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The Geiger–Marsden experiment (also called the Rutherford gold foil experiment) was an experiment to prove the structure of the atom performed by Hans Geiger and Ernest Marsden in 1909, under the direction of Ernest Rutherford at the Physical Laboratories of the University of Manchester. The unexpected results of the experiment demonstrated for the first time the existence of the atomic nucleus, leading to the downfall of the plum pudding model of the atom, and the development of the Rutherford (or planetary) model.
The popular theory of atomic structure at the time of Rutherford's experiment was the "plum pudding model". This model was developed in 1904 by J. J. Thomson, the scientist who discovered the electron. This theory held that the negatively charged electrons in an atom were floating (sometimes moving) in a sea of positive charge—the electrons being akin to plums in a bowl of pudding. The plum pudding model was the prevailing theory on the structure of the atom until it was disproved by Ernest Rutherford in his analysis of the gold foil experiment, published in 1911.
The gold foil experiment consisted of a series of tests in which positively charged alpha particles (helium nuclei) were fired at a very thin sheet of gold foil. If Thomson's Plum Pudding model was to be accurate, the big alpha particles should have passed through the gold foil with only a few minor deflections. This is because the alpha particles are heavy and the charge in the "plum pudding model" is widely spread.
Rutherford's experiment consisted of a beam of alpha particles, generated by the radioactive decay of radium, directed normally onto a sheet of very thin gold foil in an evacuated chamber. A zinc sulfide screen at the focus of a microscope was used as a detector; the screen and microscope could be swiveled around the foil to observe particles deflected at any given angle. Under the prevailing plum pudding model, the alpha particles should all have been deflected by, at most, a few degrees; measuring the pattern of scattered particles was expected to provide information about the distribution of charge within the atom. However, the actual results surprised Rutherford. Although many of the alpha particles did pass through as expected, many others were deflected at small angles while others were reflected back to the alpha source. They observed that a very small percentage of particles were deflected through angles much larger than 90 degrees. According to Rutherford:
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 15-inch shell at a piece of tissue paper and it came back and hit you. On consideration, I realized that this scattering backward must be the result of a single collision, and when I made calculations I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greater part of the mass of the atom was concentrated in a minute nucleus. It was then that I had the idea of an atom with a minute massive centre, carrying a charge.—Ernest Rutherford
The data generated from the gold foil experiment demonstrated that the plum pudding model of the atom was incorrect. The fact that some of the alpha particles were deflected or reflected meant that the atom had a concentrated centre of positive charge and of relatively large mass. The alpha particles had either hit the positive centre directly or passed by it close enough to be affected by its positive charge. Since many other particles passed through the gold foil, the positive centre would have to be a relatively small size compared to the rest of the atom - meaning that the atom is mostly open space.
Because the majority of the positive particles continued on their original path unmoved, Rutherford was forced to conclude that most of the remainder of the atom was a region of very low density. A great deal of charge was also associated with the central region of high density. Rutherford hypothesized that these two properties resided in the same physical structure and termed his discovery "the central charge", a region later named the nucleus. Thus the current view of the nuclear atom - a structure with a positively charged centre (nucleus) of high density and negatively charged electron particles moving around the nucleus at relatively large distances compared to the nuclear radius - was created.
Rutherford interpreted the experimental results in a famous 1911 paper. He was able to definitively reject J.J. Thomson's plum pudding model of the atom, since none of Thomson's negative "corpuscles" (i.e. electrons) contained enough charge or mass to deflect alphas strongly, nor did the diffuse positive "pudding" or cloudlike positive charge, in which the electrons were embedded in the plum pudding model. Instead, Rutherford suggested that a large amount of the atom's charge and mass is instead concentrated into a very physically small (as compared with the size of the atom) region, giving it a very high electric field. Outside of this "central charge" (later termed the nucleus), he proposed that the atom was mostly empty space. Rutherford was able to say from the experiment that the nuclear charge was positive and used the following language for pictorial purposes:
From energetic considerations of how far alpha particles of known mass and kinetic energy would be able to penetrate toward a central charge of 100 e (1.6022×10−17 C), Rutherford was able to calculate that the radius of his gold central charge would need to be physically smaller (how much smaller, could not be told) than 3.4×10−14 meters (the modern value for the actual radius is only about a fifth of this). The figure applied in a gold atom which was itself known to be much larger: 1.5×10−10 meters or so in radius – a very surprising finding, as it implied a strong central charge less than 1⁄4000 of the diameter of the atom.
Rutherford had used strictly Newtonian methods to analyze the relatively low-energy alpha-scattering of this experiment. Later, when full quantum mechanical methods were available, it was found that they gave the same scattering equation which had been derived by Rutherford by classical means.
Although Rutherford's model of the atom itself had a number of problems with electron charge placement and motion, which were only resolved following the development of quantum mechanics, the central conclusion from the Geiger–Marsden experiment, the existence of the nucleus, still holds.
Rutherford's description of the atom set the foundation for all future atomic models and the development of nuclear physics. Rutherford's model was later elaborated into the Bohr model by physicist Niels Bohr in 1913. The Bohr model, in turn, was soon replaced by the Schrödinger model of the atom, as the basic atomic model used today.