Atom Visibility

(Created prior to 1993)

Question: Has anyone seen an atom?
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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
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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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