A Picture's Worth a Thousand Sub-Atomic Theories

We scientists love to come up with theories to explain things. Actually, it's kind of our job. For example, in my last post I talked about light waves, but you’ve probably also heard people talking about light particles, or photons. This dual theory of light is exactly that—a theory that we have come up with to explain some of the phenomena we have observed involving light. On the one hand, these kinds of theories allow us to communicate about things we can’t see. On the other hand, we may be completely and utterly wrong about how things are actually working. While I would love to ramble on about how this is what makes science so beautiful and why it’s silly to get so worked up about theories, this post actually does have a direction and a point.

That point happens to be a carbon monoxide molecule on the tip of a nifty gadget called an Atomic Force Microscope (AFM). The basic principle of AFM relies on measuring the deflection of a lever as it is “dragged” over a molecule. Think of a diving board with an upside-down cone attached underneath the bouncy end – the cone is the sensor and any force that pushes up on the cone will cause the diving board to bounce up and down. The motion of the diving board can be pretty easily measured and quantified, given the correct use of some heavy mathematics. Of course this is all being done on an unfathomably tiny scale, where the tip of the cone sensor is just a single carbon monoxide molecule wide.

It turns out you can use this type of microscopy to get digital maps of molecules. Science magazine has recently published a couple of papers by some guys at IBM in Zurich who have created beautiful images of individual molecules using this technique. Here’s the most impressive example:

Gross, Leo et al. (2012)

Panel A is just a model of the molecule that the authors included for comparison but the other three images were created using AFM. Isn’t that incredible? The three AFM images look almost identical to the model that we’ve been using for ages to represent this particular molecule. When I see these images I feel so proud of all the scientists who have contributed to our understanding of what molecules are.

Atomic force microscope measurements are affected by different types of forces. Not your everyday forces like gravity or The Force (@LSkywalker), but atomic and molecular forces with funny names like Pauli repulsion and van de Waals forces. But here’s the thing, our explanations of these forces—where they originate, how strong they are, how we quantify them— are theories. We use these explanations because frankly, so far in our understanding of the molecular world, they work.

 Now just because there is a pretty picture of our seemingly brilliant molecular model doesn’t mean we are right about everything and can go home. Our idea of electron clouds and molecular orbitals and electrostatic interactions may still be utterly wrong. However, the images produced by Leo Gross and colleagues are simply more evidence that at least for now, these theories are still working.

Sources and further reading:

Gross, Leo et al. (2009). The Chemical Structure of a Molecule Resolved by Atomic

Force Microscopy. Science 325, 1110. DOI: 10.1126/science.1176210

Gross, Leo et al. (2012). Bond-Order Discrimination by Atomic Force Microscopy.

Science 337, 1326. DOI: 10.1126/science.1225621

Meyer, E. (1992) Atomic Force Microscopy. Progress in Surface Science, Vol. 41,

pp. 3-49.

Raiteri, R., Grattarola, M., Butt, H., Skládal, P. Micromechanical cantilever-based

            biosensors. Sensors and Actuators B, 79(2001), pp 115-126.