Chemistry Everyday for Everyone Applications and Analogies
London Dispersion Forces and “The Wave” C. Jayne Wilcox Department of Chemistry, William Rainey Harper College, Palatine, IL 60067
One of a variety of intermolecular forces of attraction, London dispersion forces arise from the motion of electrons within atoms and molecules (1, 2). As electrons shift from one region of a molecule to another, the resulting electron imbalance causes electrons in a neighboring molecule to move away from the higher electronegative region of the first molecule. Temporary dipoles are formed and the result is instantaneous, short-lived electrostatic attractions between molecules. When introducing this topic to my students, I ask them if they’ve ever done “the wave” at a sports event. Sometimes, depending on the group, I ask them to start a wave going around the room. I then suggest that there is a loose analogy between London dispersion forces and a crowd of people doing “the wave” at, say, a baseball game in a large stadium.
The movement of one person influences and changes the behavior of another, and another, and so on. Long after the original person has stood up, raised his arms, and sat down, the result of this person’s motions can be seen influencing others in the crowd as they go through the same motions. So it is with electrons. Just as London forces increase in strength with increasing numbers of electrons (increasing formula masses of atoms or molecules), “the wave” becomes more impressive as the number of people doing it increases. Literature Cited 1. Earles, T. T. J. Chem. Educ. 1995, 72, 727. 2. Pravia, K.; Maynard, D. F. J. Chem. Educ. 1996, 73, 497.
JChemEd.chem.wisc.edu • Vol. 75 No. 10 October 1998 • Journal of Chemical Education
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