Efter at den engelske fysiker [[James Chadwick]] opdagede [[neutron]]en i 1932,<ref>Chadwick announced his initial findings in: {{cite journal|author=J. Chadwick|doi=10.1038/129312a0|title=Possible Existence of a Neutron|year=1932|page=312|issue=3252|volume=129|journal=Nature|url=http://web.mit.edu/22.54/resources/Chadwick.pdf|bibcode=1932Natur.129Q.312C|access-date=22. december 2017|archive-date=27. august 2018|archive-url=https://web.archive.org/web/20180827202416/http://web.mit.edu/22.54/resources/Chadwick.pdf|url-status=live}} Subsequently he communicated his findings in more detail in: {{cite journal|doi=10.1098/rspa.1932.0112|author=Chadwick, J.|year=1932|title=The existence of a neutron|journal=Proceedings of the Royal Society A|volume=136|issue=830|pages=692–708|url=http://www.chemteam.info/Chem-History/Chadwick-1932/Chadwick-neutron.html|bibcode=1932RSPSA.136..692C|access-date=22. december 2017|archive-date=18. december 2017|archive-url=https://web.archive.org/web/20171218191523/http://www.chemteam.info/Chem-History/Chadwick-1932/Chadwick-neutron.html|url-status=live}}; and {{cite journal|doi=10.1098/rspa.1933.0152|author=Chadwick, J.|year=1933|title=The Bakerian Lecture: The neutron|journal=Proceedings of the Royal Society A|volume=142|issue=846|pages=1–25|bibcode=1933RSPSA.142....1C}}</ref> undersøgte [[Enrico Fermi]] og hans kolleger i [[Rom]] virkningerne af at bombardere uran med neutroner i 1934.<ref>E. Fermi, E. Amaldi, O. D'Agostino, F. Rasetti, and E. Segrè (1934) "Radioattività provocata da bombardamento di neutroni III," ''La Ricerca Scientifica'', vol.&nbsp;5, no.&nbsp;1, pages&nbsp;452–453.</ref> Fermi konkluderede at hans eksperimenter havde frembragt nye grundstoffer med 93 og 94 protoner, som hans team navngav [[ausonium]] og [[hesperium]]. Imidlertid var ikke alle overbeviste af Fermis analyse af resultaterne. Den tyske kemiker [[Ida Noddack]] foreslog på skrift i 1934 at i stedet for at der frembragtes en ny tungere kerne, "kan man forestille sig at kernen bryder op i mindre dele."<ref>{{cite journal|author=Ida Noddack|year=1934|page=653|issue=37|title=Über das Element 93|volume=47|journal=Zeitschrift für Angewandte Chemie|url=http://www.chemteam.info/Chem-History/Noddack-1934.html|doi=10.1002/ange.19340473707|access-date=22. december 2017|archive-date=11. december 2017|archive-url=https://web.archive.org/web/20171211223522/http://www.chemteam.info/Chem-History/Noddack-1934.html|url-status=live}}</ref><ref>[http://www.astr.ua.edu/4000WS/TACKE.html Tacke, Ida Eva] {{Webarchive|url=https://web.archive.org/web/20130304083613/http://www.astr.ua.edu/4000WS/TACKE.html |date= 4. marts 2013 }}. Astr.ua.edu. Retrieved on 2010-12-24.</ref> Der blev ikke fulgt op på Noddack's konklusioner på den tid.
Kemi