With a more direct method the English scientist C. T. R. Wilson has shown the paths of the α-particles by making use of the characteristic property of ions, that in damp air they attract the neutral water molecules which then form drops of water with the ions as nuclei. In air which is completely free of dust and ions the water vapour is not condensed, even if the temperature is decreased so as to give rise to supersaturation, but as soon as the air is ionized the vapour condenses into a fog. When Wilson sent α-particles through air, supersaturated with water vapour, the vapour condensed into small drops on the ions produced by the particles; the streaks of fog thus obtained could be photographed. [Fig. 19] shows such a photograph of the paths of a number of atoms. When a streak of fog ends abruptly it does not mean that the α-particles have suddenly halted, but that their velocity has decreased so that they can no longer break the molecules of air to pieces, producing ions. The paths of the β-particles have been photographed in the same way, although an electron of the β-particles has a mass about 7000 times smaller than that of a helium atom; the electron has, however, a far greater velocity than the helium atom. This velocity causes the ions to be farther apart, so that each drop of water formed around the individual ions can appear in the photograph by itself ([cf. Fig. 20]).
CHAPTER IV
THE NUCLEAR ATOM
Introduction.
We are now brought face to face with the fundamental question, hardly touched upon at all in the previous part of this work, namely, that of the construction and mode of operation of the atomic mechanism itself. In the first place we must ask: What is the “architecture” of the atom, that is, what positions do the positive and negative particles take up with respect to each other, and how many are there of each kind? In the second place, of what sort are the processes which take place in an atom, and how can we make them interpret the physical and chemical properties of the elements? In this chapter we shall keep essentially to the first question, and consider especially the great contribution which Rutherford made in 1911 to its answer in his discovery of the positive atomic nucleus and in the development of what is known as the Rutherford atomic model or nuclear atom.
Rutherford’s Atom Model.
Rutherford’s discovery was the result of an investigation which, in its main outlines, was carried out as follows: a dense stream of α-particles from a powerful radium preparation was sent into a highly exhausted chamber through a little opening. On a zinc sulphide screen, placed a little distance behind the opening, there was then produced by this bombardment of atomic projectiles, a small, sharply defined spot of light. The opening was next covered by a thin metal plate, which can be considered as a piece of chain mail formed of densely-packed atoms. The α-particles, working their way through the atoms, easily traversed this “piece of mail” because of their great velocity. But now it was seen that the spot of light broadened out a little and was no longer sharply limited. From this fact one could conclude that the α-particles in passing among the many atoms in the metal plate suffered countless, very small deflections, thus producing a slight spreading of the rays. It could also be seen that some, though comparatively few, of the α-particles broke utterly away from the stream, and travelled farther in new directions, some, indeed, glancing back from the metal plate in the direction in which they had come ([cf. Fig. 21]). The situation was approximately as if one had discharged a quantity of small shot through a wall of butter, and nearly all the pellets had gone through the wall in an almost unchanged direction, but that one or two individual ones had in some apparently uncalled for fashion come travelling back from the interior of the butter. One might naturally conclude from this circumstance that here and there in the butter were located some small, hard, heavy objects, for example, some small pellets with which some of the projectiles by chance had collided. Accordingly, it seemed as if there were located in the metal sheet some small hard objects. These could hardly be the electrons of the metal atoms, because α-particles, as has been stated before, are helium atoms with a mass over seven thousand times that of a single electron; and if such an atom collided with an electron, it would easily push the electron aside without itself being deviated materially in its path. Hardly any other possibility remained than to assume that what the α-particles had collided with was the positive part of the atom, whose mass is of the same order of magnitude as the mass of the helium atom ([cf. Fig. 21]). A mathematical investigation showed that the large deflections were produced because the α-particles in question had passed, on their way, through a tremendously strong electric field of the kind which will exist about an electric charge concentrated into a very small space and acting on other charges according to Coulomb’s Law. When, in the foregoing, the word “collision” is used, it must not be taken to mean simply a collision of elastic spheres; rather the two particles (the α-particle and the positive particle of the metal atom) come so near to each other in the flight of the former that the very great electrical forces brought into play cause a significant deflection of the α-particles from their original course.
Fig. 21.—Tracks of α-particles in the interior of matter. While 1 and 3 undergo small deflections by collisions with electrons, 2 is sharply deflected by a positive nucleus.
Rutherford was thus led to the hypothesis that nearly all of the mass of the atom is concentrated into a positively charged nucleus, which, like the electrons, is very small in comparison with the size of the whole atom; while the rest of the mass is apportioned among a number of negative electrons which must be assumed to rotate about the nucleus under the attraction of the latter, just as the planets rotate about the sun. Under this hypothesis the outer limits of the atom must be regarded as given by the outermost electron orbits. The assumption of an atom of this structure makes it at once intelligible why, in general, the α-particles can travel through the atom without being deflected materially by the nuclear repulsion, and why the very great deflections occur as seldom as is indicated by experiment. This latter circumstance has, on the other hand, no explanation in the atomic model previously suggested by Lord Kelvin and amplified by J. J. Thomson, in which the positive electricity was assumed to be distributed over the whole volume of the atom, while the electrons were supposed to move in rings at varying distances from the centre of the atom.
