While intramolecular bonds occur within compounds to hold atoms together, it is also possible for bonds to exist between compounds. These intermolecular bonds do not result from the transfer or sharing of electrons. Rather, they are caused by the attraction between the positive and negative parts of separate compounds.
The force of attraction between hydrogen and an extremely electronegative atom, such as oxygen or nitrogen, is known as a hydrogen bond. For example, in water (H2O), oxygen atoms are attracted to the hydrogen atoms in nearby molecules, creating hydrogen bonds. These bonds are significantly weaker than the chemical bonds that involve sharing or transfer of electrons, and have only 5 to 10 percent of the strength of a covalent bond. Despite its relative weakness, hydrogen bonding is quite important in the natural world; it has major effects on the properties of water and ice and is important biologically with regard to proteins and nucleic acids as well as the DNA double helix structure.
Van der Waals forces are electrical interactions between two or more molecules or atoms. They are the weakest type of intermolecular attraction, but if substantial amounts of these forces are present, their net effect can be quite strong.
There are two major types of van der Waals forces. The London dispersion force is a temporary force that occurs when electrons in two adjacent atoms form spontaneous, temporary dipoles due to the positions the atoms are occupying. This is the weakest intermolecular force and it does not exert a force over long distances. Interestingly, London dispersion forces are the only forces that exist between noble gas atoms; without these forces, noble gases would not be able to liquefy.
The second type of van der Waals force is dipole-dipole interactions, which are the result of two dipolar molecules interacting with each other. This interaction occurs when the partial positive dipole in one molecule is attracted to the partial negative dipole in the other molecule.