London dispersion forces exist in non polar molecules. These are molecules where the electronegativity difference between the atoms is not greater than 0.5 on the pauling scale. London dispersion forces occur when one molecule exhibits a small and temporary dipole, a slight change in the layout of the electron cloud where for a short moment it is more on one atom than another. This means that one of the atoms in the molecules has a small negative charge and the other end has a small positive charge. This is known as a temporary dipole as it happens quickly and for a very small amount of time. This is important as it has an affect on adjacent molecules. When this dipole is induced the molecule next to it will be influenced and this will cause a dipole to be induced (created) in this molecule. For example when two molecules that are non polar come close to each other one will have a dipole causing a dipole to be created in the adjacent molecule, this creates a temporary and weak attraction known as a london dispersion forces. This force can then be broken and can be reformed with another molecule as they change and occur very quickly.
As molecules get larger or longer with more electrons they have larger electron clouds. This means that the dipoles that are induced and instantaneous are larger so the molecules which are larger in mass would need more energy to break them apart. This is evident in iodine and chlorine. Both are simple covalent molecules which have different masses. Iodine has a mass of 126 with 53 protons and 53 electrons, it is a purple grey solid which has a melting point of 113.7 degrees. This is compared to chlorine which has a mass of 35.5 which is much smaller with 17 protons and 17 electrons meaning it is much smaller than iodine and this is reflected in the fact it is a yellow gas with a melting point of -101.5 degree.
A non polar molecules is a molecule that has an electronegativity under 0.5 meaning the electrons are evenly shared between the two atoms
Electronegativity is the ability of an atom to attract electrons in its bond determined by the nuclear charge, number of protons and the number of shells.
The Pauling scale is a numerical measure of the tendency of an atom to attract electrons in a bonded pair
London dispersion forces is the term given to weak inter molecular forces between molecules
A temporary dipole occurs when electrons on a molecules are temporarily unevenly distributed on a molecule creating small areas of positive and negative charge