Graphite

  • Slippery, black, and a good conductor
  • Structure based on layers of carbon atoms arranged in six-membered rings
  • Each carbon on each layer is surrounded by three other carbon atoms in a trigonal planar arrangement
  • Often used as a lubricant in locks
  • Slipperiness comes from the strong bodning within the carbon atoms but little bonding between them

Silicon

Silicon is an important compound in the earth’s crust. Similar to carbon compounds, silicon is fundamental to most of the rocks, sands, and soils found on our planet. However, even though both carbon and silicon are in Group 4A on the periodic table, the carbon-based compounds in geology and the silicon compounds in geology have very different structures. Carbon contains long strings of carbon-carbon bounds whereas silicon compounds often have chains of silicon-oxygen bonds.

The fundamental silica-oxygen compound is silica, which has the empirical formula of SiO2. When it is heated above its melting point and then cooled rapidly, the armorphous solid, glass, is formed. Glass contains a good deal of disorder and more closely resembles a very viscous solution rather than a crystalline solid.

Compounds related to silica, found in rocks, soils, and clays, are silicates. Similar to silica, the silicates are based on interconnected SiO4 tetrahedra. However, the ratio of O:Si is greater than 2:1 like in silica causing it to form neutral solids. In other words, silicates are salts that are composed of metal cations and polyatomic silicon-oxygen anions. 

Ceramics are typically made from clays and are hardened by being exposed to high temperatures. Like glass, ceramics are based on silicates. However, unlike glass, once ceramics are hardened, it becomes resistant to high temperatures and is hard to remelt. This is because glass is a homogenous structure whereas ceramic is heterogeneous. Despite their resistance at high temperatures, ceramics are brittle, causing them to be used merely as household antiquities instead of high tech materials such as on the body of car or airplane engines.

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