Rutherford saw a mystery. Becquerel had discovered in that uranium gives off invisible rays that fog photographic film. The Curies had shown that thorium acts similarly.
In November , they reported the chemical separation of two unknown materials from the ore pitchblende, both highly radioactive — the first use of the word. What they had was unclear, but it was spectacularly unexpected. The challenge these unexpected phenomena presented had been faced by scientists before. How does one explain something completely new?
Isaac Newton — had faced this with light: what is color, what produces fringes of light and shadow at the edges of objects? Michael Faraday — discovered that changes in magnetism produce electrical current and that electromagnetic forces may bend and flex. They investigated these new phenomena by exhaustive experimentation and description. Rutherford's work on radioactivity bears a strong resemblance. The method to this experimentation is simple: joyful and inventive play.
Think of every tool or technique that might tell us something about the rays given off by radioactive materials. Try them all out. Many will show no result, but some will be revealing. Three radiations Understanding the nature of alpha, beta and gamma rays Three radiations This diagram, copied from an original by Marie Curie, shows the effect a magnetic field can have on different types of radiation. Magnetic fields curve the trajectory of particles carrying an electric charge. Alpha rays, curving to the right, are positively charged, the beta rays curving to the left are negatively charged, and the unaffected gamma rays are electrically neutral.
Years later, after , beta particles were observed being curved to the right — this would herald the discovery of the positron, and beta-positive radiation. Learn more : Alpha Beta Gamma rays. Because it is unusually ductile, gold can be made into a foil that is only 0. When this foil was bombarded with -particles, Geiger found that the scattering was small, on the order of one degree.
These results were consistent with Rutherford's expectations. He knew that the -particle had a considerable mass and moved quite rapidly. He therefore anticipated that virtually all of the -particles would be able to penetrate the metal foil, although they would be scattered slightly by collisions with the atoms through which they passed. In other words, Rutherford expected the -particles to pass through the metal foil the way a rifle bullet would penetrate a bag of sand.
One day, Geiger suggested that a research project should be given to Ernest Marsden , who was working in Rutherford's laboratory. Rutherford responded, "Why not let him see whether any -particles can be scattered through a large angle?
Many years later, reflecting on his reaction to these results, Rutherford said: "It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a inch shell at a piece of tissue paper and it came back and hit you. Rutherford concluded that there was only one way to explain these results.
He assumed that the positive charge and the mass of an atom are concentrated in a small fraction of the total volume and then derived mathematical equations for the scattering that would occur.
These equations predicted that the number of -particles scattered through a given angle should be proportional to the thickness of the foil and the square of the charge on the nucleus, and inversely proportional to the velocity with which the -particles moved raised to the fourth power. In a series of experiments, Geiger and Marsden verified each of these predictions. When he published the results of these experiments in , Rutherford proposed a model for the structure of the atom that is still accepted today.
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