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I am a little confused on who first split the atom. Wikipedia says that J.J Thomspon did with his cathode ray tube experiments. In what sense does this split the atom? But then it also says Rutherford did using alpha particles and nitrogen.

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    $\begingroup$ Usually "splitting the atom" refers to splitting its nucleus (nuclear fission), which neither Rutherford no Thomson did. They split off electrons from atoms, which technically is still "splitting" the atom, I guess. Artificial nuclear fission was first done in 1938 by Hahn and Strassmann. $\endgroup$ – Conifold Jun 5 '17 at 22:25
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    $\begingroup$ I beg to differ. The first published controlled fission experiment was Cockroft and Walton in 1932, where they showed the $^{7}Li (p,\alpha) \alpha$ reaction. Nobody said fission had to be done with neutrons - protons can do it just fine. The first observed was in Rutherford's group, the $^{14}N (\alpha,p) ^{17}O$ reaction. This was seen in 1919 and explained in 1925 (with some poor student looking at some astronomical number of cloud chamber photographs to get 8 events). $\endgroup$ – Jon Custer Jun 5 '17 at 22:46
  • $\begingroup$ I see my problem. There are different levels of "splitting". Both answers are correct and useful. The problem is really my poor understanding of the process. The comments point to in the right direction. $\endgroup$ – Sedumjoy Jun 5 '17 at 23:09
  • $\begingroup$ The phrase "splitting the atom" is a term used in popularizations that has no clearly defined technical meaning. Therefore there is no way to answer this question. $\endgroup$ – Ben Crowell Jun 6 '17 at 5:02
  • $\begingroup$ I haven't find into Wiki's entry dedicated to J.J.Thomson the phrase "splitting the atom". $\endgroup$ – Mauro ALLEGRANZA Jun 6 '17 at 12:13
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The cathode ray experiments separated an electron from an atom. This is usually not considered splitting the atom.

The discovery of alpha radiation showed that the atom could be broken up, and that nature did it as a matter of course. To 'control' the energy of the alpha particle one used different source elements. The desire to vary and control the energy of alphas, as well as to use other particles such as protons, led to the technology for high voltage accelerators.

But, the $^{14}N (\alpha,p) ^{17}O$ reaction was first observed in Rutherford's lab in March of 1919, as noted in notebooks with various observers including Marsden. Final confirmation awaited a paper by PMS Blackett, also from Rutherford's lab, in 1925, where he looked at 400,000 cloud chamber pictures to pull out the 8 cases. His results are in the Proceedings of the Royal Society, Volum 107, issue 742, pages 349-360 (Feb. 1925).

Meanwhile, by 1932 Cockroft and Walton had developed their (controllable) high voltage ion accelerator. One of the first papers from it was the $^{7}Li (p,\alpha) \alpha$ reaction, where an incident proton makes a $^{8}Be$ nucleus which splits in to two $\alpha$ particles. This was published in the Proceedings of the Royal Society, Vol 137, No. 831 229-242 (July 1, 1932).

Chadwick's paper on the discovery of the neutron was published in the June 1st 1932 issue of the Proceedings. By 1935 Chadwick was publishing things like 'Disintegration by Slow Neutrons', Mathematical Proceedings of the Cambridge Philosophical Society 31(4) 612-616 (1935). There he discusses the $^{6}Li (n,T) \alpha$, $^{10}B(n,\alpha)^{7}Li$, and $^{14}N (n,\alpha) ^{11}B$ reactions.

Otto Hahn's 1944 Chemistry Nobel Prize (which did not include Lise Meitner) was 'for his discovery of the fission of heavy nuclei'.

So: J.J. Thompson had nothing to do with splitting the atom. Similarly, atoms were known to split well before Hahn and Meitner.

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