Chapter III
Constituents of the Atom
§ 1. The Electron
THE notion that matter consists of discrete particles is, as we have seen, a very satisfactory hypothesis. As opposed to the only other possible theory, that matter is continuous, the atomic theory is more successful in explaining phenomena, and it also appears to be a more natural theory, one more easily grasped. The “continuum” theory has had its supporters, however, amongst whom we may mention Goethe, besides the more serious scientific names of Mach and Ostwald. But whatever arguments there may once have been in favour of the continuous theory of matter, recent work has caused the theory to be irretrievably abandoned. But when we turn from matter to the “imponderables” such as light, heat, electricity, the case is rather different. Both heat and electricity were for a long time regarded as fluids. These fluids were regarded as imponderable and continuous. There was even a two-fluid theory of electricity according to which the two kinds of electricity, positive and negative, were manifestations of two different fluids. According to the one-fluid theory, the two kinds of electricity were manifestations of the presence of a defect or excess of the fluid. But these theories, although occasionally written about at length, were rather perfunctory. They were little more than convenient mathematical fictions. By assuming them, the mathematicians were enabled to get on with their real interests, which consisted in working out the laws according to which electrified bodies acted on one another. The whole of this early work was purely formal. Experiment had shown that the fundamental law of electrostatics was of the same form as the Newtonian Law of Gravitation. Electrified bodies were regarded as geometrical shapes carrying electric “charges” and attracting or repelling one another according to the Newtonian law. “Action at a distance” was assumed; that is to say, the change of force between electrified bodies which accompanied change of position was assumed to take place instantaneously, so that the positions and charges at a given instant gave the forces at that same instant. This was the same assumption that underlay the Newtonian Law of Gravitation, and it was open to the same objection, namely, that it made it very difficult to conceive how the action between distant bodies was propagated through the space separating them. If the notion of propagation were given up the mutual action between bodies not in contact became purely miraculous; if the notion of propagation were retained it had to be conceived as taking place with infinite velocity.
A very great advance on these conceptions was made by James Clerk Maxwell. He directed attention to the “field,” to the space separating electrified bodies, and he established mathematical equations whereby the propagation of electric and magnetic actions in space could be followed from point to point and from instant to instant. And he showed that electromagnetic effects were propagated with the velocity of light, i. e., 300,000 kilometres per second. Light itself was shown to be an electromagnetic phenomenon, and hence the theory is usually called the Electromagnetic Theory of Light. Heinrich Hertz, a brilliant follower of Maxwell, succeeded in producing electromagnetic waves some metres in length, and in showing that they could be reflected and refracted and made to behave generally in ways characteristic of light waves. Wireless telegraphy was developed directly from Hertz’s work, and electromagnetic waves were produced several kilometres in length. Thus the very important transition was made from the action of a distance-theory to the field-theory of electric and magnetic action.
In the meantime, the study of the electric charges themselves had been comparatively neglected. Certain phenomena attending the conduction of electricity in solutions had, it is true, given rise to speculations that electricity was probably atomic in constitution, but it was not until the so-called cathode rays were studied that the existence of atoms of electricity, disconnected from ordinary matter, was experimentally confirmed. The apparatus necessary to produce cathode rays consists of a glass tube in which an almost complete vacuum exists. Through the walls of this tube two metallic wires are passed, which are connected to a source generating electricity. One of these wires is terminated, on the inside of the tube, by a metallic disc. If now the potential difference between the two wires is sufficiently high (some hundreds of volts) rays emanate from the metallic disc (called the cathode) and proceed in straight lines, producing a fluorescence at the other end of the tube where they strike the glass walls. That the rays proceed in perfectly straight lines may be shown by placing an object in the path of the rays—say a cross or a circular disc—when its clear-cut shadow is thrown on the far end of the tube.
