Molecules and networks

• 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.

• 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.

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.