postulate of rutherford atomic model
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Postulates of Rutherford atomic modelbased on observations and conclusions. An atom is composed of positively charged particles. ... Atoms nucleus is surrounded by negatively charged particles called electrons. The electrons revolve around the nucleus in a fixed circular path at very high speed.
Rutherford model
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LAST UPDATED: Sep 5, 2019 See Article History
Alternative Titles: Rutherford atomic model, nuclear atom, planetary model of the atom
Rutherford model, also called nuclear atom or planetary model of the atom, description of the structure of atoms proposed (1911) by the New Zealand-born physicist Ernest Rutherford. The model described the atom as a tiny, dense, positively charged core called a nucleus, in which nearly all the mass is concentrated, around which the light, negative constituents, called electrons, circulate at some distance, much like planets revolving around the Sun.
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Rutherford model
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The nucleus was postulated as small and dense to account for the scattering of alpha particles from thin gold foil, as observed in a series of experiments performed by undergraduate Ernest Marsden under the direction of Rutherford and German physicist Hans Geiger in 1909. A radioactive source capable of emitting alpha particles (i.e., positively charged particles, identical to the nucleus of the helium atom and 7,000 times more massive than electrons) was enclosed within a protective lead shield. The radiation was focused into a narrow beam after passing through a slit in a lead screen. A thin section of gold foil was placed in front of the slit, and a screen coated with zinc sulfide to render it fluorescent served as a counter to detect alpha particles. As each alpha particle struck the fluorescent screen, it would produce a burst of light called a scintillation, which was visible through a viewing microscope attached to the back of the screen. The screen itself was movable, allowing Rutherford and his associates to determine whether or not any alpha particles were being deflected by the gold foil.
Rutherford gold-foil experiment
Rutherford gold-foil experiment
The Rutherford gold-foil experimentDiagram of physicist Ernest Rutherford's gold-foil experiment. In 1909 Rutherford disproved Sir J.J. Thomson's model of the atom as a uniformly distributed substance. Because only very few of the alpha particles in his beam were scattered by large angles after striking the gold foil while most passed completely through, Rutherford knew that the gold atom's mass must be concentrated in a tiny dense nucleus.
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Most alpha particles were observed to pass straight through the gold foil, which implied that atoms are composed of large amounts of open space. Some alpha particles were deflected slightly, suggesting interactions with other positively charged particles within the atom. Still other alpha particles were scattered at large angles, while a very few even bounced back toward the source. (Rutherford famously said later, “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.”) Only a positively charged and relatively heavy target particle, such as the proposed nucleus, could account for such strong repulsion. The negative electrons that balanced electrically the positive nuclear charge were regarded as traveling in circular orbits about the nucleus. The electrostatic force of attraction between electrons and nucleus was likened to the gravitational force of attraction between the revolving planets and the Sun. Most of this planetary atom was open space and offered no resistance to the passage of the alpha particles.
The Rutherford model supplanted the “plum-pudding” atomic model of English physicist Sir J.J. Thomson, in which the electrons were embedded in a positively charged atom like plums in a pudding. Based wholly on classical physics, the Rutherford model itself was superseded in a few years by the Bohr atomic model, which incorporated some early quantum theory.
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The Editors of Encyclopaedia Britannica
This article was most recently revised and updated by Adam Augustyn, Managing Editor.
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