Molecules and networks

Chemical Ideas 5.2

OCR Salters B F332

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  • Each carbon atom is joined tetrahedrally to four other atoms by strong covalent bonds.
  • The very strong C-C bonds and highly symmetrical network structure make diamond the hardest naturally occurring substance.
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Silicon(IV) oxide

  • Silicon bonds covalently to four oxygen atoms.
  • Quartz is a pure form of silicon(IV) oxide. It is an extended network of SiO4 units. Each silicon atom has a half-share in four oxygen atoms.
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Differences between CO2 and SiO2

Carbon dioxide

  • Molecular structure with three atoms bonded in a linear arrangement, O=C=O.
  • Little energy is needed to pull the molecules apart, so solid carbon dioxide sublimes at a low temperature.
  • The intramolecular bonds in carbon dioxide are polar, so it dissolves in water easily.

Silicon dioxide

  • Silicon atoms are larger than carbon atoms and have more elctrons.
  • Silicon is unable to make double bonds, therefore discrete SiO2 molecules are not formed.
  • It requires a lot of energy to overcome the intramolecular bonds, so silicon(IV) oxide has a high melting point, high boiling point and does not dissolve in water.
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