I reasearched about the history of the discovery of neutrons and couldn't find who found out that isotopes are a change in the number of neutrons. Can you please tell me who it was that made this discovery?

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    $\begingroup$ See also History of science and mathematics SE site hsm.stackexchange.com Btw isotopes are a change in the number of neutrons is rather sloppy formulation. Because such a change is a neutron capture or ejection and not an isotope. $\endgroup$
    – Poutnik
    Nov 13 '20 at 23:53
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    $\begingroup$ Frederick Soddy, I guess. But it is the other way around. He figured that the things that are chemically similar, but have different neutrons should be called something, and called them isotopes. He is the one who coined the term. $\endgroup$ Nov 14 '20 at 0:07
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    $\begingroup$ Chemistry knew about elements with different atomic masses in their atoms well before they knew neutrons were the reason for this. Neutrons were only discovered in the 1930s at which point a good explanation for the observations could be finally made. So who deserves credit is, perhaps, unclear as the observation existed long before the explanation. $\endgroup$
    – matt_black
    Nov 14 '20 at 1:05
  • $\begingroup$ @IvanNeretin Indeed it makes sense that before people could count the neutrons in an atom, they were not aware of isotopes as the difference is so subtle and isolation so difficult. $\endgroup$
    – Greg
    Nov 14 '20 at 2:33
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    $\begingroup$ @Greg actually Deuterium was discovered the year before neutrons were so they did know about isotopes before neutrons $\endgroup$
    – A.K.
    Nov 16 '20 at 20:04

From Pauling's "General Chemistry" (ref. 1):

In 1921 the American chemist W.D. Harkins described nuclei as being built of protons and neutrons; he used the word "neutron" for a hypothetical particle with mass equal to that of the proton and with no electric charge. Ernest Rutherford made a similar suggestion during the same year. In 1932 the discovery of tne neutron was reported by the English physicist James Chadwick (born 1891).

The original article in which Harkins (ref. 2) reported this hypothesis is linked by the Wikipedia:


  1. Linus Pauling, General Chemistry, second edition, 1970, p. 102
  2. Harkins, William (1921). "The constitution and stability of atomic nuclei. (A contribution to the subject of inorganic evolution.)". Philos. Mag. 42 (249): 305.

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