Thus, in contrast to today’s atom that has a very dense and very small (compared to the whole atom) positively charged nucleus, Thomson’s had a more dispersed positive charge. As a whole, the plum pudding representation only strived to explain why most atoms were neutral.
It’s interesting to note that this model was sometimes visualized as having a cloud of positive charge, a striking contrast to the most recent atomic model which describes the positive nucleus to be surrounded by an electron cloud.
Introduced by Thomson in the March 1904 edition of the UK’s Philosophical Magazine, this model was invalidated 5 years after during what is now known as the Rutherford gold foil experiment, an experiment designed to probe the atom.
The basic setup of the experiment was as follows: alpha particles coming from a radioactive source were directed to a gold foil. The foil was then surrounded by a wall of zinc sulfide that exhibited scintillations (tiny flashes) when hit by the alpha particles. Patterns formed by the scintillations would provide information as to how the charges were distributed inside the atom.
With the plum pudding model in mind, Rutherford expected very minimal alpha particle deflections as they were bombarded on the foil. Hence, the scintillations were only supposed to be observed right behind the foil, exactly opposite to the source of alpha particles. Alas, although majority of the flashes were indeed observed behind the foil, a few were also seen near the source.
According to Rutherford, it was like firing a cannonball on a tissue paper and seeing it bounce against the paper right back at him on certain occasions.
Since alpha particles are just helium nuclei, they are positively charged. The only logical explanation therefore was that the positive charge in the atom were not as dispersed as you would imagine in a pudding’s soft crust. Instead, Rutherford guessed that they should be concentrated in a very small volume in the center. This specific portion of Rutherford’s model still holds even up to this day.
We have some articles in Universe Today that are related to the this article. Here are two of them:
Some more from Physics World:
- 100 years of the electron: from discovery to application
- Proton and neutron masses calculated from first principles
Tired eyes? Let your ears help you learn for a change. Here are some episodes from Astronomy Cast that just might suit your taste: