Chemistry AS ES CI 5.3 and 3.1

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  • ES Bonds between molecules: temporary and permanent dipoles CI 5.3 and 3.1
    • Polar Molecules
      • A dipole occurs when a molecule (or part of a molecule) has a positive end and a negative end. When a molecule has a dipole, we say it is polarised. Molecules with a permanent dipole are polar molecules.
    • Instantaneous (temporary) dipoles
      • The electron  density in a molecule may be unevenly distributed  at any one time (if no permanent dipole)
        • The electron density distribution changes and so the polarity will change
      • If other molecules are close to a molecule with a dipole these may cause an effect and produce an induced dipole.
    • Intermolecular bonds
      • Molecular substances which contain dipoles attract each other. Two of these  kinds of attraction are instantaneous dipole-induced dipole bonds and permanent dipole-permanent dipole bonds.
      • Instantaneous dipole - Induced dipole
        • very weak
        • can happen even if permanent dipole already
      • Permanent dipole - permanent dipole bonds
        • stronger than ID-ID but weaker than hydrogen bonds
    • Electronegativity
      • The degree to which an atom of an element attracts electrons is called its electronegativity. The more electronegative an element an element is than the greater its attraction for electrons
        • The order of electronegativity values for some common elements is
          • F > O > Cl > Br
          • N > I > S > C > H
            • Difference between C and H is so small that molecules are said to be non polar
    • Bond polarity and polar molecules
      • A polar molecule is one that has a permanent dipole
      • Some molecules with a polar bond may not be a dipole if the bonds have a symmetrical arrangement


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