5. Polarisation

• Electronegativity is ability to attract bonding electrons in covalent bond
• Electronegativity is measured on Pauling Scale. Higher number means element is better able to attract the bonding electrons. Fluorine is most electronegative element. Oxygen, nitrogen and chlorine are also strongly electronegative
• Covalent bonds in diatomic gases (H2, Cl2) are non-polar because atoms have equal electronegativites, so bonds between them are essentially non-polar
• In covalent bond between 2 atoms of different electronegatives, bonding electrons are pulled towards more electronegative atom. Makes bond polar. Greater difference in electronegativity, more polar bond is
• In polar bond, difference in electronegativity between 2 atoms causes dipole. Dipole is difference in charge between 2 atoms caused by a shift in electron density in bond
• If charge is distributed unevenly over whole molecule, then molecule will have permanent dipole. Molecules that have permanent dipole are called polar molecules. Whether or not molecule is polar depends on whether it has any polar bonds, and its overall shape.
• In simple molecules, such as hydrogen chloride, one polar bond means charge is distributed unevenly across whole molecule, so it has permanent dipole
• More complicated molecules might have several polar bonds. Shape of molecule will decide whether or not it has overall permanent dipole. If polar bonds are arranged symmetrically so dipoles cancel out, such as CO2, then molecule has no permanent dipole and is non-polar
• If polar bonds are arranged so that they all point in roughly same direction, and they don’t cancel out, then charge will be arranged unevenly across whole molecule. Results in polar molecule - molecule has permanent dipole