One form of interaction between molecules are London Dispersion forces, or induced dipole forces. These exist between any two molecules, and should not be confused with the permanent dipole forces between polar molecules

To understand London dispersion forces, think of the electrons in a molecule as a constantly changing cloud. In a non-polar molecule, on average the electrons are distributed equally over the molecule, but occasionally one side or the other will gain a small excess of electron density. When this occurs, the molecule has a temporary dipole- one side of the molecule has a slight + charge, the other a slight - charge.

When another molecule approaches the first, it can feel this dipole. The electrons around the second molecule can then rearrange so that there is a favorable interaction between the two molecules.

All molecules have dispersion forces. In general, they are quite weak and their effects do not show up in molecules that are held together by stronger forces such as network covalent bonds or large permanent dipoles. Since the strength of dispersion forces depends on the electron cloud around the molecule being able to move, London forces are increased by:

1. How easy it is to form a dipole: larger molecule or atom size helps.
2. The number of electrons in the atom or molecule

Since melting and boiling points are directly affected by the strength of intermolecular bonds, we can see how the strength of the London forces varies as the molecule becomes larger

Example: Which of the following has a higher melting point: Cl₂, Br₂, or I₂?

Solution: Each of the above molecules is a diatomic halogen, and thus similar in how the molecule behaves. Each sample of the bulk is held together by London dispersion forces. Since iodine is larger than bromine which is larger than chlorine, we expect that the London forces are largest in iodine and smallest in chlorine, so iodine should have the highest melting and boiling points. This is borne out by observation: at room temperature, chlorine is a gas, bromine a liquid and iodine a solid.

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