Structure and Bonding

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  • Structure and Bonding
    • Ionic Substances
      • Mainly metal compounds and ammonium compounds
      • Form giant ionic lattice structures, with alternating positive and negative ions
      • Usually have high melting and boiling points because a lot of energy is needed to break the strong ionic bonds
      • Conduct electricity when in molten of aqueous solution because the ions are then mobile. However, in the solid state there are no mobile electrons and the ions are locked together so they are not mobile and the material does not conduct
    • Covalent Substances
      • Mainly non-metallic elements and compounds involving only non-metals
      • Can either have a;
        • Simple molecular structure
          • Each molecule is made up of a defined number of atoms, covalently bonded together. These molecules are then attracted by intermolecular forces
          • Have low melting and boiling points because only th eintermolecular forces have to be broken in theses processes and these are relatively weak.
          • Low electrical condictivity in the solid and molten state as there are no mobile ions or electrons
          • Usually low solubility in water unless either they react with water to form ions or form hydrogen bonds with water
        • Giant molecular structure
          • The covalent bonding extends throughout the whole crystal, usually in three dimensions
          • Tend to have high melting and boiling points because the covalent bonding extends throughout the whole crystal so covalent bonds have to be broken to melt the substance
          • Diamond
            • Three dimensional lattice of carbon atoms, each covalently bonded to and surrounded tetrahedrally by four others
            • Melting point very high because strong C-C covalent bonds have to be broken for diamond to melt
            • Very hard because of the giant lattice holding the atoms together and the strong covalent bonds
            • Non-conductor because it has no mobile electrons or ions
            • Uses of diamond and graphite
              • Diamond-tipped drill bits
                • Very hard and high melting point due to giant lattice
              • Graphite lubricants
                • Layers able to slide over each other
              • Graphite pencils
                • Layers able to slide over each other
              • Graphite electrodes in arc lamps
                • Mobile electrons allow graphite to conduct electricity. High melting point
              • Graphite re-entry shields for missile nose cones
                • High melting point due to gaint covalent structure
          • Graphite
            • Another ofrm of carbon and contains a giant molecular structure but is a layer, the covalent bonding extends in two dimensional which are held together by van der Waals forces
            • Each carbon is bonded to three othes in the sheet
            • Can be used as a lubricant as it is slippy due to relatively weak van der Waals forces holding the layers, so they can slide over each other
            • High melting point because strong covalent bonds have to be broken to melt the substance
            • One electron per carbon is delocalised, meaning it can move freely within a layer, hence graphite conducts electricity
            • Uses of diamond and graphite
              • Diamond-tipped drill bits
                • Very hard and high melting point due to giant lattice
              • Graphite lubricants
                • Layers able to slide over each other
              • Graphite pencils
                • Layers able to slide over each other
              • Graphite electrodes in arc lamps
                • Mobile electrons allow graphite to conduct electricity. High melting point
              • Graphite re-entry shields for missile nose cones
                • High melting point due to gaint covalent structure
    • Metallic Bonding
      • Metallic elements have giant metallic structures. The metal atoms form a lattice structure, held together by metallic bonding
      • 'Metallic bonding is the sttraction between a lattice of positive ions and a sea of delocalised electrons'
      • Metals are very good conductors of electricity. This is because the delocalised electrons are mobile
      • Metals generally have relatively high melting points. This is because a lot of energy is needed to break the strong metallic bonding
      • Generally the more delocalised elecrons, the stronger the metallic bonding

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