Van+der+Waals

Van der Waals' Forces.

Van der waal's force - The van der Waals, named after Dutch scientist Johannes Diderik van der Waals, is the sum of the attractive or repulsive forces between molecules of the same substance.

Van der Waals' Forces are also known as temporary dipoles. These dipoles are called temporary dipoles because the polar bonding is impermanent; for example, a Fluorine molecule can have temporary dipoles whereas, for example, Hydrogen Fluoride is permanently polar. Molecules with more electrons (for example Iodine compared to Fluorine) have a larger chance of having temporary dipoles as they have more electrons to move to one side, making the electron density unequal due to the difference in electron distribution between the two atoms. Van der Waal's forces need relatively little energy required to break the forces and separate the molecules from each other. This is why Iodine (which has a larger molar mass and electron count than Fluorine) is a solid at room temperature. To generalize this point, as Mr of molecules or compounds attracted together by Van der Waal's forces increases (which suggests more interaction), boiling point increases.

Substances with Van der Waals' forces generally have low melting and boiling points, as not a lot of energy is needed to break the forces and separate the molecules from each other. The boiling points increase with increasing molecular mass.