Bonding and structure

• Particle model


  The three states of matter can be represented in the particle model
• This model assumes that there are no forces between the particles, that all particles in a substance are identical, and that the spheres are solid
• The amount of energy needed to change the state of a substance depends on the forces between the particles. The stronger the forces between the particles, the higher the melting or boiling point of the substance
• Covalent bonding


  Atoms share electrons to form strong chemical bonds
• Single bond


  Each atom shares one pair of electrons
• Double bond


  Each atom shares two pairs of electrons
• Giant covalent structures


  • High melting and boiling points because the strong covalent bonds between the atoms must be broken to melt or boil the substances. This requires a lot of energy.
  • Solid at room temperature
• Small molecules


  • Low melting and boiling points because only the intermolecular forces need to be overcome to melt or boil the substances, not the bonds between the atoms. This does not require a lot of energy as the intermolecular forces are weak.
  • Normally gaseous or liquid at room temperature
• Large molecules


  • Melting and boiling points are low compared to giant covalent substances but higher than for small molecules. Large molecules have stronger intermolecular forces than small molecules, which require more energy to overcome.
  • Normally solid at room temperature
• Most covalent structures do not conduct electricity because they do not have delocalised electrons or ions that are free to move to carry charge
• Graphite


  A giant covalent structure made only of carbon, where each carbon atom bonds to three others and forms hexagonal rings in layers. Each carbon atom has one spare electron, which is delocalised and therefore free to move around the structure
• Graphite


  • The layers can slide over each other because they are not covalently bonded, making graphite softer than diamond
  • The delocalised electrons are free to move through graphite, so can carry charges and allow an electrical current to flow, making graphite a conductor of electricity
• Graphene


  Consists of only a single layer of graphite. Its strong covalent bonds make it a strong material that can also conduct electricity. It is used in composites and high-tech electronics
• Fullerenes


  Hollow cages of carbon atoms bonded together in one molecule. Can be arranged as a sphere or a tube (called a nanotube). Molecules held together by weak intermolecular forces, so can slide over each other. Conduct electricity
• Buckminsterfullerene


  The first fullerene to be discovered, with 60 carbon atoms arranged in rings that form a hollow shape
• Nanotubes


  • The carbon atoms are arranged in cylindrical tubes. Their high tensile strength (they are difficult to break when pulled) makes them useful in electronics
• Key terms


  • boiling point
  • covalent bond
  • delocalised electrons
  • double bond
  • large molecules
  • melting point
  • nanotube
  • non-metal
• Ions are particles that have a different number of protons and electrons, giving them a charge
• Ionic bonding


  When metal atoms react with non-metal atoms, the metal atoms transfer electrons to the non-metal atoms, forming positive and negative ions that are attracted to each other by the strong electrostatic force of attraction
• Giant ionic lattice


  • The electrostatic force of attraction works in all directions, so many billions of ions can be bonded together in a 3D structure
• Metallic bonding


  Metal atoms form layers, with the electrons in the outer shells being delocalised and free to move through the whole structure. The positive metal ions are then attracted to these delocalised electrons by the electrostatic force of attraction
• Metals


  • Malleable (soft) because the layers can slide over each other
  • Good conductors of electricity and thermal energy because the delocalised electrons are free to move through the whole structure
  • Have high melting and boiling points because the electrostatic force of attraction between metal ions and delocalised electrons is strong, so lots of energy is needed to break it
• Alloys


  Mixtures of a pure metal with atoms of a different element, which makes the resulting material harder because the new atoms disturb the regular arrangement of the layers, preventing them from sliding over each other
• Key terms


  • conductor
  • delocalised electron
  • ion
  • lattice
  • layer
  • malleable
  • electrostatic force of attraction
  • transfer
  • conductivity