Intermolecular bonds
Intermolecular bonds are found between molecules. They are also known as Van der Waals forces, and there are several types to consider.
London dispersion forces
London dispersion forces are the weakest type of intermolecular bond. They exist between all atoms and molecules.
Molecular elements (oxygen, nitrogen etc) and monatomic elements (the noble gases) will condense (move closer together) forming solids if cooled to sufficiently low temperatures.
This shows that there must be an attraction between the individual molecules (or atoms in the case of monatomic substances) that is being overcome.
London dispersion forces are caused by an uneven distribution of electrons within an atom. This results in a slightly negative (\(\delta-\)) and slightly positive \((\delta+)\) charge on either side of the atom. A temporary dipole has been established. This temporary dipole can induce a temporary dipole on a neighbouring atom/molecule. London dispersion forces are the electrostatic attractions set up between the slightly positive end of one atom/molecule and the slightly negative end of one atom/molecule.
The opposite charges then attract each other.
The strength of London dispersion forces depends on the size of the molecule or atom.
Larger atoms and molecules have more electrons. This leads to larger dipoles being established.
London dispersion forces increase the larger the atomic size.
Permanent dipole-permanent dipole interactions
Molecules with a permanent dipole are polar.
Polar molecules display attractions between the oppositely charged ends of the molecules.
This type of intermolecular bond is stronger than London dispersion forces with the same number of electrons.
Hydrogen bonding
Hydrogen bonding is the strongest type of intermolecular bond.
It is a specific type of permanent dipole to permanent dipole attraction that occurs when a hydrogen atom is covalently bonded to a highly electronegative element such as nitrogen, oxygen or fluorine.
As with permanent dipole to permanent dipole attractions, the oppositely charged ends of molecules attract.
In the diagram below, the hydrogen bonds are shown as the \(\delta+\) hydrogen atoms of one molecule are attracted to the \(\delta-\) oxygen atoms of another.
Water, ammonia, alcohols and alkanoic acids all contain hydrogen bonding.