Modes of Bonding

• Explain how ionic bonds form


  1. When a metal and a nonmetal react, the electrons in the outer shell of the metal atom are transferred to the nonmetal

  2. The electron transfer can be represented using a dot and cross diagram
• Ionic compounds


  A giant structure of ions held together by electrostatic forces of attraction between oppositely charged ions
• Ionic compounds have high melting points


  The electrostatic forces between ions within the lattice are strong
• Ionic compounds conduct electricity

  • Only when molten or dissolved, as ions are able to move around and therefore conduct electricity
  • When solid, ionic compounds are not able to as ions are fixed in place
• Bonds in polymers


  Covalent bonds
• Explain how a covalent bond forms

  Covalent bonds form when 2 atoms share a pair of electrons in order to gain full outer shells, e.g. H2, O2 or polymers
• Bonds in diamond


  Covalent bonds
• Properties of simple covalent molecules

  • Usually gases or liquids
  • Low boiling and melting points
  • Do not conduct electricity as they do not have an overall electric charge
• Forces broken during boiling of simple covalent molecules

  Weak intermolecular forces are overcome during boiling, but the covalent bonds remain intact
• Larger molecules have higher melting and boiling points


  Intermolecular forces increase with the size of molecules and therefore require more energy to overcome during melting/boiling
• Properties of giant covalent structures

  • Solids with very high melting points
  • The atoms are linked to other atoms by strong covalent bonds
• Properties of polymers


  • Very large molecules, linked to atoms by strong covalent bonds
  • The intermolecular forces between polymer molecules are relatively strong, so substances are solids at room temperature
• Metallic bonding


  • The electrons in the outer shell metal atoms are delocalised and so are free to move through the whole structure. This is referred to as a 'sea' of electrons
  • The sharing of delocalised electrons gives rise to strong metallic bonds
• Properties of metals


  • High melting and boiling points
  • Conduct heat and electricity as delocalised electrons in their structure - move throughout the metal in the 'sea'
  • Able to be bent and shaped as the layers of atoms are able to slide over each other
• Limitations of dot and cross diagrams

  • Shows how atoms are bonded and the electrons
  • Does not show the 3D arrangement of molecules
  • Does not include intermolecular forces (the ones broken when boiling/melting simple molecules)
• Limitations of ball and stick diagrams

  • Shows how atoms are bonded and the 3D shape
  • Does not show electrons or chemical symbols
  • Does not include intermolecular forces (the ones broken when boiling/melting simple molecules)
• Limitations of 3D diagrams

  • 3D arrangement shown
  • Does not show bonding or electrons
  • Does not include intermolecular forces (the ones broken when boiling/melting simple molecules)
• Explain the formation of diamond

  1. Each carbon is joined to 4 other carbons joined by covalent bonds

  2. This is the maximum number of bonds each carbon atom can make

  3. Extremely hard, very high melting points, does not conduct electricity
• Explain the formation of graphite

  1. Each carbon is covalently bonded to 3 other carbons

  2. Layers of hexagonal rings are formed, with no covalent bonds between layers and weak intermolecular forces

  3. The layers can slide over each other and so graphite is soft and slippery

  4. One electron from each carbon atom is delocalised, making graphite similar to metals and able to conduct electricity
• Fullerenes


  • Fullerenes contain different numbers of carbon atoms
  • Molecules form hollow shapes, based on hexagonal rings of carbon atoms
  • Cylindrical fullerenes with very high length:diameter used as carbon nanotubes
• Graphene


  • Single layer of graphite
  • Properties make it useful in electronics and composites
• Particle size and surface area:volume

  Larger particles have a smaller surface area to volume ratio and so become less reactive
• Nanoparticles


  • 1-100 nanometers across
  • Include fullerenes
  • Different properties to the 'bulk' chemical its made from due to their high surface area:volume
  • Smaller quantities are needed to be effective
• Uses of nanoparticles and their relevant properties

  • Catalysts - high surface area to volume ratio
  • Stronger, lighter building materials
  • High selective sensors
  • New cosmetics - titanium dioxide nanoparticles so small that they do not reflect visible light so no white marks
  • Lubricant coatings for artificial joints and gears - reduce friction
• Risks of using nanoparticles

  • So small that they could potentially enter the bloodstream
  • Many feel that the risks are not yet known so more testing is required