Van der Waals force
In chemistry, van der Waals' forces is a type of intermolecular force. An intermolecular force is a relatively weak force that holds molecules together, and van der Waals' forces are the weakest type of intermolecular force. They are named after the Dutch scientist Johannes Diderik van der Waals (1837-1923).
Van der Waals' forces occur between molecules because negatively charged electrons, which orbit the molecule, move to create slightly different charges from one end of the molecule to the other, these slight differences are called partial charges and are noted as δ- or δ+.
Each electron exists in an orbital around the atom/molecule, and has a probability of being at any point inside that orbital. Electrons may be unevenly distributed for a short period of time - appear to gather in one area for an instant, due to their random movement, as in the diagram. A slightly negative charge, δ-, will appear in the area where there are more electrons, as electrons are negatively charged. It follows that in the area where there are less electrons, there is a slightly positive charge, δ+. We know that opposite electrical charges attract, so a molecule which is positively charged at one end, will attract electrons in a neighbouring molecule. As as result, forces of attraction form between molecules, due to opposite electrical charges.
Van der Waals forces can be classified as:
- Forces that act between two dipoles, named Keesom after Willem Hendrik Keesom
- Forces that act between a dipole, and a charged molecule, named Debye after Peter Debye
- Forces that act between two polarised particles, called London Dispersion forces after Fritz London