Van der Waals Forces
Van der Waals forces are weak attractive forces between electrically neutral atoms or molecules. They are much weaker than the electrostatic forces which bind charged atoms or molecules (ions) of opposite sign or the covalent forces that bond neighboring atoms by sharing electrons. These forces develop because the rapid shifting of electrons within molecules causes some parts of the molecule to become momentarily charged, either positively or negatively. For this reason, weak, transient forces of attraction can develop between particles that are actually neutral. The magnitude of the forces is dependent on the distance between neighboring molecules. Van der Walls forces cause gas molecules to condense first to a liquid and finally to a solid as the gas is cooled.
The forces are named for the Dutch physicist Johannes Diederik van der Waals (1837-1930). The discovery of these forces evolved from van der Waals's research on the mathematical equations describing the gaseous and liquid states of matter. These equations are generally known as gas laws and relate the temperature, pressures and volume of gases. Originally derived for an idealized gas, these equations assumed that gas molecules had zero volume and that there were no attractive forces between them. In 1881, van der Waals proposed an empirical gas law which included two parameters to account for molecular size and attraction. This more accurate model was the first serious attempt to formulate gas laws for real gases, and it earned van der Waals the Nobel Prize for physics in 1910.
See also Gases, properties of.
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