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5.6: Molecules, London Dispersion Forces, and van der Waals interactions

We have briefly (admittedly absurdly briefly) defined what energy is and begun to consider how it can be transformed from one form to another. Now we need to consider what we mean by matter, which implies an understanding of the atomic organization of the molecules that compose matter. As you hopefully know by now, all matter is composed of atoms. The internal structure of atoms is the subject of quantum physics and we will not go into it any depth. Suffice to say that each atom consists of a tiny positively charged nucleus and cloud of negatively charged electrons154. Typically atoms and molecules, which after all are collections of atoms, interact with one another through a number of different types of interactions. The first are known as van der Waals interactions, which are mediated by London Dispersion Forces (LDF). These forces arise from the fact that the relatively light negatively-charged electrons are in continual movement, compared to the relatively massive and stationary positively-charged nuclei. Because charges on the protons and electrons are equal in magnitude the atom is electrically neutral, but because the electrons are moving, at any one moment, an observer outside of the atom or molecule will experience a small fluctuating electrical field.

As two molecules approach one another, their fluctuating electric fields begin to interact, this interaction generates an attractive LDF, named after its discoverer Fritz Wolfgang London (1900–1954). This force varies as ~1/R6 where R is the distance between the molecules; this relationship means that LDFs act only over very short distances, typically less than 1 nanometer (1 nm = 10-9m). As a frame of reference, a carbon atom has a radius of ~0.07 nm. The magnitude of this attractive force reaches its maximum when the two molecules are separated by what is known as the sum of their van der Waals radii (the van der Waals radius of a carbon atom is ~0.17 nm. If they move closer that this distance, the attractive LDF is quickly overwhelmed by a rapidly increasing, and extremely strong repulsive force that arises from the electrostatic interactions between the positively charged nuclei and the negatively charged electrons of the two molecules155. This repulsive interaction keeps atoms from fusing together and is one reason why molecule can form.

Each atom and molecule has its own characteristic van der Waals radius, although since most molecules are not spherical, it is better to refer to a molecule’s van der Waals surface. This surface is the closest distance that two molecules can approach one another before repulsion kicks in and drives them back away from one another. It is common to see molecules displayed in terms of their van der Waals surfaces. Every molecule generates LDFs when it approaches another, so van der Waals interactions are universal. The one exception involves pairs of small, similarly charged “ionic” molecules, that is molecules with permanent net positive or negative charge, approach each other. The strength of their electrostatic repulsion will be greater than their LDFs.

The strength of the van der Waals interactions between molecules is determined primarily by their shapes. The greater the surface complementarity, the stronger the interaction. Compare the interaction between two monoatomic Noble atoms, such as helium, neon or argon, and two molecules with more complex shapes. The two monoatomic particles interact via LDFs at a single point, so the strength of the interaction is minimal. On the other hand, the two more complex molecules interact over extended surfaces, so the LDFs between them is greater resulting a stronger van der Waals interaction.

References

154 Wonder why electrons do become localized to the positively charged nucleus; the answer is because of quantum principles: see http://chemwiki.ucdavis.edu/Textbook...the_Nucleus%3F

155 explored further at http://besocratic.colorado.edu/CLUE-...actions-0.html

Contributors

  • Michael W. Klymkowsky (University of Colorado Boulder) and Melanie M. Cooper (Michigan State University) with significant contributions by Emina Begovic & some editorial assistance of Rebecca Klymkowsky.