The Model that Survived

Many models were proposed to explain this phenomenon. Some explained it better than others. The model which explained it in a way which is consistent with other experiments developed as follows: Mandeleev's periodic table implied that elements were atomic, that everything had a smallest distinguishable piece, an atom. In 1874, Stoney predicted that if atoms existed, then electricity should be composed of tiny particles, rather than existing as a continuous flow. In 1881, Helmholtz performed some experiments to show that this was an accurate description. In 1891, Stoney named these particles "electrons.'' In 1898, physicist J.J. Thompson showed not only that electricity is the flow of electrons, but that you could isolate the electrons separately from the atoms. These were not atoms. They had a mass very different than atoms and the electrons came from atoms. Atoms are not uncuttable. It turns out that if you remove these electrons from an atoms, then you are creating an ion of the element. So, apparently, the number of electrons was not the characteristic of the atom which made this atom that element. Somehow a gold atom was a small piece of uncuttable gold with electrons stuck inside. This was dubbed the "plum pudding'' atomic model: The atom is envisioned as a gelatinous clump of positive charge with plums of negative charge (electrons) inside. Later, in 1909, two graduate students, Geiger (as in the counter) and Marsden, found that if they looked¹ at an atom with alpha particles², then most of the alpha's passed through the gold with almost no scattering. A few alphas, however, didn't just feel a glancing blow and get knocked to the side, they bounced back at the alpha-emitter! This was very peculiar and was inconsistent with the plum pudding model. It took three years of manipulation for the graduate students with their advisor, Ernest Rutherford to figure out that the way the alphas scattered off of the gold atoms was consistent with essentially all of the mass confined to a region 10,000 times smaller than the radius of the atom! Thusly was born the nuclear atom, in which the electrons orbited very, very far from the nucleus.