Chemistry Unit 2.2 - How structure influences the properties and uses of substances

Chemistry Unit 2.2 - How structure influences the properties and uses of substances

1. Molecules

   • Most non-metal elements are molecular
     • Most consist of molecules with just 2 atoms joined together
       • E.g. Oxygen (O2), Nitrogen (N2)
   • Most compounds between non-metals are molecular
     • E.g. water (H2O), Carbon Dioxide (CO2)
   • Intermolecular Forces
     • All of these substances have very strong covalent bonds, but much weaker forces holding the molecules together
       • When one of these substances melts or boils, it is the weak 'intermolecular forces' that break, not the strong covalent bonds
   • Simple Molecular Substances do not conduct electricity
     • They do not have any free electrons and the molecules do not have electric charges.

2. Ionic Compounds

   • Have regular structures (GIANT IONIC LATTICES)
     • In these there are STRONG ELECTROSTATIC FORCES in ALL directions between oppositely charged ions
   • Have high melting points and high boiling points
     • Due to the large amounts of energy needed to break the strong bonds
   • When melted or dissolved in water they can conduct electricity
     • The ions are free to move and carry the current

3. Covalent Structures

   • Atoms that share electrons can also form giant structures
     • Diamond, Graphite and Silicon Dioxide are examples of giant covalent structures (lattices)
       • All of the atoms in these structures are linked to other atoms by strong covalent bonds
         • Therefore they have VERY high melting points
   • Diamond
     • Made of carbon atoms, which are joined to FOUR other atoms by strong covalent bonds
       • Therefore, it is VERY strong and hard
   • Graphite
     • Made of carbon atoms, where each atom is joined to three others, forming layers
       • The bonds are much weaker than covalent bonds
         • This enables the layers to slide across each other, making graphite soft
     • One electron is delocalised
       • This allows graphite to conduct heat and electricty
     • Used as pencil lead - As the pencil moves across the page, layers of graphite rub off
       • Also used as a lubricant and an electrode in electrolysis

4. Metals

   • Metals conduct heat and electricity
     • Due to the delocalised electrons in their structures
   • They layers of atoms in metals are able to slide over each other
     • Therefore, metals can be bent and shaped
   • Alloys are usually made from 2+ different metals
     • Different sized atoms of the metals distort the layers in the structure making it more difficult for them to slide over each other
       • This makes alloys harder than pure metals
     • Shape memory alloys can return to their original shape after being deformed
       • E.g. Nitinol used in dental braces

5. Polymers

   • Very large molecules made from many smaller molecules (monomers)
     • E.g Ethane -> Polyethene, Propene -> Polypropene
   • Properties of polymers depend on what they are made from and the conditions under which they are made
     • Low Density (LD) and High Density (HD) Polyethene are produced using different catalysts and reaction conditions
   • Thermosoftening polymers consist of individual, tangled polymer chains
     • Thermosetting polymers consist of polymer chains with cross-links between them so that they do not melt when heated

6. Nanoscience

   • Refers to structures that are 1-100nm (one billionth of a metre)
   • Nanoparticles have a VERY large surface area compared to their volume, so they are often able to react very quickly
     • This makes them useful catalysts
   • Used in sunscreens to block harmful ULTRAVIOLET light
   • Can lead to the development of...
     • New catalysts
     • New coatings
     • New computers
     • Stronger and lighter building materials
     • Sensors that detect individual substances in tiny amounts
     • Sunscreens and deodorants

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