Rutherford’s atomic model

In the 100th anniversary of the Borh’s atomic model, let’s talk, of course, about the… Rutherford’s atomic model! It’s not that I don’t like Bohr, but I’ve always been more in experimental Physics rather than in theoretical Physics, and it just so happens that Rutherford’s atomic model is directly based on the experimental results of Rutherford himself and his team, while Bohr (I recognize his merits) he got others problems, combined them, gave a quantum touch to them, and after thinking about it for quite a while and (I’m sure that after) many sleepless nights, he presented his model to the world.

Ernest Rutherford was born in 1871 in New Zealand, and after graduating in the Canterbury College in Christchurch in his country of origin, he moved to Cambridge first, later to Montreal, and then back to Manchester and Cambridge again where he has responsible for the Cavendish laboratory succeeding J.J. Thomson, who had worked with during his first stay in Cambridge. It was during his stay in Manchester that he developed his atomic model with the help of two more great experimenters, Hans Geiger, who developed the famous particle counter, and Ernest Marsden.

Before starting with the Rutherford’s atomic model, it is necessary start with a previous model, Thomson’s one (yes, the same Thomson as earlier). The truth is we could go back in time even more, and forwards too, to analyse the historical evolution of the atomic model, but I don’t want you to stop reading yet.

J.J. Thomson was, amongst many other things, the person who discovered the electron and the first in measuring its mass-charge relation. Regarding the atomic model, Thomson proposed that the electrons were in a positively charged sphere, electrically uniform, that produces an attractive radial force for each electron. In summary, Thomson proposed as atomic model what it’s known as plum pudding, being the electrons them plum of the pudding. This model worked well with the hydrogen atom, or even with elements with two or three electrons, but with more complex atoms, things got more complex because the electrons had to be located in a way that they were in electrostatic equilibrium with the positively charged sphere, and Thomson himself admitted that from eight or nine electrons on, to locate the electrons in the pudding to make the atom stable and to find the equilibrium distribution was too much difficult to calculate it.

I think that the post name should have been Thomson’s atomic model… but here we go with Rutherford. While Rutherford was in Montral, the nature of α, β and γ particles was known of at least good hints of what their nature was were in progress. When he arrived Manchester, he was lucky to find Geiger there, who was a key person in the establishment of his model. Rutherford and Geiger focused on the research of α particles and its use in different experiments. One of the first results of such research was the development of an ‘electrical’ model to count the particles emmited by radioactive substances. Twenty years after, Geiger together with Müller improved the electrical method to count giving rise to the famous Geiger counter. Before that, counting particles was made using a more traditional method. A zinc sulfide screen was put in front of the particles source and the sparkles produced were counted one by one. However this method was not fully reliable as, among other things, depended on the observer patience. Parallel research, such as the one by Erich Regener in the Berlin University, improved counting techniques in a zinc sulfide screen to an extent that the results obtained didn’t differ to the ones using that primitive Geiger counter. Therefore Rutherford abandoned his electrical method and started to count sparkles one by one again.

As a result of the good job Geiger was doing, Rutherford promoted him so now he could have under his responsibility other students to teach them about radioactivity techniques. It was this way how, during the academic course of 1908-1909, Ernest Marsden started to collaborate with Geiger and Rutherford. The experiment they were working on was about α particles diffusion when they went through thin aluminium, gold, or other elements, sheets. To that purpose they put a sheet between the zinc sulfide screen and the tube where the α particles went through. Rutherford’s boys realised that the α particles diffusion going through the sheets was important. A large number of particles went through the sheet in the initial movement direction, but many others where deflected in a large angle or even they bounced off.

When Geiger and Marsden informed Rutherford about this result, and considering that he didn’t like the plum pudding model, he realised that the data didn’t match what was expected from the Thomson model because, if the positive charge was uniformly distributed (the pudding part) the interaction between the electric charge and the inner atom’s electric charge would not be strong enough to deflect the particle in a large angle or even to make it bounce off. The results could only be explained if the positive charge was located in a very tiny area in the centre of the atom, being the electrons at a certain distance from the centre.

However, despite Geiger and Marsden published the results of his experiment in 1909, Rutherford didn’t publish his final atomic model until two years later, in 1911. In this publication he explained the why of Geiger and Marsden’s results. To explain the deflection and to model the atom, Rutherford had to find a mathematical formula to explain the results. According to rumours Rutherford, as many other experimenters, didn’t like maths, so to solve his problem he had to look for the help of a young mathematician named R.H. Fowler, who later became his son-in-law.

When the movement of the electrons around the positive nucleus of this model was studied, it was found that it is unstable. This is because when the electrons move around the nucleus they emit energy, so step by step, they would fall into the nucleus due to the loss of energy. But then 100 years ago, Bohr appeared in the scene, he applied the quantum hypothesis and… but that is another story.

References:

Sanchez Ron, J.M., Historia de la Física Cuántica. I. El periodo fundacional (1860-1926)

Gamow, George, Biografía de la Física

Thomson, Joseph J., Electricidad y Materia

http://en.wikipedia.org/wiki/Ernest_Rutherford

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