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A.P. Chemistry
Period: 5
September 3, 2012
Summary of the Models of an Atom form Democritus to Rutherford
It was of this primary matter, modified in various ways, that allother things were created. Democritus expanded the idea to state that matter was composed of small particles called "atoms" that could be divided no further. These atoms were all composed of the sameprimary matter with the only differences between them being their size, shape and weight. The differences in these characteristics explained the differences in the properties of the matter around us.Unfortunately for Democritus and mankind in general, his ideas were largely ignored for the next 2000 years. While others had proposed very similar theories, John Dalton is usually credited withdeveloping the first coherent atomic theory. In addition to helping to explain the laws of chemical reactivity, Dalton's theory also helped further the concept of atomic weights. Work by Amedeo Avogadro,Stanilao Cannizarro and J.J. Berzulius in the first half of the 19th century led to the accurate determination of the atomic weights of the elements and chemical formulas of compounds made from them.In 1886, Eugen Goldstein pierced several holes in the cathode of a cathode ray tube and noticed a stream of particles in the space behind the cathode. He called these streams canal rays. Usingpowerful magnetic fields in 1898, Wilhelm Wien verified that these particles were positively charged and had masses comparable to a hydrogen atom. Additionally, he concluded that, as the particles were notall influenced by the magnetic field to the same extent, they must have different masses. These two results were in stark contrast to the normal cathode ray that was composed solely of electrons.During the 1920's and early 30'sthe center of this kind of work was in Germany. Later, in the 1940's it shifted to the United States. Had Rutherford pursued his proposal about the existence of the...