“I have broken the machine (the atom) and touched the ghost of matter.” – Ernest Rutherford
Rutherford continued his study of how alpha particles were scattered by thin metal sheets, the work he had begun at McGill University. According to the plum pudding model of the atom, with the mass spread out in a thin and diffuse way, an alpha particle would be far more massive than anything it was likely to encounter in the atom. Thus, as the high-speed alpha particles emitted from a radioactive material, such as radium, passed through the target material, there should be expected little deflection of their paths. Geiger made a quantitative study of these small deflections and observed that they increased with the mass of the target atom and the thickness of the foil, as should be the result in the plum pudding model. Now he would make a key discovery on the nature of the atom. In his experimentation, he fired alpha particles at a sheet of gold foil only one-fifty-thousandth of an inch thick, thus the gold was only a few hundreds of atoms thick. The results of the experiment showed that most of the alpha particles passed through without being affected by the gold. Rutherford gave his young assistant, Ernest Marsden, the task of measuring large deflections of alpha particles passing through the thin gold film. No one knows for sure why Rutherford put Marsden on this task; maybe he had a hunch to expect the unexpected. The scintillation counter registered that the path of some of the alpha particles through the gold film were scattered through large angles, indicating they had collided with a gold atom and the path of travel was deflected—much like a collision of billiard balls.
Figure – Schematic diagram of the Geiger and Marsden gold scattering experiment.
The results of the scattering experiments weren’t immediately obvious to Marsden, Geiger, or Rutherford, who would ruminate on the results for over a year. At the March 7, 1911, meeting of the Manchester Literary and Philosophical Society, he gave a brief description of his concept of the nuclear atom. The results of his experiment with Geiger and Marsden were revealed in a paper later that year titled, “The Scattering of Alpha and Beta Particles by Matter and the Structure of the Atom.” The paper would later be regarded as one of the seminal papers on the structure of the atom.
In his paper he concluded that since the gold foil was extremely thin (0.00004 cm) and the majority of the alpha particles passed through undeflected, it would seem that the atoms were mostly empty space. The alpha particles that were deflected through large angles, sometimes greater than ninety degrees, seemed to indicate that within the gold atom there were very massive positively charged regions capable of turning back the alpha particles—much like a tennis ball bouncing off a wall. Rutherford worked out the probability of a given direction of a single scattering with an alpha particle of a given velocity striking a foil of material of known atomic weight and known thickness. In his view, the atom contains a very small central positive charge where virtually all the mass of the atom was contained. Surrounding the central core were the much lighter corpuscles (electrons) that have an equal number of negative charges. This model of the atom was much closer to the modern view of the atom and replaced the concept of the featureless, indivisible spheres proposed by the ancient Greek philosopher Democritus, which had held sway for over two millennia.
Initially, Rutherford’s model of the atom didn’t garner much attention in the physics community. Acceptance grew as the Danish theoretical physicist Niels Bohr, who spent time with Rutherford in 1911, was very interested in the “nuclear” concept of the atom. Whereas Rutherford worked with experiments to understand the atom, Bohr worked with pencil and paper as he was a theorist. Bohr and Rutherford were an unlikely pair but became lifelong friends and colleagues. In a letter to his fiancé, Bohr wrote of Rutherford, “I received a deep impression of the charm and power of his personality by which he had managed to achieve almost the incredible wherever he worked.” Rutherford spoke fondly of the young Dane, “Quite the most intelligent chap I’ve ever met.” Bohr’s model of the atom combined Rutherford’s nucleus model with the new quantum physics developed by Max Planck and Albert Einstein.