Thomson atomic theory
Department of Energy, Nuclear Physics and Reactor Theory. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.
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Lamarsh, Introduction to Nuclear Reactor Theory, 2nd ed., Addison-Wesley, Reading, MA (1983). The most important experiment, which led to the need to formulate another model, was the Geiger-Marsden Experiments (also called the Rutherford gold foil experiment). This model could not explain several problems. The Thomson model of the atom was the useful predecessor of the correct Rutherford model of the atom, especially the solar-system-like Bohr model of the atom. The plum pudding model usefully inspired his student, Ernest Rutherford, to further explore the composition of atoms. Thomson proposed that the corpuscles were distributed in a uniform sea of positive charge, in which they are scattered like “the plums in a pudding.” This fact explains the overall neutral charge of the atom. The Plum pudding model represented an attempt to consolidate the known properties of atoms at the time:ġ) Electrons are negatively-charged particlesĪccording to this model, an atom consists of a sphere of positive matter within which electrostatic forces determine the positioning of the negatively charged corpuscles. It must be noted that this model was proposed before discovering the atomic nucleus. Thomson in 1904 and is known as the Plum pudding model or the Thomson model of the atom. The original model of an atom, based on the discovery of the electron, was proposed by J. Electrons are located in an electron cloud, which is the area surrounding the nucleus of the atom.īut the original idea was different. Their rest mass equals 9.109 × 10 −31 kg ( 510.998 keV/c 2) (approximately 1/1836 that of the proton). According to the current state of knowledge, the electrons are negatively charged (-1e), almost massless particles that account for most of the size of the atom. Thompson won the Nobel Prize in physics for establishing the existence of electrons. Thomson thus concluded that atoms are divisible and that the corpuscles are their building blocks.
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It was evident that electrons are a component of all neutral atoms, the counterbalancing positive charge being carried by one or more of their other constituent parts. Thomson originally called these particles “ corpuscles.” The Irish physicist George Johnstone Stoney proposed the name electron for these particles. For example, by the action of ultra-violet light on metals, by heating metal wires, or by the ionizing action of X-rays. He showed that these charged particles could be obtained from just about any material. He calculated he must have bodies much smaller than atoms and a very large value for their charge-to-mass ratio. Thomson showed that cathode rays were composed of previously unknown negatively charged particles.
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Eventually, experimentation into electricity and radioactivity indicated that particles of matter smaller than the atom did exist. All of the results of chemical experiments during this time indicated that the atom was indivisible. This fact explains the overall neutral charge of the atom.įor almost 100 years (from Dalton’s A New System of Chemical Philosophy), atoms were thought to be the smallest possible division of matter. According to the plum pudding model, an atom consists of a sphere of positive matter within which electrostatic forces determine the positioning of the negatively charged corpuscles.