Types of van der Waal (intermolecular) Forces
Dipole-dipole forces are weak, van der Waals forces that occur between polar molecules.
The region of a molecule that is slightly negatively charged is attracted to the region of a second molecule with a slightly positive charge. Note that the below example is showing a hydrogen bond, which is an extreme form of a dipole-dipole force.
These forces are very weak, since the charges involved are small. The more polar a molecule is (i.e. the greater the difference in electronegativity), the stronger the dipole-dipole forces are.
These forces arise when non-polar molecules become temporary dipoles. Since electrons are constantly in motion, there are times when the electron distribution of an atom is asymmetrical, so that it has a small charge.
If such an atom or molecule becomes close to neutral molecule, it will induce the other molecule to form a temporary dipole too, as its electrons are repulsed by the first molecule.
The force between these two temporary dipoles is called dispersion forces.
Dispersion forces are the only intermolecular forces for noble gasses and non-polar molecules.
The strength of the dispersion force depends on two factors:
- Size of the atom – the number of electrons increase as well as the atomic radius. This means there are more electrons and a greater space in which to move, increasing the chance that there is an imbalance of charge, as well as making it more likely that the charge is greater.
- Shape of the molecule – long chain molecules have a greater surface area meaning that it is easier to develop an instantaneous dipole, as well as the fact that they can fit together more easily. This increases the dispersion force. For example, butane has stronger dispersion forces than its isomer methylpropane.
Combination of the two forces
Polar molecules usually have a higher boiling and melting point than non-polar molecules because they have both forces working for them. However, this is only true for molecules with a similar number of electrons.
If compared with a molecule with significantly more electrons, the dispersion forces are usually larger than the dipole-dipole.
Molecules that contain NH, OH or FH groups have a significantly higher dipole-dipole force than normal. This is because N, O and F are the most electronegative atoms. In addition, the atomic radii of these atoms are very small, so the dipole-dipole forces are concentrated, making them stronger.
The requirements for hydrogen bonding:
- The presence of a molecule containing at least one NH, OH or FH covalent bond
- The presence of another polar molecule containing at least one N, O or F atom with a lone pair of electrons (so that an attraction can occur.)
- Note that the lone pair of electrons does not necessarily have to be attached to an N, O or F atom attached to hydrogen.
Thus, a hydrogen bond is a strong intermolecular force between one lone pair of electrons on the N, O or F atom of one polar molecule, and the partially positively charged hydrogen atom bonded to a N, O or F atom on a second polar molecule.