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Description of Dispersion Forces

Dispersion forces are part of the van der Waals forces of attraction or repulsion between molecules. Also known as London dispersion forces, they occur during the temporary changes in electron cloud density around atoms and molecules, whether polar or nonpolar. The London dispersion force was named after the German American physicist, Fritz London, who first explained the attraction between noble gas atoms. This force is sometimes called an induced dipole-induced dipole attraction. It is the weakest intermolecular force because it is only a temporary attraction that results when the electrons in two adjacent atoms make the atoms form temporary dipoles. By contrast, intramolecular bonds are much stronger.

Polarizability
- Polarizability (the ease with which electron distribution around an atom or molecule can be distorted) leads to stronger dispersion forces between molecules. The electrons in a molecule form a constantly changing cloud. In a nonpolar molecule, the electrons are distributed equally, but sometimes one side or the other will gain an excess of electron density. When another molecule approaches it, it can feel this dipole. The electrons around the second molecule then rearrange themselves so there is a favorable interaction between the two. Although all molecules have dispersion forces, they usually remain very weak and their effects do not manifest if the molecules are held together by stronger forces, e.g., network covalent bonds or large, permanent dipoles. However, a temporary or transient dipole moment can induce a dipole moment in a nearby molecule, causing it to be attracted to the first molecule.
Molecular Shape
- The shape of molecules affects how strong dispersion forces will be between them. Molecules that form a liquid at room temperature show stronger dispersion forces than equivalent molecules of the same molecular weight that are nonpolar. For example, cylindrical n-pentane molecules (which comprise a liquid) come into contact with each other more effectively than more spherical neopentane molecules (which remain as a gas).
Molecular Size
- The strength of dispersion forces varies as the molecule becomes larger. Larger, heavier atoms and molecules exhibit stronger dispersion forces than smaller, lighter ones because larger atoms and molecules have valence electrons that are more distant from the nucleus. Their less-tight construction enables them to more readily form temporary dipoles.
Physical Effects
- Dispersion forces determine whether, and under what conditions, a substance becomes a liquid or solid. They cause nonpolar substances to condense to liquids and to freeze into solids when the temperature is lowered sufficiently. For example, boiling point indicates how strong the intermolecular forces are. Molecules containing large atoms (e.g., bromine, iodine) are highly polarizable and subject to strong dispersion forces. Thus, going down the halogen group in the periodic table, the melting and boiling points increase. The more electrons in the molecule, the greater the intermolecular dispersion forces. Iodine, which has 106 electrons per molecule, causes more temporary dipoles than other molecules and therefore has the highest melting point: 183 degrees Celsius.

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