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2-2: The Number of Electrons in an Atom

As learned in Part 1, it turned out that an electron exists in an atom.
Are there a fixed number of electrons in each atom? The answer to this question was given by the experiment of the scattering of X rays from an atom. Let's explain briefly below.

[The Property of X Rays]
In 1895, X rays were discovered by W. C. Roentgen (Germany, 1845 - 1923), while he was doing experiment on vacuum discharge. He found that a kind of "rays" which had extremely high penetrability were emitted from a Crookes tube covered by a sheet of thick black paper, and it made some of fluorescent materials at a distant place in the room fluoresce (emit light), and it exposed photographic plates in a drawer of a desk. It was clear that this phenomena is not due to the cathode rays, because the cathode rays could not penetrate a thick paper or a glass. This "rays" were named X rays for the reason that they were unknown.
The reason why X rays come out of a Crookes tube is because the cathode rays (electrons) with a very high speed collide with glass tube or nearby metal. According to the theory of electromagnetism, when a charged particle suddenly accelerate or decelerate, it radiates an electromagnetic wave (or light). In the vacuum discharge tube, a large number of high speed electrons collide against the glass tube or metal and are enforced to stop. This is sudden deceleration. Then an intensive electromagnetic wave is emitted. This is the reason of the radiation of X rays.
The evidence that X rays are electromagnetic waves were shown by a diffraction phenomenon that occurred when it passed through a very thin slit. This phenomenon was quite similar to the case of an ordinary light. A polarization phenomenon was confirmed as well. These phenomena showed that X rays are electromagnetic waves like an ordinary light.

[Scattering of X Rays by an Atom, the Number of Electrons]
An atom is electrically neutral. We know on the other hand that an atom contains electrons which are negatively charged. Accordingly there must exist "something" with positive charge which cancels the negative charge of the electrons. The mass of electron is quite light, i.e. about 1/1800 of that of the lightest atom, the hydrogen atom. This means that the above "something" with positive charge must carry almost all of the mass of an atom. You should keep this in mind.
As mentioned above, it was clarified that X rays are electromagnetic waves. If X rays are irradiated to an atom, their electric fields exert forces on the charged objects in it (electrons or "something" in the atom) and oscillate them. An oscillating charged object which repeats acceleration and/or deceleration rapidly emits electromagnetic waves around. Thus the incident X rays are scattered in various directions as schematically shown in the following figure.
According to the theory of electromagnetism, a charged particle moving with an acceleration emits an electromagnetic wave whose strength is proportional to the square of the acceleration. The acceleration of a particle is obtained by dividing the force acting on the particle by the mass. Therefore the strength of the emitted electromagnetic wave is inversely proportional to the square of the mass. Hence the strength of the electromagnetic wave emitted from the "something" is 1/1000000 less than what is emitted from an electron, so that it is negligible. Namely, we can consider that X rays are scattered only by the electrons in the atom.

[The Number of Electrons in an Atom]
Experiments to illuminate X rays on various atoms were carried out as schematically drawn in the above figure. In these experiments, a part of the incident X rays was scattered. The remaining X rays would penetrate the atoms, and the strength of the penetrated X rays would be weakened (or decreased) by the scattered X rays. This decreasing rate depends on the number of electrons in the atom. By investigating this in detail, the number of electrons could roughly be estimated. The results told that the number of electrons in hydrogen atom is 1 and in helium 2, and so on. People have known that the number of electrons in an atom is about a half of the atomic weight in general.
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