polarisation and intermolecular forces

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  • polarisation and intermolecular forces
    • electronegativity
      • electronegativity depends on...
        • atomic radius
        • nuclear charge
        • shielding
    • permanent dipole-dipole forces
      • polar bond
      • stronger than VDWs
      • weak electrostatic forces of attraction
    • dipole is the difference in charge between two atoms caused by a shift in e- density in the bond
    • covalent bond between two atoms of the same element are non-polar
    • atoms with similar electronegativities are essentially non-polar
    • not all molecules that have polar bonds are polar
      • if polar bonds are symmetrical then the charges cancel out
    • intermolecular forces are weaker than covalent, ionic and metallic bonds
      • they affect the physical properties of a compound
    • induced dipole-dipole (vander waals) forces
      • e- in charge clouds are always moving
        • dipoles are constantly being created and destroyed
      • weakest
      • straight molecules can lie closer together than beached ones and so have stronger VDWs
        • higher boiling point
          • larger molecules have larger e- clouds so stronger VDWs
      • larger molecules have larger e- clouds so stronger VDWs
      • evidence
        • halogens are non-polar and diatomic
          • as their Mr increases so does their bp/mp
    • hydrogen bonding
      • strongest
      • occur between H and either F, O or N
        • FON=very electrogenagtive
      • ice floats because as the water cools to form ice, the molecules make more H bonds and arrange themselves in a lattice
        • H2O molecules are further apart in ice and so ice is less dense


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