Until the 1980s, direct evidence was all that scientists had of the structure of an atom. For example, when an iodine atom was linked to any one of the six carbon atoms in a benzene molecule, the resulting molecule behaved the same way as the original molecule, thus suggesting a ring structure for benzene. (If benzene were a chain, its molecular behavior would have changed depending on where the iodine atom was added.)
Then, in June, 1988, the world got its first view of the benzene molecule in an article in the journal Physical Review Letters. The picture resembled a tray of lumpy doughnuts with tiny dark smudges where holes should have been. The photograph was taken with a highly sophisticated device called a scanning tunneling microscope.
The scanning tunneling microscope is an outgrowth of the 1981 invention of two Swiss-based IBM researchers, Gerd Binning and Heinrich Rohrer. Binning and Rohrer used a tiny, needlelike probe so finely milled that its tip was only two or three atoms wide. They brought the tip to within forty billionths of an inch of the surface to be scanned and applied a low voltage. Under these circumstances, a quantum mechanical effect called tunneling allows a faint current to pass from the needle to the sample. Because the strength of the electron current increases as the gap narrows, the current strength serves as an indicator of the distance between needle tip and surface. The needle thus scans the sample's atomic surface much like an airplane tracing a mountain. Using a computer, the needle's motion is translated to a three-dimensional image—a bump or hill registers as white, a dip or hole as black.
For Binnig and Rohrer, a 1986 Nobel Prize in Physics was in their future. For chemists, there beckoned an opportunity to finally obtain visual images of molecules. And so it was, in June, 1988, that the benzene molecule was revealed. By 1990, the first snapshots of DNA were available. Still more startling photographs continue to appear, verifying the indirect evidence of molecular details and giving grand insights into how chemicals react.
