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1-4: The Constituents of Nuclei, Summary

[The Constituents of Nuclei]
Up to the 1930s, people had thought that the atomic nucleus consists of protons and electrons. For example, they considered that the nitrogen 14 of the atomic number Z = 7 consists of 14 protons and 7 electrons. This idea appeared quite plausible, but it was made clear that it contained serious difficulties as explained on the last page.
Just after the discovery of the neutron by Chadwick, D. Ivanenko (Soviet Union) and W. K. Heisenberg (Germany, 1901-76) independently proposed an idea that "the atomic nucleus consists of protons and neutrons" (1932). This was nothing else than the first step of Nuclear Physics. Hereafter this idea has been established; namely, a nucleus of an atomic number Z is a composite many-particle system consisting of Z protons and N neutrons.
Accordingly, for example, the nitrogen 14 nucleus of the atomic number 7 consists of 7 protons and 7 neutrons. And we can easily understand that there exist some nuclei (isotopes) of the same atomic number Z but different neutron numbers N.

[The Nucleon]
The proton and the neutron have almost the same mass. Different is their electric charges. Their currently accepted properties are listed in the following Table.

From these properties, Heisenberg and E. P. Wigner (US, 1902-95) thought that these two kinds of particles are of two different states of the same particle, called "nucleon" today.
We can therefore say, in a word, that the constituent of the atomic nucleus is nucleon.

[Summary of Part 1]
Let us summarize what we have learned in Part 1.
(1) After the success of the Rutherford Model of the Nuclear Atom, the existence of the atomic nucleus was recognized for the first time.
(2) The mass of the nucleus is almost equal to that of the whole atom. Its diameter is about , which is extremely smaller than the size of the whole atom, about .
(3) Due to the discovery of the neutron by Chadwick, it was clarified that the constituents of nuclei are protons and neutrons (generically nucleons).
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