Dipole-dipole forces

Intermolecular and intramolecular bonding

molecules with intermolecular and intramolecular bonding shown The bonding which holds giant structures and molecules together is likely to either ionic or covalent or metallic in the case of metals. This is type of bonding is very strong, it is called the intramolecular bonding. That is the bonding "within" the molecule/structure. However there is another type of bonding which acts between different covalent molecules and atoms. This type of bonding is called intermolecular bonding and it is very weak when compared to covalent or ionic bonding.
In gcse chemistry we simply learned about the existence of this bonding between different molecules and atoms and the affect it had on some of the physical properties of small molecules, properties such as melting and boiling points.

In A-level chemistry you will need to be more familar with the different types of intermolecular bonding and how each types acts and the affects it has on many of the properties of atoms and molecules.
The way in which different molecules interact with each other depends on the type of bonding present within the molecule, that is the intramolecular bonding and also on the atoms present in the molecule. Some molecule have a permanent dipole (charged ends) while other molecules are non-polar. Some molecule may have a hydrogen atoms bonded to nitrogen, oxygen or fluorine atoms, in this case a very strong type of intermolecular bonding called hydrogen bonding can be present between these molecules. Even non-polar molecules can interact with each other by the presence of temporary induced dipoles which are produced by the movement of electron within atoms or molecules or the movement of electrons as the molecules or atoms approach each other.

There are 3 types of intermolecular bonding that you need to be aware of. These are:

Permanent dipole-dipole interactions

Molecules with permanent dipoles are one where there are differences in the electronegative values of between 0.5 and 1.9 between the bonding atoms. This difference in electronegativity leads to polar molecules which have charged ends (dipoles). For example hydrogen chloride gas consists of small covalent molecules. Hydrogen has an electronegativity value of 2.2 and chlorine has a value of 3.0, this means that the difference in electronegativity is large enough to produce a permanent dipole in the hydrogen chloride molecule. The electrons in the H-Cl bond will spend most of their time closer to the chlorine atom; this means it will have a partial (δ-) negative charge and the hydrogen atom will have a partial positive charge (δ+). This is shown in the molecule opposite:

This means that when different hydrogen chloride molecules approach each other there will be an electrostatic attraction between the partially charged δ+ and δ- ends or poles of the molecule. This is shown in the diagram below:

Key Points

Practice questions

Check your understanding - Questions on dipole-dipole bonding