Intermolecular Forces
Intermoleculr bonding is a form of bonding that exists between molecules. The strength of the intermolecular bonds determines how easily the molecules will separate and hence the melting and boiling points. There are different strengths of intermolecular bond, and we shall look at them in order. Starting with the weakest:
Van der Waals
Also known as induced dipoles and even London Forces. This is a weak force caused by the attraction of temporary dipoles, the diagram below shows how they form.
Electrons are constantly moving about in a random pattern in their energy level. This means that different parts of the molecule carry a very slightly negative charge, known as δδ- (delta delta minus) since it is so small. This movement induces a dipole in neighbouring molecules.
Because electrons are always moving around very quickly, the charges switch around all the time. It is also important to note that the more electrons in a molecule / atom, the stronger these Van der Waals or London forces are. This is seen in the increasing boiling points of the noble gases as you go down the group.
Permanent Dipoles
In bonding you will have learned about polarisation and how a permanent dipole is produced. In this type of molecular bonding, the opposite delta charges are attracted. This type of bonding is stronger than the above. Below is an example of permanent dipole interactions in HCl
Hydrogen Bonding
Hydrogen bonding is much stronger than the previous two and is a special type of permanent dipole-dipole. It only happens in molecules where hydrogen bonds with the following: Oxygen (O), Nitrogen (N) and Fluorine (F). This is because these molecules have high electronegativities (4, 3.5 and 3).
As you can see in the above diagram. When these atoms bond to hydrogen, it leaves lone pairs of electrons that are not being used in bonding. This creates electrostatic forces between molecules.
Hydrogen bonds exist in our most abundant molecule on earth: water. This gives it all sorts of properties that are essential for life.