 The Thompson model seemed very promising. It could explain how an atom could emit radiation and a characteristic frequency, but then an experiment performed by Rutherford Geiger and Marsden in 1909 pulled the rug out from under this picture. Their gold foil experiment investigated the interaction of atoms with the so-called alpha particles emitted from the recently discovered radioactive element radium. They built an alpha particle source using a lead chamber with a window in one wall. On the opposite wall they mounted a radium sample which emitted alpha particles, which we now know are helium nuclei. Most of the particles were absorbed by the lead but those that made it through the window formed the narrow beam. Rutherford's team aimed their alpha source at a very thin gold foil. Encircling the gold sample was a detector sensitive to alpha particles, for example, photographic film. What they expected to see was essentially all of the alpha particles passed through the foil and end up more or less in one spot on the detector. This is what you would expect from the Thompson model, where the great majority of the mass of the gold atoms is spread out in clouds of positive charge. A positively charged alpha particle traveling through this positively charged region should never feel a very strong force and its path should not deviate significantly. Their mass being very small the electrons in the gold should have little effect on the alpha particles. An alpha particle approaching the positive charge cloud of a gold atom should feel a repulsive force that increases with decreasing distance between the two. But once the alpha particle penetrates the charge cloud the force would actually decrease. So there should be a maximum force an alpha particle could be subjected to and it was thought that this would only be enough to at most slightly modify its path. While most alpha particles did indeed pass through the foil Rutherford was shocked to find that a few were actually scattered at large angles. He's quoted as saying this is as if you fired a 15 inch shell at a piece of tissue paper and it came back and hit you. The inescapable conclusion was that the positive charge and most of the mass of the gold atoms are concentrated in a very small region what we now call the nucleus. Most alpha particles would still pass more or less directly through the foil but occasionally a collision would scatter a particle at a large angle. The smaller the region of positive charge the larger is the maximum force between it and an alpha particle. Knowing the force required for scattering allowed Rutherford's team to estimate the size of the nucleus. They found it was only about 10 millionths the size of an atom. An unbelievably small and dense region of matter. The Thompson model of the atom had been overthrown but what would take its place? With the positive charge and the great majority of its mass confined to a tiny nucleus the electrons would somehow have to explain the size of an atom. But electron scattering experiments had demonstrated that they also were not clouds of charge but instead tiny particles. One possibility was suggested by the similarity between gravitational and electrical forces. Both fall off as one over distance squared. The Rutherford or planetary model resembled the solar system. The nucleus like the sun formed the center and contained the great majority of the mass. Around this move the electrons in orbits analogous to those of the planets. In place of gravity we have the electrical attraction between the positive nucleus and the negative electrons. Like the solar system most of the atomic system is empty space and its extents are determined by orbits not the sizes of its individual components. But the attractiveness of this model was tempered by some serious problems. The Thompson model explained the existence of at least one discrete spectral frequency. An orbit however seemingly could be of any size in any period. So where do we get discrete frequencies in this model? But even more troubling is that electromagnetic theory predicts that electrons in such an atom will rapidly radiate away their orbital energy and crash into the nucleus. Here we show two views of an electron's electric field. It starts at rest and then we move it along a circular orbit. As it orbits its electric field spirals outward carrying away energy as electromagnetic radiation. This leaves the electron with less and less energy with the result that its orbit spirals into the nucleus. The atom will rapidly collapse. An interesting solution to the radiation problem had earlier been suggested by Nagaoka. His model predated the discovery of the nucleus so like Thompson he assumed the bulk of the atom's mass formed a large cloud of positive charge. He didn't like the idea of negative charge moving through positive charge so he suggested a model motivated by the rings Saturn. The electron was spread out into a ring of negative charge orbiting the positive charge. An orbiting ring of charge unlike an orbiting charge particle does not produce radiation and so does not have the problem with the radiation collapse that the planetary model does. However as we've seen Millikon later showed that the electron is indeed a discrete particle and we never see a piece of an electron. Moreover Nagaoka found that a ring of charge is unstable due to the mutual repulsion of its pieces unlike the rings of Saturn in which the pieces gravitationally attract each other. So he discarded his model. Despite the theoretical difficulties it created the discovery of the nucleus clarified the structure of the atom. The size of an atom roughly one ten millionth of a millimeter represents the space somehow occupied by one or more negatively charged electrons. At the center ten millionths the size of the atom itself lies almost all of the atom's mass in the positively charged and unfathomably dense nucleus. Outside of this tiny nuclear realm atomic physics and by extension chemistry and biology is primarily the physics of a single particle, the electron.