4 Periodic trend in structure and melting point

Macromolecular: diamond Tetrahedral arrangement of carbon atoms. 4 covalent bonds per atom Macromolecular: Graphite Planar arrangement of carbon atoms in layers. 3 covalent bonds per atom in each layer. 4th outer electron per atom is delocalised. Delocalised electrons between layers. Both these macromolecular structures have very high melting points because of strong covalent forces in the giant structure. It takes a lot of energy to break the many strong covalent bonds. Bonding Structure Examples Covalent : shared pair of electrons Macromolecular: giant molecular structures. Diamond Graphite Silicon dioxide Silicon Metallic: electrostatic force of attraction between the metal positive ions and the delocalised electrons Magnesium, Sodium Giant metallic (all metals) lattice. Only use the words molecules and intermolecular forces when talking about simple molecular substances Property Macromolecular Giant Metallic boiling and melting points high- because of many strong covalent bonds in macromolecular structure. Take a lot of energy to break the many strong bonds high- strong electrostatic forces between positive ions and sea of delocalised electrons Solubility in water insoluble insoluble conductivity when solid diamond and sand: poor, because electrons can’t move (localised) graphite: good as free delocalised electrons between layers good: delocalised electrons can move through structure conductivity when molten poor (good) general description solids shiny metal Malleable as the positive ions in the lattice are all identical. So the planes of ions can slide easily over one another -attractive forces in the lattice are the same whichever ions are adjacent. Melting and boiling points For Na, Mg, Al- Metallic bonding : strong bonding – gets stronger the more electrons there are in the outer shell that are released to the sea of electrons. A smaller positive centre also makes the bonding stronger. High energy is needed to break bonds. Si is Macromolecular: many strong covalent bonds between atoms high energy needed to break covalent bonds– very high mp +bp Cl2 (g), S8 (s), P4 (S)- simple Molecular : weak London forces between molecules, so little energy is needed to break them – low mp+ bp S8 has a higher mp than P4 because it has more electrons (S8 =128)(P4=60) so has stronger London forces Ar is monoatomic weak London forces between atoms Similar trend in period 2 Li,Be metallic bonding (high mp) B,C macromolecular (very high mp) N2 ,O2 molecular (gases! Low mp as small London forces) Ne monoatomic gas (very low mp)