What Happens on a Molecular or Atomic Level when Two Objects Touch?

Fred Wudl, a professor of chemistry and materials at the University of California, Santa Barbara, replies:

"The answer to the first question is diffuse, pun intended. Quantum mechanics teaches that the farther you move away from the nucleus of an atom, the lower the probability of finding electrons that may belong to that atom, but that even far from the nucleus there is still a finite probability of finding the electrons. So, strictly speaking, electrons on the 'surface' of molecules are very diffuse but carry an electric charge.

"Because like charges repel, two molecules will resist being brought close together. The shorter the distance between them, the more energy it takes to push them to get even closer. If the molecules approach each other with sufficient energy, however, they may exchange atoms or actually form a bond (chemical reaction). Otherwise, their electron clouds will just deform to avoid the large electrostatic repulsion, and the two will then fly apart again."

So the answer really depends on what the surfaces are made of and how they interact. Atoms on the two surfaces may chemically bond to one another, and in that sense touch; in other circumstances, they may just avoid one another.

--Originally published: Scientific American Online, October 21, 1997