Rutherford model
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The Rutherford model of the atom was devised by Ernest Rutherford. He performed his famous Geiger-Marsden experiment (1909), which showed that the Plum pudding model ( of J. J. Thomson) of the atom was incorrect. In the experiment, Rutherford allowed alpha particles to pass through a thin piece of gold foil. He predicted that most of the particles would pass through the foil or be deflected slightly. This is what happened most of the time, but a few particles, 1 in 8000, bounced back towards the source. This directly supported the hypothesis that atoms have a dense region containing most of their mass, associated with a highly concentrated electric field (probably positive in nature), instead of spread-out positive or negative field. Rutherford thought it likely, on purely symmetric and aesthetic grounds, that such a region of dense charge and mass, would be located in the atom's core, or center.
In 1911, Rutherford came forth with his own physical model. In it, the atom is made up of an atomic nucleus surrounded by a cloud of orbiting electrons. In this 1911 paper, Rutherford only commits himself to a region of very high positive or negative charge in the atom, but uses this language for pictorial purposes: "For concreteness, consider the passage of a high speed a particle through an atom having a positive central charge N e, and surrounded by a compensating charge of N electrons." However, the Rutherford model did not attribute any structure to the orbits of the electrons themselves, though it did mention atomic model of Hantaro Nagaoka, in which the electrons are arranged in a ring (this is the ONLY previous atomic model mentioned in the 1911 paper).
The Rutherford model of the atom was soon superseded by the Bohr atom, which used some of the early quantum mechanical results to give locational structure to the behavior of the orbiting electrons, confining them to certain circular (and later eliptical) planar orbits. Since the Bohr model is an improvement on the Rutherford model, some sources combine the two, referring to the Bohr model as the Rutherford-Bohr model.
The Rutherford model was important because it essentially proposed the concept of the nucleus, although this word is not used in the paper. What Rutherford notes as the (probable) concomitant of this results, is a "concentrated central charge" in the atom: "Considering the evidence as a whole, it seems simplest to suppose that the atom contains a central charge distributed through a very small volume, and that the large single deflexions are due to the central charge as a whole, and not to its constituents." The central charge containing most of the atom's positive charge, invariably later become associated with a concrete structure, the atomic nucleus.
After the Rutherford model and its confirmation in the experiments of Henry Moseley and its theoretical description in the Bohr model of the atom, the study of the atom branched into two separate fields, nuclear physics, which studies the nucleus of the atom, and atomic physics which studies atom's electronic structure.
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[edit] Symbolism
Despite its inaccuracy, the Rutherford model caught the imagination of the public in a way that the more correct Bohr model did not, and has continually been used as a symbol for atoms and atomic energy, even by physicists who should know more than others. Examples of its use over the past century include:
- The logo of the United States Atomic Energy Commission, which was in part responsible for its later usage in relation to nuclear fission technology in particular.
- The flag of the International Atomic Energy Agency is a Rutherford atom, enclosed in olive branches.
- The US minor league baseball Albuquerque Isotopes' logo is a Rutherford atom, electron orbits forming the "A."
- The Unicode Miscellaneous Symbols codepoint U+269B (⚛), ATOM SYMBOL, uses a Rutherford atom.
- On maps, it is generally used to indicate a nuclear installation.
[edit] Key points of Rutherford model
- The electron clouds of the atom do not influence alpha scattering.
- A large number of the atom's charges, up to a number equal to about half the atomic mass in hydrogen units, are concentrated in very small volume at the center of the atom. These are responsible for deflecting both alpha and beta particles.
- The mass of heavy atoms such as gold is mostly concentrated in the central charge region, since calculations show it is not deflected or moved by the high speed alpha particles, which have very high momentum in comparison to electrons, but not with regard to heavy atoms (such as gold) on the whole. This suggests that much of the mass of atoms is concentrated in their centers.
[edit] Rutherford's contribution on the modern science
However, after this discovery, scientists start to realize that the atom is not ultimately a single particle but is made up of tiny subatomic particles. So they started their research to figure out the actual atomic structure which lead to Rutherford’s gold foil experiment later on and from that experiment he discovered that atoms have a positively charged nucleus in the centre, and also that it consists of mostly empty space.
Later on, scientists figured out the expected number of electrons in an atom by using the X-ray. Since, when X-ray passes through an atom, some of it gets scattered, while the rest of it passes through the atom. The X-rays loses its intensity due to scattering, and by figuring out the rate of decrease they found out the number of electrons contained in an atom. Note that the intensity also decreases due to the number of electrons in an atom.
