Learn how London dispersion forces are created and what effect they have on properties such as boiling and melting points.
Discover this weak intermolecular force and how it is one of the Van der Waals forces.
London Dispersion Force
You know that every atom and molecule has electrons and that these electrons are in constant motion. At any one instant in time, these electrons can be more towards one side of a molecule than another. When the electrons are concentrated more at one end of a molecule, that end becomes slightly negative.
The other end, where the electrons are not as concentrated, becomes slightly positive. At this instant, this molecule is a temporary dipole. This dipole can encourage a nearby molecule to also become dipole because the negative side of the first molecule will cause the electrons to run away on the other molecule (since negative (-) detests negative (-)).
|molecules. The London dispersion force is sometimes called a ‘Van der Waals force.’ Van der Waals force is a general term that describes any attractive intermolecular force between molecules and includes both the London dispersion force and the dipole-dipole force discussed elsewhere.
Weakness of Force
As you can imagine, this is an extremely weak force – much weaker than hydrogen bonding or any other intramolecular force. As I mentioned, this force works between all atoms and molecules. This is the only intermolecular force that works on noble gases and nonpolar molecules.
A London dispersion force works because of the movement of electrons. As you can imagine, the more electrons in the atoms, the stronger the force. Also, in a larger atom, the electrons are going to be filling the higher energy shells, which means they will be farther away from the nucleus and thus be less tightly held by the nucleus’ attraction. The electrons farthest away are able to move more freely, creating a dipole easier. This is why the London dispersion force increases with increasing atomic mass.
Effects on Properties
Intermolecular forces affect the properties of a substance. The stronger the bonds between – and among – molecules in a substance, the greater that substance sticks together. For instance, helium, He, is a noble gas.
The only intermolecular force it has between molecules is a London dispersion force. This force is very weak, so it doesn’t hold those molecules together very strongly. That is why helium has such a low boiling point of -452° F.Another example is the nonpolar molecule methane, CH4.
Because it is nonpolar, it only experiences London dispersion forces to keep it together in a liquid. Its boiling point is -263° F. Now, look at NH3, ammonia.
It has only one fewer hydrogen atom, but it is polar and experiences hydrogen bonding. Its boiling point is a much higher -28° F. Contrast this with water, H20, which is not only a polar compound, it also experiences hydrogen bonding and has some other special properties. Its boiling point is +212° F. The boiling point of hydrogen alone, without the attached oxygen, is over 600 degrees lower, at -423° F.