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Chemistry Revision (Ionic Bonding (The behaviour of ionic compunds (They…

- - - - At any particular moment, the electrons in an atom are likely to be more to one side than the other. At this moment, the atom would have a temporary dipole. This dipole can cause another temporary dipole in the opposite direction on a neighbouring atom. The two dipoles are then attracted to each other.
      - The second dipole can cause yet another dipole in a third atom because the electrons are constantly moving, the dipoles are being created and destroyed all the time. Even though the dipoles keep changing, the overall effect is for the atoms to be attracted to each other.
    - - The delta positive and delta negative charges on polar molecules cause weak electrostatic forces of attraction between molecules. These are called permanent dipole dipole interactions. Permanent dipole dipole interactions happen in addition to induced dipole dipole interactions.
    - - Hydrogen bonding can only happen when hydrogen is covalently bonded to FLUORINE, NITROGEN OR OXYGEN
      - Hydrogen has a high charge density because it's so small, and fluorine, nitrogen and oxygen are very electronegative. The bond is so polarised that a weak bond forms between the hydrogen of one molecule and a lone pair of electrons on the fluorine, nitrogen or oxygen in another molecule.
      - Molecules which have hydrogen bonding usually contain -OH or
        -NH groups
      - Hydrogen bonding has a huge affect on the properties of substances. They are soluble in water and have higher boiling and freezing points than molecules of a similar size that are unable to hydrogen bonds.
      - In ice, molecules of water are held together in lattice by hydrogen bonds. When ice melts, hydrogen bonds are broken, so ice has more hydrogen bonds than liquid water. Since hydrogen bonds are relatively long, this makes ice less dense that liquid water.
    - - Low melting and boiling points - the intermolecular forces that hold together the molecules in simple covalent compounds are weak so don't need much energy to break. So the melting and boiling points are normally low - they are often liquids or gases at room temperature. As intermolecular forces get stronger, melting and boiling points increase.
      - Polar molecules are soluble in water - water is a polar molecule, so only tends to dissolve other polar substances. Compounds with hydrogen bonds can form hydrogen bonds with water molecules, so will be soluble. Molecules that only have induced dipole dipole forces will be insoluble.
      - Don't conduct electricity - even though some covalent molecules have permanent dipoles, overall covalent molecules are uncharged.
    - - Covalent bonds don't break during melting and boling
      - To melt or boil a simple covalent compound you only have to overcome the intermolecular forces that hold the molecule together.