Intermolecular forces are the forces of attraction between molecules with in a compound. These forces cause the compound to exist in a state of matter which is either a solid, liquid or a gas. The forces we are required to know are as follows Van Der Waals, dipole-dipole and Hydrogen bonds. Now lets look at each of these in more detail.

Van Der Waals

Van Der Waals are non polar and are only temporary as they occur between the similar elements, for example Cl2, I2 and CH3. This means the amount of proton pull from the nucleus is exactly the same; there is only a slight change from time to time because of electron movement this causes the Van Der Waals to occur, being temporary it is known as a temporary Dipole. There strength is equivalent to 1% the force of a covalent bond, although the bigger the compound the stronger the forces due to more distortion because of a bigger electron cloud.

Dipole-Dipole

These are polar molecules and have permanent dipoles. The negative end of one element is permanently attracted to the positive end of another element. They have 1/100 of the strength of a covalent bond. An example is: Hs+ ------- Cls-

Hydrogen Bonds

These can only occur between F (fluorine) N (Nitrogen) and O (Oxygen). They occur because of a large difference in electronegativity (see article). They have 1/10 the strength of a covalent bond and prove to be the strongest of the three forces.