The Rutherford atomic model is one of those fantastic cultural objects where everything comes together in a strangely harmonious, indelible, yet more-or-less unconscious way: the experimental events, the simple image and its real impact on the development of atomic physics. It’s all there and let’s go through piece-by-piece:
First of all, to reiterate: the Rutherford atomic model is the basic image of an atom that most people seem to have in mind when they say “atom” these days – or since the model was first proposed in 1911. It has electrons and a nucleus and you can visualize it without much effort.
Still, how did Rutherford come up with the archetypical “atom”?
Summarizing Kragh on Bohr (pages 27 to 31): apparently, his archetypical atom evolved out of scattering experiments. In 1910, Thomson worked out the scattering of low energy beta particles (electrons) off of atoms. Thomson scattering has remained the standard low-energy approximation for electron scattering for over a century. Rutherford, having already gotten the Nobel Prize in 1908 for chemistry, found himself wondering how the scattering of alpha particles – the particle he used in his work on radioactivity and which he correctly understood to be a Helium atom that was missing its 2 electrons – could be similarly approximated.
In working on how alpha particles scattered, Rutherford and his team observed what is probably the single most classic experimental event in physics (and of course that classic event was instrumental in imagining the archetypical atom): one out of about 8,000 alpha particles bounced straight back off the atomic scattering target. So the atom had a tiny, but very substantial, nucleus, solid enough to bounce one in 8,000 Helium ions straight back to the radioactive source from whence it came. So Rutherford modeled his atom: nucleus, electrons, and a lot of empty space. As a scattering theory and an easily understood and visualized image, it worked perfectly. However, it had some definite problems with its electrons. First of all, they were no longer available to each atom in thousands so they could not be arranged to spin and oscillate without radiating away all their accelerated classical electromagnetic energy and falling into the nucleus – which was now the location of most of the mass and all of the atom’s positive charge. So, yes, all the pieces were there and you could picture the whole thing, give it a weight and even isotopes, but there was no way to picture how its electrons worked – and, by 1911, electrons were assumed to be doing quite a number of things in atoms: staying with the atoms and not radiating away all their energy, but (at the same time) and absorbing and emitting light at very specific wavelengths for example. For two years the atom stayed like that: perfectly atomic, completely imaginable, but missing a lot of the dynamics required in its electrons. Then in 1913, all that changed when Niels Bohr described the first version of a quantum-structured atom. The electrons did more of what they were supposed to do but it was hard to picture how they did it. And it only got harder as the quantum picture was elaborated. Still here is an image of the first Bohr atomic model (Hydrogen – where the model works best):
