2.2.7 Covalent Network Structures

1. Molecular structure 2. Covalent networks ( giant covalent )

• Both are joined by covalent bonds

1. Molecular substances

• Consists of separate groups of covalent bonded atoms called molecules

1. Covalent Networks ( Giant Covalent )

• Contain covalently bonded atoms in a contiuous three-dimesional lattice.

  • Silicon

  • Silicon Dioxide

  • And allotropes of carbon

Allotropes of Carbon

• Some elements have different structural forms called allotropes like Carbon

  • Diamond

  • Graphite

  • Graphene

  • Group of substances called fullerenes

• Despite all being made from carbon atoms, they have different chemical and physical properties due to their differing structural arrangemnets.

a- diamond b - graphite c - graphene d- nanotubes e- buckminsterfullerene (C_60)

1. Diamond

• Each carbon bonded to four others in tetrahedral arrangement

• Poor electrical conductor due to no mobile charged particles ( electrons are localized )

• Excellent Thermal Conductor

  • Vibrational energy carriers called phonons travel welll through the highly egular lattice and strong covalent bonds

1. Graphite

• Made of sheets of carbon atoms

• Each carbon bonded to three others in a hexagonal arrangement where the geoemtry around each carbon atom is trigonal planar

• Are bonded such that one eelctron per carbon atom is delocalized

  • Which are free to move up and down each sheet and $\therefore$a good electrical conductor

• The sheet itself is strong, but the attraction between the sheets are not, ( called London( dispersion ) Forces )

  • Easy to separate; thus graphite is a good lubricant, and ideal for pencils --> writing causes sheets to fall, leaving a mark

1. Graphene

• Single sheet of graphite

  • Thus 1 atom thick ( therefore said to be 2D)

• Good Electrical Conductor

• Flexible

• Lightweight

• transparent

• Very strong

• Applications huge

  • Bendable electronic displays & desalination technologies

1. Fullerenes

• Groups of carbon allotropes arranged in interlinking hexagonal and pentagonal rings

• Some form long hollow cylinders ( Carbon Tubes )

  • Used in nanotechnology ( use of objects with dimesions of $< 100$nm ( 1000 atoms or less across )

• Applications huge

  • Used to reinforce composite materials ( combination of two materials with different physical and chemical properties )

    • Strong due to the covalent bonds

  • electronics

    • Presence of delocalised electrons

      • Conductive

1. Buckminsterfullerenes ( buckyballs )

• Covalent Molecular structure

• $C_{60}$

• Atoms are arranged in hexagons and pentagons to resemble a football

• Applications huge

  • Medicine as drug carriers

  • Low boiling point ( why ? )

Silicon and Silicon Dioxide

• Silicon forms 3 dimesional lattice

  • Each atom --> 4 other atoms in tetahedral arrangement

    • SImialr to Diamond

• Note Si-Si bond is weaker than its C-C counterpart

• Despite belonging to same group they are different

  • Silicon is a metalloid, carbon is non-metal

    • Thus diamond is poor electrical conductor

    • Silicon is semiconductor

    • Double & triple bonds common in C, not in Si

• Note that Si-O is very strong

  • Silicon Dioxide / Silica ( $SiO_2$)

    • Quartz ( crystalline form )

    • sand

    • glass