Fig. 22.—Photograph of the paths of two α-particles
(positive helium ions).
One collides with an atomic nucleus.
The same characteristic phenomenon made evident in the passage of α-particles through substances by the investigations of Rutherford appears in a more direct way in Wilson’s researches discussed on [p. 81]. His photographs of the paths of α-particles through air supersaturated with water vapour ([see Fig. 22]) show pronounced kinks in the paths of individual particles. Thus in the figure referred to, there are shown the paths of two α-particles. One of these is almost a straight line (with a very slight curvature), while the other shows a very perceptible deflection as it approaches the immediate neighbourhood of the nucleus of an atom, and finally a very abrupt kink; at the latter place it is clear that the α-particle has penetrated very close to the nucleus. If one examines the picture more closely, there will be seen a very small fork at the place where the kink is located. Here the path seems to have divided into two branches, a shorter and a longer. This leads one at once to suspect that a collision between two bodies has taken place, and that after the collision each body has travelled its own path, just as if, to return to the analogy of the bombardment of the butter wall, one had been able to drive two pellets out of the butter by shooting in only one. Or, to take perhaps a more familiar example, when a moving billiard ball collides at random with a stationary one, after the collision they both move off in different directions. So, when the α-particle hits at random the atomic nucleus, both particle and nucleus move off in different directions; though in this case, since the nucleus has the much greater mass of the two, it moves more slowly, after the collision, than the α-particle, and has, therefore, a much shorter range in the air than the lighter, swifter α-particle. Had the gas in which the collisions took place been hydrogen, for example, the recoil paths of the hydrogen nuclei would have been longer than those of the α-particles, because the mass of the hydrogen nucleus is but one quarter the mass of the α-particle (helium atom).
The collision experiments on which Rutherford’s theory is founded are of so direct and decisive a character that one can hardly call it a theory, but rather a fact, founded on observation, showing conclusively that the atom is built after the fashion indicated. Continued researches have amassed a quantity of important facts about atoms. Thus, Rutherford was able to show that the radius of the nucleus is of the order of magnitude 10⁻¹² to 10⁻¹³cm. This means really that it is only when an α-particle approaches so near the centre of an atom that forces come into play which no longer follow Coulomb’s Law for the repulsion between two point charges of the same sign (in contrast to the case in the ordinary deflections of α-particles). It should be remarked, however, that in the case of the hydrogen nucleus theoretical considerations give foundation for the assumption that its radius is really many times smaller than the radius of the electron, which is some 2000 times lighter; experiments by which this assumption can be tested are not at hand at present.
The Nuclear Charge; Atomic Number; Atomic Weight.
It is not necessary to have recourse to a new research to determine the masses of the nuclei of various atoms, because the mass of the nucleus is for all practical purposes the mass of the atom. Accordingly, if the mass of the hydrogen nucleus is taken as unity, the atomic mass is equal to the atomic weight as previously defined. The individual electrons which accompany the nucleus are so light that their mass has relatively little influence (within the limits of experimental accuracy) on the total mass of the atom.
On the other hand, a problem of the greatest importance which immediately suggests itself is to determine the magnitude of the positive charge of the nucleus. This naturally must be an integral multiple of the fundamental quantum of negative electricity, namely, 4·77 × 10⁻¹⁰ electrostatic units, or if we prefer to call this simply the “unit” charge, then the nuclear charge must be an integer. Otherwise a neutral atom could not be formed of a nucleus and electrons, for in a neutral atom the number of negative electrons which move about the nucleus must be equal to the number of positive charges in the nucleus. The determination of this number is, accordingly, equivalent to the settling of the important question, how many electrons surround the nucleus in the normal neutral state of the atom of the element in question.
The answer to the question is easiest in the case of the helium atom. For when this is expelled as an α-particle, it carries, as Rutherford was able to show, a positive charge of two units—in other words, two electrons are necessary to change the positive ion into a neutral atom. At the same time there is every reason to suppose that the α-particle is simply a helium nucleus deprived of its electrons; it follows, therefore, that the electron system of the neutral helium atom consists of two electrons. Since the atomic weight of helium is four, the number of electrons is consequently one-half the atomic weight. Rutherford’s investigation of the deflections of α-particles in passing through various media had already led him to believe that for many other elements, to a considerable approximation, the nuclear charge and hence the number of electrons was equal to half the atomic weight. Hydrogen, of course, must form an exception, since its atomic weight is unity. The positive charge on the hydrogen nucleus is one elementary quantum, and in the neutral state of the atom, only one electron rotates about it. [Fig. 23] gives a representation of the structure of the hydrogen atom, and the structures of the two types of hydrogen ions formed respectively by the loss and gain of an electron. In the picture, the position of the electron is, of course, arbitrary, and for the sake of simplicity its path is supposed to be circular.
Fig. 23.—Schematic representation of the nuclear atom.