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2-6: What is the Atomic Nucleus?

As discussed in the preceding page, 2-5, the success of the Rutherford model of the nuclear atom taught us that an atom consists of the positively charged massive nucleus and the surrounding light electrons.
An atom contains Z electrons whose total charge is -Ze, so that the positive charge +Ze cancelling this negative charge of the electrons is considered to be concentrated into the nucleus.
It has been explained in detail in the previous pages that the total mass of the electrons in an atom is extremely smaller than the total mass of the atom. This means that almost all mass of an atom is carried by the nucleus.
Then, what is an atomic nucleus made of? What are the constituents of the nucleus? What is the structure of the nucleus?

[The Size of a Nucleus]
Rutherford's formula of angular distribution shown in the preceding page is obtained by assuming that the charge of the nucleus is concentrated on a point, i.e. a point charge. However, even if the charge spreads over an area narrower than the minimum of the closest approach shown in the following figure, Rutherford's formula must still hold. Therefore, if the experimental data are well reproduced by Rutherford's formula, the nuclear radius of that atom is understood to be smaller than the minimum of the closest approach.
Let us look back the trajectories of Rutherford scattering shown in the following figure, where the red arrow at the central part denotes the minimum of the closest approach.

In the experiment of the alpha particle scattering for the target of a copper foil, the resultant data are well fit to Rutherford's formula up to the scattering angle The incident energy in this case is E = 5.3 MeV, and the atomic number of copper is Z = 29. The minimum of the closest approach is given by

Consequently, we can say that the radius of the copper nucleus is less than Comparing this nuclear radius with that of an atom, the size of a nucleus is less than 1/5000 of that of an atom. You must be able to realize how small the atomic nucleus is.

[Artificial Transmutation of Elements, Proton]
In 1919, Rutherford succeeded in changing nitrogen into oxygen artificially. This was the first artificial transmutation of elements.
As part of the alpha particle scattering experiment, Rutherford placed a radioactive material in nitrogen gas as shown in the following figure, and he observed the scintillations on the fluorescent substance. He found very bright scintillations whose directions were not considered to be those by alpha particles.

Similar phenomena in various kinds of elements, for example, boron, fluorine, neon, sodium, phosphorus, sulfur, argon, and so on were observed later.
These phenomena were considered that high speed alpha particles collide with atoms and knock on an "unknown" particle to rush out with high energies.
Photographs of these phenomena were successfully taken with Wilson's cloud chamber. By putting the cloud chamber in a magnetic field and analyzing the results precisely, it was clarified that these "unknown" particles are the same as the hydrogen ions.
Since a hydrogen atom consists of an electron and a nucleus, a hydrogen ion is the hydrogen atom having lost an electron. This is nothing else than the nucleus of hydrogen atom, which was named proton by Rutherford.
This phenomenon was considered as the following process:

This means that the nucleus of the nitrogen atom is disintegrated under the strong forces exerted by a high speed alpha particle, the hydrogen nucleus (proton) is liberated, and the rests are rearranged to form the nucleus of oxygen. Namely, it implies that Rutherford carried out the first artificial transmutation of element; this is a realization of alchemists' dream in a modern form.

[The Constituents of Atomic Nuclei]
Thus, it turned out that the proton is one of the constituents of atomic nuclei. However, it was not clear whether a nucleus is made only of protons. It appears quite reasonable to consider that the helium nucleus (alpha particle) consists of four protons and two electrons. On the early stage, people thought that the nucleus is made of protons and electrons. It has however been clarified that this idea has a serious inconsistency.
After the discovery of the neutron by J. Chadwick (UK, 1891 - 1974), it has finally elucidated that the true constituents of nuclei are protons and neutrons.
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