Rutherford's model of the atom was no doubt appealing in part because it seemed to mimic the larger world people could see. As appealingly simple as it is, Rutherford's model contained a serious flaw. Rutherford's vision of the atom as a kind of "solar system" in miniature is what most people probably carry around in their heads. However, the developing techniques for measuring relative atomic masses seemed to contradict this, suggesting instead that less than 100 electrons would be more likely.įinally in the years between 19 Ernest Rutherford and his students Hans Geiger and Ernest Marsden performed the famous gold-foil experiment in which the nuclear arrangement of the atom was discovered. Thomson's calculations indicated that to remain stable atoms might need to have hundreds, if not thousands, of electrons. His conclusion has become known as the "plum pudding" model of the atom and was the first to include sub-atomic particles with charges. With the ingredients needed to keep matter neutral, Thomson speculated and calculated just how it might all hold together. Thomson's work (and the work of others) with cathode ray tubes eventually identified a much more massive positive particle apparently common to all matter. Based on this evidence Thomson proposed the first atomic model with sub-atomic particles. Thomson's dilemma: how could matter containing electrons be neutral and where was all the mass? Other experiments with discharge tubes suggested the existence of a positive particle with much greater mass (the proton). With this value, and the charge/mass ratio that Thomson had measured earlier it was possible to calculate the mass of the electron. His eventual conclusion was that the charges on the drops were either equal to or multiples of one number which he decided was the charge of a single electron. Millikan used his apparatus to make many measurements of the effect of the electric field vs. Thus there are obviously particles smaller than atoms. Using the smallest charge obtained and Thomson's charge/mass ratio the electron mass is roughly 1/2000 the mass of the lightest atom. In 1909 Robert Millikan used the classic oil drop experiment to determine the charge on these particles. He was able to measure the charge/mass ratio of these particles and found this to be the same regardless of what gas was in the tube or what metal the electrodes were made from. Thomson demonstrated in 1897 that the rays consist of a stream of negatively charged particles which he called electrons. From careful experiments with cathode ray tubes J.J. Thus these tubes have been called cathode ray tubes. Thomson (seen here in his lab with one of his many hand-made cathode ray tubes)Īssociated with the flow of electricity in such a tube are rays which originate from the negative electrode-the cathode. The first evidence for sub-atomic particles came from experiments with the conduction of electricity through gases in sealed glass tubes at low pressures.įigure : J.J. An eminent organic chemist of the time, Adolf Kolbe, said in 1877, "Dalton's atoms are no more than stupid re table-tapping and supernatural explanations." Some contemporaries of Dalton remained unimpressed.
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