Now the fact that these rays are deflected when the tube is placed in an electric or magnetic field shows that they consist of small electrified particles in movement. Further, the nature of the deflection shows that the electric charges carried by these small particles are charges of negative electricity. The question arises: What is the nature of the small electrified particles? Are they, for instance, atoms of matter carrying electric charges? Are they, perhaps, larger than atoms? Can it be that they are smaller than atoms, that in the cathode rays we have matter existing in a sub-atomic state? Certain measurements were made which allowed this question to be answered without ambiguity. Each little electrified particle or corpuscle carried a charge e and had a mass m. The measurements did not determine either e or m directly, but they did determine the ratio e/m of these two quantities. The ratio turned out to have the extraordinary value of 1·77 × 107 (electromagnetic) units. Let us see just why this value was so extraordinary.
If X-rays or the rays emitted by radium are allowed to penetrate a gas, they have the power of enabling that gas to conduct electricity. The rays, in their passage through the gas, produce positively and negatively charged carriers of electricity. These carriers are called Ions. Now the most important characteristic of an ion is its electric charge, and an ingenious experimental method enables this charge to be determined. If air be saturated with water vapour and the air be then suddenly expanded, the resultant cooling causes a cloud of small drops of water to be formed. These drops coalesce round the tiny dust particles present in the air. If the air has been purified of dust particles it is possible for a considerable expansion to take place without the formation of a cloud of drops. It was found, however, that if ions are present they play the part of dust particles. Small drops condense round the ions and a cloud is formed. By taking suitable precautions, a single drop can be observed under the microscope. These drops fall under their own weight. Now the rate of fall of such a drop will depend on its size, its density, and on certain properties of the gas through which it is falling. The mathematical problem of determining the velocity of the drop from these other factors was solved by Sir George Stokes. Now when the drops are formed round the little electrified bodies called ions each drop carries an electric charge. If, therefore, we cause an electric force to act on the drop, say by letting the drop fall between two parallel electrified plates, we can cause the electric force to act either with or against the gravity of the drop and so either hasten or retard its descent. Knowing the rate of fall under gravity alone, and also the rate of fall under a known electric force, the actual charge carried by the drop can be calculated. In this way it was found that the smallest charge, the charge carried by a single ion, is 4·77 × 10-10 (electrostatic) units. Now the ion, besides having a charge has also, of course, a certain mass. The lightest ion known, the hydrogen-ion, which consists of a single hydrogen atom carrying the above charge, has for the ratio e/m, the ratio of charge to mass, the value 9649·4 (electromagnetic) units. Let us contrast this with the ratio 1·77 × 107 obtained for the electrified corpuscles of the vacuum tube. This latter value is more than 1800 times greater than the value for the hydrogen-ion. How is this to be explained?
We might suppose that each electrified corpuscle is an atom carrying a great many of the elementary electric charges—i. e., the smallest charge carried by a single ion. But if we suppose the corpuscles to be single atoms on which many charges are heaped, we should hardly expect the ration e/m to be always the same for every corpuscle. It would seem that there ought sometimes to be more and sometimes fewer charges. But a grave objection is that the ratio e/m for the corpuscles is quite independent of the nature of the gas of which a residuum is always left in the vacuum tube, and is quite independent of the material constituting the cathode. If the corpuscles are electrified atoms, where do these atoms come from? Atoms of different substances have different weights. How, then, does it happen that the ratio e/m always remains the same? The only possible hypothesis which explains all the facts is that the corpuscles consist of elementary charges of electricity linked to a mass about 1800 times smaller than the mass of a hydrogen atom. The corpuscles are of sub-atomic dimensions. These tiny particles are called Electrons.
Of the existence of these bodies there can no longer be any doubt. A great number and variety of phenomena are now known which point to their existence; electric currents, radioactive processes, the generation of X-rays, various optical effects, all bear witness to the existence of these sub-atomic electrified bodies. Precise measurements enable us to give the mass of an electron. It is 0·903 × 10-27 gramme. As a comparison, we give also the mass of a hydrogen atom, which is 1·650 × 10-24 gramme, a value about 1830 times greater than that of the electron. The figure giving the mass of an electron may be expressed by saying that one thousand million million million million electrons would have a mass rather less than one gramme.
