giant covalent structures

giant covalent structures

1. allotropes of carbon

   • diamond
     • structure
       • three dimensional lattice (network) of tetrahedral carbon atoms
       • All of the carbon atoms are bonded to four other carbon atoms in a tetrahedral arrangement
       • All of the valence electrons are used for bonding
     • properties
       • no valence electrons free for conduction of electricity
       • high melting and boiling points
       • the hardest naturally occurring substance due to its covalently bonded structure
       • insoluble in covalent solvents
   • Graphite
     • structure
       • giant two dimensional layers
       • held together in place by van der waals attractions
       • layers consist of hexagons of carbon atoms
         • each carbon bonded to three others
       • one free electron per carbon atom that can be delocalised
       • trigonal planar geometry
       • 120 degrees bond angle
     • properties
       • black slippery solid
         • as layers can slip over each other
       • high melting and boiling point
   • Graphene
     • innovations
       • newly discovered
       • trying to develop graphene sheets
       • transparent and flexible computers
     • structure
       • contains single layers of graphite
         • arranged hexagonally
         • planar sheet of carbon atoms
       • one atom thick
       • arranged in a honeycomb crystalline lattice.
       • covalently bonded to three other carbons
       • can form nanotube, fullerene
     • properties
       • good conducter of electricity
         • because of free to move delocalised electrons
       • nearly transparent
       • 200 times stronger than steel
   • C60 fullerenes
     • structure
       • graphite layer can be folded and bent into balls and tubes if the numebr of carbons is large enough
         • bucky balls and nano tubes
       • effectively two hemispherical geodhesic domes stuck together to make a sphere.
       • not neccesarily a network solid
       • composed of individual molecules with strong covalent bonds but with weak London forces between the molecules
       • like a small sheet of graphite curved into a ball
       • not a giant structure
     • discovery
       • developed from the discovery by Kroto of spherical molecules
         • 60 carbons
       • named after Richard Buckminster Fuller
     • properties
       • soluble in some polar substances
       • do not conduct electricity
         • because of symetrical nature

2. silicon dioxide (quartz)

   • structure
     • giant macromolecular
       • similar to diamond
     • all valence electrons used to link silicon atoms
     • tetrahedral
     • oxygen atoms interposed between each pair of silicon atoms
     • silicon bonded covalently to four oxygen atoms
     • oxygen bonded covalently to two silicon atoms.
     • bent as oxygen has two pairs of non bonding electrons
   • properties
     • hard solid
     • very high melting and boiling point
       • due to strong covalent bonds
     • non conductor
       • as all valence electrons are used in bonding
       • molten silicon dioxide can conduct electricity as electrons are free to move
     • soluble in water