Network covalent bonding

Network covalent bonding

A network solid or covalent network solid is a chemical compound in which the atoms are bonded by covalent bonds in a continuous network. In a network solid there are no individual molecules and the entire crystal may be considered a macromolecule. Formulas for network solids as those for ionic compounds are simple ratios of the component atoms represented by a formula unit.

Examples of network solids include diamond with a continuous network of carbon atoms and silicon dioxide or quartz with a continuous three dimensional network of SiO2 units. Graphite and the mica group of silicate minerals structurally consist of continuous two dimensional layers covalently bonded within the layer with other bond types holding the layers together.

Properties

  • Electrical conductivity: Poor, as there are no free electrons. When molten, unlike ionic compounds, the substance is still unable to conduct electricity as the macromolecule consists of uncharged atoms, not ions as seen in previously mentioned ionic compounds. Contrary to this rule is graphite. Graphite is an allotrope of Carbon. In this allotrope, each atom of Carbon forms three covalent bonds, leaving one electron in each outer orbital de-localised, creating multiple "free electrons" within each plane of Carbon. This grants graphite electrical conductivity.
  • Melting point: High, due to the large amount of energy required to rearrange the covalent bonds.
  • Hardness: Hard, due to the strong covalent bond throughout the lattice (however, the layers of carbon atoms in graphite can be easily displaced, allowing the substance to be malleable).
  • Solubility: Generally insoluble in any solvent due to the difficulty of solvating a very large molecule.

Examples

  • Quartz (SiO2)
  • Diamond (Carbon)
  • Graphite (Carbon)

References

  • Zumdahl, Steven S., and Susan A. Zumdahl, Chemistry, Houghton Mifflin, 5th ed., 2000, pp. 470-476 ISBN 0-618-03591-5

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