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Why do Van der Waal forces include all of the intermolecular forces? Why are they called intermolecular forces? What is their purpose?
Question Date: 2012-01-12
Answer 1:

The name intermolecular is because, if two molecules have dipoles, the result will be an attraction, where the high density of electrons of one molecule (partially negatively charged) is attracted to the low electron density area of a second molecule (partial positive charge) and vice versa.

Physically these forces arise from variations in the electron density around a molecule. Due to the different atoms, a molecule may have a permanent dipole where one end has a higher electron density than the other, or an induced dipole, which arises from random motion of the electrons in larger, longer molecules.

These intermolecular forces are incredibly important in chemistry as they determine many physical properties of a substance (boiling point, vapor pressure, freezing point, viscosity, etc.)

Answer 2:

Van der Waals (vdW) forces are the forces between molecules. Because these forces are between molecules, they are inter-molecular. (This is the same root found in "inter-national", which you know means between nations or countries.) Depending on who you ask, vdW forces are usually all the intermolecular forces that aren't ionic (between ions, such as the attractive force between positive sodium ions and negative chloride ions in table salt).

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