Polarisation

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  • Electronegativity
    • Polarisation
      • Polar & Non-polar bonds
        • covalent bonds in diatomic gases are non-polar because the atoms have equal electronegativities & so the electrons are equally attracted to both nuclei
        • some elements such as carbon & hydrogen have similar electronegativities so bonds between them are essentially non-polar
        • in a covalent bond between 2 atoms of different electronegativities, the bonding electrons are pulled towards the more electronegative atom
          • the greater the difference in electronegativity the more polar the bond
        • in a polar bond, the difference in electronegativity between 2 atoms is called a dipole
          • a dipole is a difference in charge between 2 atoms caused by a shift in electron density in the bond
      • Polar molecules
        • if charge is distributed unevenly over a whole molecule then the molecule will have a permanent dipole
          • molecules that have a permanent dipole are called polar molecules
        • whether or not a molecule is polar depends on whether it has any polar bonds & its overall shape
        • in simple molecules, such as hydrogen chloride, the 1 polar bond means charge is distributed unevenly across the whole molecule so it has a permanent dipole
        • more complicated molecules might have several polar bonds
          • shape of molecule decides whether or not it has an overall permanent dipole
            • if the polar bonds are arranged symmetrically so that the dipoles cancel eachother out, such as in carbon dioxide, then the molecule has no permanent dipole & is non-polar
            • if polar bonds are arranged so that they all point in roughly same direction & don't cancel eachother out, then charge will be arranged unevenly across the whole molecule
              • this results in a polar molecule - the molecule has a permanent dipole
    • ability to attract bonding electrons in covalent bond
    • measured on the Pauling scale
      • higher number means element is better able to attract the bonding electrons
    • fluorine is the most electronegative element.
    • oxygen, nitrogen & chlorine are also very strongly electronegative

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