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 I2 is a solid whereas Cl2 is a gas. 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.
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.
These are also called transient, induced dipole-dipole interactions. They 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 dipoles 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 factor 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
Van der Waals forces occur between all molecular substances and noble gases. They do not occur in ionic substances.
18.104.22.168 Forces between molecules
Forces between molecules:
• induced dipole–dipole (van der Waals, dispersion, London) forces
The melting and boiling points of molecular substances are influenced by the strength of these intermolecular forces.
Students should be able to:
• explain the existence of these forces between familiar and unfamiliar molecules
• explain how melting and boiling points are influenced by these intermolecular forces.