Among the intermolecular forces, the induced dipole-induced dipole forces are the only ones not studied by the Dutch physicist Johannes Diederik Van der Waals (1837-1923). They were elucidated by the German physicist Fritz Wolfgang London (1900-1954), which is why these forces are also called london forces or London dispersion forces. Another name given to these forces is instantaneous dipole-induced dipole.
This type of force occurs in nonpolar substances, such as H2, O2, F2, Cl2, CO2, CH4 and C2H6, among others. And they can also occur between noble gas atoms, when they approach, causing repulsion between their electrospheres. Thus, the electrons accumulate on one side, which is negatively polarized and the opposite side positively, due to the deficiency of negative charge.
Nonpolar molecules can pass from the gaseous state – in which they are very far apart and there is no interaction, as there are no poles – to the liquid and solid state. In these states of aggregation, the molecules are closer and the electronic attractions or repulsions between their electrons and nuclei can lead to a deformation of their electronic clouds, momentarily, originating temporary positive and negative poles.
Instantaneous dipoles can induce polarization of neighboring molecules, resulting in attractive forces.
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This induction can also occur between different molecules and usually these forces are weaker in intensity than dipole-dipole and hydrogen bonding forces. Therefore, solids with this interaction strength, such as dry ice (carbon dioxide – CO2) and iodine (I2), which are in the solid state, sublimate (pass to the gaseous state); because the energy required to break their interactions is small.
An example of intermolecular forces between polar and nonpolar molecules occurs between oxygen gas (nonpolar) and water (polar). It turns out that the negative end of the water approaches the O2, repelling each other and, thus, the electronic cloud of the non-polar molecule moves away. The oxygen is then momentarily polarized and begins to interact with the water, becoming solubilized in it.
Since these forces are weak, the solubility of this gas in water is small. Even so, its presence is essential to preserve the life of various aquatic organisms.
This interaction force also occurs in nature, giving the adhesion between the legs of the geckos and the surface where they walk. That’s why they can walk on walls and ceilings without falling or sticking.
By Jennifer Fogaça
Graduated in Chemistry
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