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Name: Unknown
Status: N/A
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Date: Around 1993


Question:
Has anyone seen an atom?



Replies:
We have seen atoms in many ways. Some of them are very indirect. Large microscopes that use electrons instead of light have been able to see single atoms as fuzzy pictures on film. The atoms we have seen this way are the larger atoms that contain a lot of protons and neutrons in their nuclei. This was a good question. I hope you have as much fun with science as we do.

Sam Bowen


A very good question. The answer is Yes. And No. The reason for No is that it is actually impossible for anybody to "see" an individual atom, since all atoms are thousands of times smaller than the smallest light waves we can see using our eyes. The reason for Yes is that, even though they cannot be seen directly with our eyes there is so much evidence for atoms, and we know so much about them, that it is impossible to say they do not exist. One of the greatest achievements of the last few years (which won a Nobel prize) was a new kind of microscope called the "scanning tunneling microscope", which allows an extremely sensitive "probe" (basically a rod with a very fine tip) to wander around on the outside of some solid materials, and actually feel the bumps that are caused by the atoms there, and then a computer can convert those bumps into a picture of the surface, showing the individual atoms lying there, and the patterns they form, the steps as one layer of atoms gives way to another, and all sorts of details that could never be seen before. So, in the expanded sense of the word "see", using these new instruments and computers, people have now actually "seen" atoms. Let me just list a few of the other bits of evidence for atoms. The very first real evidence came in the 1800's when people were first able to measure the details, such as pressure, volume, and weight, for gases. They discovered that gases obeyed certain laws such that at a standard temperature and pressure, a certain volume of Oxygen, for example, weighed almost exactly 16 times as much as the same volume of Hydrogen gas, and that when you combined two volumes of Hydrogen gas with one volume of Oxygen, you got pure water out of it with nothing left over. Similarly, a lot of other elements seemed to have a ratio of mass to Hydrogen that was almost exactly a whole number, and that when these elements were combined to form compounds, they always did it in whole number ratios (such as 2 H's and 1 O forming water, which is H_2 O). All of this led to the idea that the molecular compounds actually were composed of a collection of atoms bonded together, and that these atoms had specific weights, which were often whole number multiples of the weight of Hydrogen. This led to the periodic table of the elements, first devised by Mendeleev, at the end of the 1800's, and since improved and copied into every Chemistry classroom in the world. Other evidence for atoms comes from radioactivity. With the Geiger counter, every time you hear a "tick" you know an atom just decayed. Modern particle detectors allow people to see the tracks left by high-energy particles that are even much smaller than atoms, even though it is impossible for them to see these particles directly. These techniques are possible because the high energy particles can lose a little bit of their energy to the atoms around them, and this energy can be magnified by making those surrounding atoms a little unstable to start with. Therefore a little energy goes a long way to making a noise, or visible tracks. The discovery of X-rays at the end of the 1800's, and the realization that X-rays are just a very short-wavelength version of light, meant that X-rays could in principle be used just like light, to look at things much smaller than could ever be seen with visible light. Unfortunately, nobody has been able to make an X-ray microscope that works like a regular light microscope. However, it was soon discovered that a beam of X-rays shot into a solid would notice layers of atoms in that solid and the resulting "diffraction pattern" can be used to figure out exactly where the atoms were located, at least for special types of solids known as crystals. Many common solids can then be understood as just regular arrangements of atoms, and the exact distances between the layers of atoms can be determined from the X-rays, telling us how big the individual atoms must be. The layering of the atoms also show up on the outside sometimes, when the crystals form "facets", or very nice flat faces, with sharp edges and fixed angles between neighboring faces. People really like gems with facets, especially diamond, but a lot of other materials can form facets also (ordinary table salt for example). Another kind of microscope is the electron microscope. This works by sending a beam of electrons, instead of light, onto the material you want to look at, and since we can easily make electrons with very short wavelengths, these electron microscopes are almost strong enough to see atoms. They can see large molecules relatively easily, and are also used to look at the tiny features on the latest computer chips, since those are now also too small to see with regular light. I think the Guinness Book of World Records lists the most powerful microscope, and at least until a few years ago it was some kind of electron microscope. Another way in which individual atoms or electrons can be "seen" is in special traps which trap only one, or a few, at a time. It is then possible to shine light or other types of radiation at these trapped atoms or electrons, and to notice the effects of the individual particles on the radiation. For example, I believe one recent experiment was able to make the atoms fluoresce (that is, send out their own radiation) so that their positions could be seen using a sensitive camera. There are also special semiconductor devices (on a chip) that can trap and look at individual electrons. From all these experiments come details of how much atoms weigh, and how big they are, and in the 1920's and 30's, a theory was finally developed that actually explained all these things. This theory is called quantum mechanics, and is extremely precise in its predictions, especially for the interactions of atoms with light (both visible and invisible). The many agreements on numbers, often with 10 or more digits, between the theory of quantum mechanics and experiments involving spectroscopy (the study of different frequencies of light) is perhaps the most convincing evidence that atoms exist, and that we do know an awful lot about them.

Arthur Smith



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