Editing Nuclear physics (section)

=== James Chadwick discovers the neutron ===
{{Main|Discovery of the neutron}}
In 1932 Chadwick realized that radiation that had been observed by [[Walther Bothe]], [[Herbert Becker (physicist)|Herbert Becker]], [[Irène Joliot-Curie|Irène]] and [[Frédéric Joliot-Curie]] was actually due to a neutral particle of about the same mass as the proton, that he called the [[neutron]] (following a suggestion from Rutherford about the need for such a particle).<ref>{{cite journal|last=Chadwick|first=James|author-link1=James Chadwick|title=The existence of a neutron|journal=[[Proceedings of the Royal Society A]]|year=1932|volume=136|number=830|pages=692–708|doi=10.1098/rspa.1932.0112 |bibcode=1932RSPSA.136..692C|doi-access=free}}</ref> In the same year [[Dmitri Ivanenko]] suggested that there were no electrons in the nucleus — only protons and neutrons — and that neutrons were spin {{frac|1|2}} particles, which explained the mass not due to protons. The neutron spin immediately solved the problem of the spin of nitrogen-14, as the one unpaired proton and one unpaired neutron in this model each contributed a spin of {{frac|1|2}} in the same direction, giving a final total spin of 1.

With the discovery of the neutron, scientists could at last calculate what fraction of [[binding energy]] each nucleus had, by comparing the nuclear mass with that of the protons and neutrons which composed it. Differences between nuclear masses were calculated in this way. When nuclear reactions were measured, these were found to agree with Einstein's calculation of the equivalence of mass and energy to within 1% as of 1934.