For the last several posts, we’ve gone through the atomic nature of the physical world and the model of the atom. Now let’s turn to modern evidence that these ideas are correct. Remember, atoms are too small to see with the eye, even with the best microscope. So how do we know the model is correct?
There is lots of evidence the picture is correct. In most cases, there are layers of physics between the observation and the interpretation which is fine if you take the time to understand the physics. Here I’ll talk about what is – in my opinion – the observations with the smallest amount of physics between what is measured and what we describe as seen.
That is evidence gathered using what is called an atomic force microscope (AFM), or a scanning tunneling microscope(STM). This figure is a schematic representation of an ATM.
This technique provides the most direct route to visualizing what an atom looks like, because there is only a single physical equation that is used to transform observation into something we use as a representation of the atom. In this case, the instrument makes use of the known fact that the amount of current that flows between two charged points depends strongly on the distance between them. This relationship is more common than you might think. Consider an overhead light that is a long tube – this common light source is called a fluorescent lamp. The lamp is simply two charged metal plates, one with a positive voltage and the other with a negative voltage. The distance between the plates is large so the voltage needs also to be large to make current flow, and it is the current – the electrons moving through space – that creates the light. It is not much difference in principle with a lightning storm. The distance between the clouds and the ground is much larger, so the voltage must be higher to cause the current, or lightning, to flow. What is important is that when at constant voltage, the amount of current that flows between to charged points is proportional to the distance between the points.
In the figure, the STM is built in a way that the tip of the probe can be held very very close to a surface. A voltage difference is then applied to the tip and the surface, and the amount of current that flows is measured. From the voltage and the current, the distance can be calculated. As the microscope tip is moved along a surface, the tip is held very steady and the amount of current flowing is measured. When the tip is over an atom, the space between is small and current flows relatively large. When it is over the “valley” between two atoms, the current flow drops because the distance is larger. As you might imagine it is very hard to hold the tip steady enough to maintain a constant distance that varies less than an atom diameter, but it can be done by carefully avoiding vibrations and using the correct materials. I’ve included another picture that shows a more artistic rendering of the process, but remember, these details are too tiny for the eye to see. Humans cannot see nor feel the roughness of a smooth metallic surface as shown in the artist’s drawing. But what might not be so obvious is the difficulty in creating a tip of the microscope that is a single atom in diameter. If the tip is larger than a single atom, say 100 atoms across, the same current will flow as that entire surface passes over an atom. It will give an observation that says all single atoms are the size of the tip, which is really 100 atoms. The real trick of making observations with an AFM is to make a tip that is very very sharp – on the order of a single atom at the end. As you might also guess, that makes the tip very fragile and they break easily. These are not measurements that are easy to make! There are now commercial sources for tips and even for the microscopes so it is easier than it was ten years ago. Here are a couple of examples of what the tips look like. If you follow this link, you will see a general purpose tip, that at 8 nm is broader than a single atom. Much more fragile and expensive is the high resolution tip, that is on the order of 1 nm across.
The results show images that look very much what we expect given the models described in earlier posts. The first scans of this type of instrument were made by IBM. Here is a famous image that shows not only can single atoms be imaged, they can be pushed around to spell out words. This shows an AFM image of xenon atoms on a very smoothly polished silicon surface. This shows it doesn’t matter what language is used. Note that this surface is not as well-polished as the IBM image. These two images should give you a general idea of the data that can be collected but keep in mind, what is really measured is a current, which is converted to a distance, which is converted to an image. To be more confident of the images, a carefully designed experiment can be done. Quantum mechanics predicts that when a single atom is confined to a space close to the size of that atom, the position of the atom becomes delocalized. This is the true meaning of the wave-particle duality nature of matter you might have heard about. To test this idea, scientists used an AFM to place a single atom in the center of a “corral of iron atoms. When this structure was constructed, an AFM was then scanned over the corral with this image being the result. This image shows steps along the way during construction of the corral. When two atoms are placed within a quantum confining corral, the waves of each atom should start to interfere with the other. Here is an image of that situation. This is the beginning of teleportation. Not quite ready for Star Trek, but someday… An over view of the IBM work and more images can be seen by going to the IBM STM/AFM page.
I hope you will agree that these types of images provide compelling evidence that atoms are the building blocks of the physical world we can touch and sense in other ways. For simplicity sake, as your own “physical model” of an atom, these images show you can visualize atoms as spherical marbles. There are details that make that model to simple but the purpose of a model is not to be absolutely correct. Most models are meant to be used and to do that they are almost always a simplification of reality. For example you now know that atoms can be broken down into electrons, protons and neutrons and they really are made of a nucleus surrounded by electrons. And now you know that when contained, atoms behave like waves and particles as shown in one of the images above. But most useful is that they can be visualized in most cases by a simple spherical marble. In future posts we will talk about the ways the marbles come together to form molecules, and it is the shape of those molecules that give rise to the complexity we see in macroscopic shapes that make the world.