2.1 Chemistry- Bonding Structure and properties

• Compounds


  Substances in which 2 or more elements are chemically combined
• Types of strong chemical bonds

  • Ionic
  • Metallic
  • Covalent
• The type of bonding accounts for many of the physical and chemical properties of the compound / molecule
• Ionic bonding


  • Bonding occurs between metals and non-metals
  • Ionic bonds are electrostatic attractions between oppositely charged ions
  • The ions form a giant ionic lattice
  • Electrons in the outer shell of the metal are transferred
• Ionic bonding example


  • Sodium chloride
• Ionic compounds


  • Have high melting and boiling points
  • Don't conduct electricity when solid but do when molten/dissolved
  • Are brittle
• Metallic bonding


  • Metals consist of giant structures of positive metal ions
  • The electrons in the outer shell of metal atoms are delocalised and so are free to move through the whole structure
• Metallic compounds


  • The layers of ions can slide over each other, so metals can be bent and shaped making them malleable and ductile
  • The delocalised electrons can move through the metal and carry charge, so metals conduct electricity and heat
  • The metallic bonds are very strong and require large amounts of energy to be broken, so most metals have very high melting and boiling points
• Covalent bonding


  When atoms share pairs of electrons, they form covalent bonds
• Covalently bonded compounds


  • Small molecules (e.g. HCl, H2, O2, Cl2, NH3, CH4)
  • Very large molecules (polymers)
  • Giant covalent structures (macromolecules) (e.g. diamond, silicon dioxide)
• To show covalent bonding you can draw: dot and cross diagrams, repeat units for polymers, ball and stick and two- and three-dimensional diagrams
• Simple molecules


  • Substances that consist of simple molecules are usually gases or liquids that have low boiling and melting points
  • Substances that consist of simple molecules have weak intermolecular forces between the molecules
  • The intermolecular forces increase with the size of the molecules, so larger molecules have higher melting and boiling points
  • Substances that consist of simple molecules don't conduct electricity, because simple molecules do not have an overall electric charge
• Giant covalent structures


  • Substances that consist of giant covalent structures are solids with very high melting points
  • All of the atoms in these structures are linked to other atoms by strong covalent bonds
• Types of giant covalent structures

  • Diamond
  • Graphite
  • Graphene
  • Fullerenes
  • Carbon nanotubes
• Diamond


  • Each carbon is joined to 4 other carbons covalently
  • Very hard
  • Very high melting point
  • Does not conduct electricity
• Graphite


  • Each carbon is covalently bonded to 3 other carbons, forming layers of hexagonal rings which have no covalent bonds between the layers
  • The layers can slide over each other due to no covalent bonds between the layers, but weak intermolecular forces (meaning that graphite is soft and slippery)
  • One electron from each carbon atom is delocalised
  • Can conduct electricity
• Graphene


  • Single layer of graphite
  • Has properties that make it useful in electronics and composites
• Fullerenes


  • Molecules of carbon atoms with hollow shapes
  • Based on hexagonal rings of carbon atoms, but may also contain rings with five or seven carbon atoms
  • The first fullerene to be discovered was Buckminsterfullerene (C60), which has a spherical shape
• Carbon nanotubes


  • Cylindrical fullerenes with very high length to diameter ratios
  • Their properties make them useful for nanotechnology, electronics and materials
• Uses of fullerenes and nanotubes

  • Fullerenes can be used as lubricants, to deliver drugs in the body and catalysts
  • Nanotubes can be used for reinforcing materials, for example tennis rackets
• Nanoparticles


  • 1-100 nanometers across
  • Contain a few hundred atoms
  • As the side of cube decreases by a factor of 10 the surface area to volume ratio increases by a factor of 10
  • Include fullerenes
  • Have different properties to the 'bulk' chemical they're made from, because of their high surface area to volume ratio
• Uses of nanoparticles


  • Nano-silver kills bacteria, so used in wound dressings and deodorants; also used to line socks and fridges to kill bacteria that cause bad smells
  • Nano-titanium dioxide is used in sunblock creams to block harmful ultraviolet light without appearing white on the skin, and in self-cleaning windows to help break down dirt
• Risks of nanoparticles include potentially entering the bloodstream, long term effects not known, and potential to damage the environment
• Smart materials


  Responsive to certain stimuli, such as temperature and moisture
• Types of smart materials

  • Shape memory alloys and shape memory polymers (can be bent and deformed but return to original shape when heated)
  • Thermochromic materials (change colour when they reach a certain temperature)
  • Photochromic pigments (change colour when exposed to light)
  • Polymer gels (hydrogels absorb up to 1,000 times their volume in water, and certain stimuli can cause the water to be released)