Experimental Details

I’ve been guilty of glossing over details. I suspect, but can’t remember, that I had educated myself on the experimental details that went into the discovery of the structure of the atom. Thomson, Rutherford and Millikan led teams that performed painstaking experiments that I now summarize in less than an hour. Worse still, over time I have conflated multiple experiments to make the conclusions easier to digest for my students. I’m reminded of my shortcomings while reading The Matter of Everything, written by the experimental physicist Suzie Sheehy. As a theorist, I acknowledge my blinders and lack of experience when it comes to appreciating the experimental details.

 

Sheehy focuses on those details! In the first three chapters, she covers three stories: the cathode ray tube, the gold foil experiment, and the photoelectric effect. I cover all three in my General Chemistry course. Today’s post is to remind myself where I have over-glossed the details, and do a better job when I resume teaching in the fall semester. I’d like to blame the G-Chem textbook which also glosses over the details, but it’s my own fault for not remembering. As the expert in the classroom, I should know better.

 

The Cathode Ray Tube. Sheehy begins her story with the discovery of X-rays and the use of the Crookes tube. I skip this story and go straight to J. J. Thomson’s experiments. I had forgotten how crucial vacuum is to this story. If most of the gas particles had not been removed (which was no easy task back then), they would interfere with the cathode ray, and it would be difficult to observe the crucial bending of the ray that led Thomson to conclude that this ray of “light” was made up of negatively charged particles (electrons!). The observable green glow was because stray electrons would hit gas particles or the wall, so-called “braking radiation” because electrons stopped by the glass would emit light (i.e., electromagnetic radiation). While Thomson tried different gases, the results were the same. I had conflated this by telling the students that this allowed him to conclude that no matter what the element, what all atoms had in common was the electrons, and where they differed was in the nature of the positive particle (the nucleus with different numbers of protons). Thomson did change the identity of the metal electrodes, which would support what I said, but I’ve been failing to mention this to the students. I resolve to do better.

 

The Gold-Foil Experiment. Thomson proposed a plum-pudding model for the atom. I tell the students that Rutherford wanted to test the model. What I fail to also say is that Rutherford had previously observed that shooting alpha-particles through a thin metal piece produced a fuzzy image (on a photographic plate), i.e., the particles were scattered in some way but no one knew why. His assistants Geiger and Marsden performed a series of experiments where they measured the deflection of alpha-particles against a metal slab, and then decreased the thickness of the metals so that more alpha-particles could pass through. Thus, they would need detectors for both the wide-angle reflection and for those that passed through. G-Chem textbooks show a detector “ring”. I have been erroneously telling the students that having detectors that picked up wide-angle deflections was fortuitous – but that’s wrong. They were there from the beginning because of how the experiment was carried out. I will get this right next time.

 

The Photoelectric Effect. I present the full set of experimental details before providing Einstein’s explanation; so does the G-Chem textbook. That’s backwards. Lenard’s early experiments puzzlingly showed that changing the light intensity had no correlation with the speed of ejected electrons. I fail to mention Millikan’s early experiments showing that the photoelectric effect was independent of temperature, adding to the puzzle. Einstein then came up with his strange “quantum explanation”, making the crucial predictions that the speed of ejected electrons would vary linearly with the frequency of light, that there would be a threshold frequency, and that changing the intensity would change the number of electrons emitted (but not their speeds). It took Millikan a good ten years to verify all this, and it took so long because he was trying to disprove Einstein’s weird hypothesis. Einstein deservedly won the Nobel prize for his insight; and I need to tell the story in the correct sequence to underscore this point.

 

In G-Chem, I don’t discuss the “cloud chamber” experiments of Wilson that supported the discoveries of the Thomson and Rutherford teams. How does one take a picture or a snapshot of a tiny particle too small to be seen by a light microscope? By seeing their tell-tale effects as they whiz as a wisp through the vapor – akin to the trails we see behind a jetplane. I don’t foresee myself covering this in G-Chem. I do spend all of five minutes on Millikan’s Oil Drop experiment, briefly going over the setup, and then quickly telling the students the result. I used to do more, but I think my truncated version is about right given the goals of the course. But I would like to incorporate changes to the three stories as recounted above, and I’m reminded that it is important to sweat the experimental details.