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It's certainly tempting to giggle about how very much physicists and chemists claim to know about atoms, given the fact that they've never laid eyes on one. |
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If atoms are criminals at large, physicists can produce a pretty good sketch of any given hoodlum: Size, weight, energy level, collaborators and enemies. . |
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"Yeah," confesses University of Maryland physics Professor Ellen Williams, "we've done everything by
doing indirect measurements and deducing how atoms behave. We thought there had to be atoms and
molecules, so we designed experiments to test that. Then we made new predictions and tested those. It
becomes a very circular process."
One popular indirect measurement, just for an example, is spectroscopy. If you shine a beam of infrared light on a sample of carbon dioxide and a sample of water, the two molecules will respond by vibrating at different speeds. Their vibration rates will absorb different wavelengths of the light, and let others pass through. When the escaping light registers on a detector, it makes one distinctive pattern for water and another for carbon dioxide. Every atom and molecule has a spectroscopic "fingerprint," and when you're curious about what breed of atom you're chasing, you compare its spectroscopic "fingerprint" to a reference book. |
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So, using such sly investigations, we learned an astonishing amount about atoms. . |
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Then, in the 1940s, microscopes began inching closer to atomic-scale resolution, by bouncing electrons
off or through a sample material, and recording where their ricochet ends. But the images were poor
enough that scientists couldn't say definitively that they were looking at a pattern of atoms. In 1983,
that changed.
Two guys working at an IBM research center in Switzerland invented the Scanning Tunneling Microscope (STM). The scanning part is that you give it a flat sample of material, and it makes you a contour map of the entire surface. The tunneling part is that the contours represent atoms, detected by invasive little electrons that fly off the scanner's atom-wide "probe," and tunnel into the shell of each atom on the surface of the sample. A sample of aluminum gallium aresenide looks like egg-crate foam, with little atom-peaks and atom-valleys. Atoms on a sample of gold are lined up like the ribs of corrugated cardboard. In an explosion of invention, the STM has become a sprawling family of Scanning Probe Microscopes (SPMs) that, depending on the composition of the probe, can investigate such atomic properties as magnetism and electrical potential. Williams, who uses these microscopes to study the properties of atoms, gets bubbly about it. "It's an amazing story. This is a miracle technique. You can drag an atom off the surface and move it around -- it's an amazing revolution." And you can print out an interpretation of the tunneling results to make a "picture" of the atoms on the surface of your sample. |
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But you still can't see atoms. At least, not like you saw them in your chemistry textbook, little, yellow electrons whizzing neatly around an orderly nucleus. . |
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"'Looks like' raises the question of what is an atom," Williams says. "It's not a little marble. When
push comes to shove, electrons aren't necessarily like little, round planets. They move from a particle
state to an energy state, back and forth."
The elusive nature of electrons aside, will we ever "see" atoms, I asked her. See little mug shots of individuals? She thought about it. Not with optical light, she concluded, but perhaps, some fine day, we'll be able to make X-ray images of them. "And in your wildest imagination," I asked her, "what would they look like?" "I think," she said slowly, chuckling, "they'd look like little marbles." |
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Hannah Holmes ponders the molecular mysteries of the universe from her home in Portland, Maine.
A fresh "Skinny On ...
" arrives here every other Friday, adding to her extensive work for Discovery Online. Hannah's writing also appears on the pages of
Escape, Outside, Sierra, Backpacker, Eco Traveler and Women's
Sports and Fitness. Send her a note at skinny@online.discovery.com.
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Main WebLinks Picture: Jack Mortensbak | Copyright © 1998 Discovery Communications, Inc. |