Dispersion forces

 
Instantaneous dipoles come about due to the random movement of electrons through an atom or a molecule. Such movement distorts the the negative charge density within the molecule and creates spontaneous and short lived dipoles, as shown on the right.

The creation of instantaneous dipoles generates weak electrostatic forces of attraction between the molecules. These intermolecular forces are known as dispersion forces and are present in all molecules where electrons move at random. Dispersion forces are very weak because the charges that generate these forces are not long lasting and disappear as quickly as they are formed. Large molecules tend to have a greater number of instantaneous dipoles forming and therefore the dispersion forces acting between the molecules, weak as they may be, are slightly stronger than the forces between smaller molecules. Below is a table of the boiling points of molecules that rely only on dispersion forces to attract molecules together. Boiling points are an indicator of how strong the intermolecular forces are.

Molecule
Boiling point
oxygen
-183
nitrogen
-196
chlorine
-35
iodine
183
Creation of instantaneous dipoles which generates elecrostatic force between the molecules. These electrostatic forces are known as dispersion forces.

 
The animation above shows how instantaneous dipoles are created and generate dispersion forces. Instantaneous dipoles forming in one molecule will generate dipoles in neighbouring molecules, this is known as induced polarity. The result of the development of so many temporary dipoles is a brief, weak force of attraction (dispersion forces)
The table above shows the relationship between size of a molecule and the relative strength of the dispersion forces. It is clear that the more electrons in the molecule the greater the dispersion forces acting between the molecules. Iodine with 106 electrons in each molecule, clearly creates more instantaneous dipoles than the other molecules and therefore has the highest melting point.
Negative chlorine ions are attracted to the anode and form chlorine gas. Positive sodium ions are attracted to the cathode and form sodium metal. Power source Water molecules orient themselves to expose their negative end to the positive ion.Water molecules orient themselves to expose their negative end to the positive ion. Water molecules orient themselves to expose their negative end to the positive ion.Water molecules orient themselves to expose their negative end to the positive ion. Water molecules orient themselves to expose their negative end to the positive ion. Water molecules orient themselves to expose their negative end to the positive ion.Water molecules orient themselves to expose their negative end to the positive ion. The water molecules orient themselves to expose their positive end to the negative ion. The water molecules orient themselves to expose their positive end to the negative ion. The water molecules orient themselves to expose their positive end to the negative ion. The fluorine atom attracts the electrons towards it unevenly and results in this end of the molecule having a slightly negative charge. The fluorine atom attracts the electrons towards it unevenly and results in this end of the hydrogen atom having a slightly positive charge. Hydrogen molecule Hydrogen molecule Hydrogen molecule