Skulls in the Stars

Here’s a little obscure physics trivia for you: who first suggested that an atom might have a structure consisting of a positively-charged “nucleus” surrounded by orbiting electrons?

The easy, and mostly correct, answer is Ernest Rutherford.  In 1909, he supervised two of his students (Geiger and Marsden) in an experiment to probe the structure of atoms using alpha-particles.  Surprisingly, they found that occasionally one of the alpha-particles would be almost completely reflected from their sample, which should not have happened according to the Thomson “plum-pudding” model of the era.  Rutherford himself later remarked, “It was almost as incredible as if you fired a fifteen-inch shell at a piece of tissue paper and it came back and hit you.”  In 1911, Rutherford published a paper (“The Scattering of Alpha and Beta Particles by Matter and the Structure of the Atom,” Phil Mag, ser 6, 21 (1911), 669-88) in which he argued that the results suggested that the atom consisted of electrons orbiting a very small, dense, positively-charged nucleus which contained most of the atom’s mass.  This realization was a major breakthrough in atomic physics and eventually led to our modern picture of atomic structure.

When I was working on my post on “failed atomic models” some time ago, however, I encountered an off-hand remark that Jean Baptiste Perrin, another giant in atomic theory, had first proposed a nucleo-planetary atomic model in 1901.  Curiously, though, I was unable to find a reference to it, and I’ve always wondered why.

Perrin’s own Nobel lecture of 1926 provides most of the answer:

I was, I believe, the first to assume that the atom had a structure reminding to that of the solar system where the “planetary” electrons circulate around a positive “Sun”, the attraction by the centre being counterbalanced by the force of inertia (1901). But I never tried or even saw any means of verifying this conception. Rutherford (who had doubtless arrived at it independently, but who also had the delicacy to refer to the short phrase dropped during a lecture in which I had stated it) understood that the essential difference between his conception and that of J.J. Thomson was that there existed near the positive and quasi-punctual Sun, enormous electrical fields as compared with those which would exist inside or outside a homogeneous positive sphere having the same charge, but embracing the whole atom.

In other words, Perrin first proposed the nucleo-planetary model, but never pursued the idea beyond some basic speculations.  Rutherford is rightly given most of the credit for the development of the model, as he supervised the experiments which led to its verification and worked out the rigorous theory behind it.

One thing I would like to find, though, is the place where Rutherford referred to Perrin’s planetary model!  I’ve been searching through Rutherford’s papers on the nuclear model, but so far have not found any reference to Perrin.  If I find it, I’ll write an additional post on the subject…

Michael Faraday (1791-1867), whom I’ve talked about numerous times, has a reputation as being a bit of a theoretical lightweight, namely because he had little formal mathematical training.  In spite of this, however, he had an ability to think abstractly and, yes, theoretically about problems in a way that, when examined, is nothing short of amazing.

In my previous researches on Faraday, I came across a reference to an article he wrote in 1844 in volume 24 of Philosophical Magazine, pp. 136-144, “A speculation touching electric conduction and the nature of matter.”  Faraday, already a distinguished and even famous scientist, shared some thoughts about the nature of atomic structure, based on the paucity of knowledge that was available at the time.  His observations, though still off the mark according to current understanding, were remarkably forward thinking; furthermore, they provide a lovely snapshot of the ‘state of the art’ in 1844.   Let’s take a look at his paper:

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