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London dispersion forces are a particular type of weak van der Waals intermolecular forces . In fact, they represent the weakest intermolecular interactions of all. They are the kind of short-range attractive forces that arise between any pair of molecules or atoms when they are very close to each other. These types of interactions are formed by the presence of instantaneous dipoles on the surface of molecules that attract other instantaneous dipoles on neighboring molecules.
Being such weak forces, they are difficult to measure or observe in ionic compounds and in polar molecules, since these present other types of stronger interactions that mask them. This is why the London forces are only manifested in a measurable way in nonpolar molecules and in monatomic species such as noble gases.
In fact, London dispersion forces are the only type of intermolecular (or interatomic) interactions exhibited by noble gases and apolar molecules, since these do not exhibit stronger types of interactions such as hydrogen bonds (formerly bridges). hydrogen), dipole-dipole or induced dipole-dipole interactions.
Finally, it could be said that the London forces are responsible for the fact that noble gas atoms and nonpolar molecules can condense to form liquids or solidify, even at very low temperatures.
How do London forces work?
Like all other forms of intermolecular interactions, London dispersion forces are also forces of electrostatic attraction.
However, it is worth asking the question: how is it possible that there are forces of electrostatic attraction between neutral and apolar atoms or molecules?
The answer to this question has to do with the fact that electrons are in constant motion around the nucleus and along chemical bonds. Despite the fact that they move very quickly and are, on average, evenly distributed, it can happen that, during a short period of time, there are more electrons on one side of the nucleus or on one side of the bond than on the other. As a consequence, an electric dipole is formed, since one part of the atom (or molecule) will have an excess of positive charges, while the other will have an excess of negative charges.
These dipoles are called instantaneous dipoles since they last a very short time, but they can form anywhere in a molecule or a neutral atom . When two molecules are very close to each other, the spontaneous formation of a dipole in one of the molecules induces the formation of a second dipole in the other molecule, thus generating an attractive force between the two dipoles, which is precisely the London dispersion force.
The reason why the London forces are so weak is because the dipoles responsible for the attraction are very brief and appear and disappear constantly. However, multiple instantaneous dipoles can form at a given time, so while some dipoles disappear on one side, others can appear on the other side, holding the two molecules or two atoms together.
Determinants of London dispersion forces
Just as there are many factors that determine how strong hydrogen bonds, dipole-dipole interactions, and all the rest are, there are also factors that allow you to determine when the London forces are stronger or weaker:
The larger the atom, the greater the London dispersion forces.
The larger the atoms, the further their valence electrons are from the nucleus, so they are more loosely bound to it. This makes it easier to warp the electron clouds to generate induced dipoles. In other words, these atoms are more polarizable.
The more polarizable an atom is, the greater the induced dipoles that can be formed, so the greater the London forces between the two atoms. This is why, at room temperature, bromine is a liquid while chlorine and fluorine are gases, and iodine is a solid, despite the fact that all halogens form nonpolar diatomic molecules with the same shape.
contact surface
As a general rule, the greater the contact surface between two molecules, the greater the London dispersion forces between them.
The reason this happens is that the larger the contact surface between two molecules (or even any two surfaces), the more instantaneous dipoles will be forming at any one time. Although instantaneous dipoles are very weak, the formation of many instantaneous dipoles that add up at a given time creates a large net force of attraction between the two molecules.
This is the reason why the linear isomers of alkanes always have a higher boiling and melting point than their branched counterparts, since the less branched a compound is, the longer it will be and, therefore, the greater the contact surface. will have with another similar molecule.
References
Brown, T. (2021). Chemistry: The Central Science. (11th ed.). London, England: Pearson Education.
Chang, R., Manzo, Á. R., Lopez, PS, & Herranz, ZR (2020). Chemistry (10th ed.). New York City, NY: MCGRAW-HILL.
Rutherford, J. (2005). van der Waals Bonding and Inert Gases. Encyclopedia of Condensed Matter Physics , 286–290. https://doi.org/10.1016/b0-12-369401-9/00407-1
