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The London forces also become stronger with larger amounts of surface contact. Fluorine and chlorine are gases at room temperature, bromine is a liquid, and iodine is a solid. This trend is exemplified by the halogens (from smallest to largest: F 2, Cl 2, Br 2, I 2). This is due to the increased polarizability of molecules with larger, more dispersed electron clouds. London forces become stronger as the atom in question becomes larger, and to a smaller degree for large molecules. London forces are present between all chemical groups, and usually represent the main part of the total interaction force in condensed matter, even though they are generally weaker than ionic bonds and hydrogen bonds.
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This is frequently described as the formation of instantaneous dipoles that attract each other. Because the electrons in adjacent molecules "flee" as they repel each other, electron density in a molecule becomes redistributed in proximity to another molecule (see quantum mechanical theory of dispersion forces). London forces are exhibited by nonpolar molecules because of the correlated movements of the electrons in interacting molecules.
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They can therefore act between molecules without permanent multipole moments. The LDF is a weak intermolecular force arising from quantum-induced instantaneous polarization multipoles in molecules. The LDF is named after the German-American physicist Fritz London. They are part of the van der Waals forces. London dispersion forces (LDF, also known as dispersion forces, London forces, instantaneous dipole–induced dipole forces, or loosely van der Waals forces) are a type of force acting between atoms and molecules. The long-range part is due to London dispersion forces