Intermolecular forces

• Van der Waals forces occur between all molecular substances and noble gases. They do not occur in ionic substances
• Van der Waals are also called transient induced dipole-dipole interactions
• Van der Waals occur between all simple covalent molecules and the separate atoms in noble gases
• In any molecule the electrons are moving constantly and randomly. As this happens the electron density can fluctuate and parts of the molecule become more or less negative i.e small temporary or transient dipole form. These instantaneous dipoles can cause dipoles to form in neighbouring molecules. These are called induced dipoles. The induced dipole is always the opposite sign to the original one
• Main factors affecting size of van der Waals

  The more electrons there are in the molecule the higher the chance that temporary dipoles will form. This makes the van der waals stronger between the molecules and so boiling points will be greater
• The increasing boiling points of the halogens down the group 7 series can be explained by the increasing number of electrons in the bigger molecules causing an increase in the size of the van der Waals between the molecules. This is why l, is a solid whereas Cl, is a
• The increasing boiling points of the alkane homologous series can be explained by the increasing number of electrons in the bigger molecules causing an increase in the size of the van der Waals between molecules.
• The shape of the molecule can also have an effect on the size of the van der Waals forces. Long chain alkanes have a larger surface area of contact between molecules for van der waals to form than compared to spherical shaped branched alkanes and so have stronger van der waals.
• Permanent dipole-dipole forces occur between polar molecules
• Permanent dipole-dipole is stronger than van der waals and so the compounds have higher boiling points
• Polar molecules have a permanent dipole (commonly compounds with C-Cl, C-F, C-Br, H-Cl, C=O bonds)
• Polar molecules are asymmetrical and have a bond where there is a significant difference in electronegativity between the atoms
• Permanent dipole forces occurs in addition to van der waals

  
• Hydrogen bonding occurs in compounds that have a hydrogen atom attached to one of the three most electronegative atoms of nitrogen, oxygen and fluorine which must have an available line pair of electrons e.g -O-H, -N-H, F-H bond.
• There is a large electronegativity difference between the H and the O, N, F
• • Always show theblone pair of electrons on the O, F, N and the dipoles and all the delta positive and delta negative charges

  
• Hydrogen bonding occurs on addition to van der waals forces
• Hydrogen bonding Is stronger than the other two types of intermolecular bonding
• • The anomalously high boiling points of H2O, NH3 and HF arare caused by the hydrogen bonding between the molecules
  • The general increase in boiling point from H2S to H2Te is caused by increasing van der waals forces between molecules due to an increasing number of electrons
  • Alcohols, carboxylic acids, proteins, amides all can form hydrogen bonds