But the mass of an electron, although so small, is not zero. What are we to suppose are the origin and nature of this mass? Here we are led to a very startling and novel conception. We have always supposed that only matter had mass; electricity has been classed as an “imponderable,” that is, as something possessing no mass. But this notion cannot be maintained. An electric current in a wire, for instance, is produced by the application of an electromotive force, but the current does not attain its full strength instantly when the force is applied. Similarly, when the force generating the current is suppressed the current does not instantly vanish. It shows a tendency to persist. It seems to be endowed with inertia, and inertia is a property of mass. Further, Sir J. J. Thomson showed that an electrified material sphere requires a greater force to set it in motion than if it were unelectrified. The electric charge acted as if it imparted some extra mass to the sphere. Part of the mass of the sphere could be attributed to its ordinary matter and part to its electric charge. If we regard our electrons, therefore, as small electrified spheres, how much of their mass is to be referred to their electric charge? We reach the startling conclusion that the whole of the mass of an electron is to be attributed to its electric charge. This conclusion, we must mention, is not absolutely proved. It is a very convenient and plausible assumption to make, however, and leads to a very simple conception of matter. We shall see that all atoms may be conceived as built up out of electrons, and since electrons consist of nothing but electricity, we see that we reach an electric theory of matter, where matter is held to consist of nothing but electric charges, and to have no mass except the mass that results from these charges.
The initial difficulty of this conception resides wholly in its unfamiliarity. When we become accustomed to the idea of attributing mass to an electric charge, we shall find that it has thereby acquired just the “materiality” necessary for it to figure as what we mean by matter. On the hypothesis that the mass of an electron is due wholly to its electric charge we find, assuming the electron to be a sphere, that its radius is approximately 2 × 10-13 cm., or two ten million-millionths of a centimetre. This is about 50,000 times smaller than the radius of an atom. As compared with an atom, an electron would be like a fly in a cathedral, to use Sir Oliver Lodge’s vivid image.
Although we have said that matter is built up out of electrons, we cannot suppose it to consist of nothing but such negatively electrified corpuscles as are produced in a vacuum tube. Ordinary matter is electrically neutral; it does not exist in a state of permanent negative electrification. These negative charges must therefore be somehow associated with exactly compensatory positive charges. Now the elementary positive charge of electricity, which is of the same magnitude as the elementary negative charge, is never found associated with a smaller mass than that of the hydrogen atom. The hydrogen-ion carries the same positive charge that the electron carries negative charge, but that positive charge is never found in a “dissociated” state. We shall find, indeed, that the elementary positive charge plays quite a different rôle in the constitution of matter from that played by the negatively charged electron.
§ 2. Radium
The theory we are introducing, that atoms of matter are built up out of electric charges, is magnificently illustrated by the phenomena of radioactivity. It was in 1896 that the French scientist Becquerel found that uranium salts spontaneously emitted a radiation which could, to some extent, pass through matter, whether transparent or opaque, could influence a photographic plate, and could make air and other gases conductors of electricity. Further investigation showed that other substances also had the power of emitting these radiations, and some of them, such as Radium, possessed this property in an extraordinary degree. About forty radioactive substances are known at the present day.
The question arises, What is the nature of these radiations? To answer this question the method of analysis was adopted that we have already mentioned. The radiations, if they consist of electrically charged particles, will be deflected both by a magnetic and by an electric field. Each of these deflections gives us some information about the ratio e/m of the charge to the mass of the particles, and also about the velocity V with which the particles are moving. From the information supplied by the two sets of deflections we can determine these two quantities, i. e., we can find e/m and also V. When this method of analysis was applied it was found that the radiations from radium consisted of three kinds of rays having entirely different properties. These three types are called α-, β-, and γ-rays. The γ-rays continued without deflection; it became apparent that they did not consist of electrically charged particles at all. The β-rays proved to be negatively charged, and the amounts of the electric and magnetic deflections proved that they were of exactly the same type as the streams of electrons in a vacuum tube. The α-rays behaved very differently. They turned out to be positively instead of negatively charged, and also to possess much greater masses than the β-rays. It was found that the ratio e/m for an α-particle was the same for all α-particles, from whatever radioactive substance they were obtained. This value was found to be 4823 (electromagnetic) units. Now this value is one-half the value of the ratio e/m of a hydrogen-ion. How is this to be explained? There are three possibilities. We might say that the α-particle carries a unit positive charge, the same as the hydrogen-ion, but that this charge is united with two hydrogen atoms. Or we might say that it has a unit positive charge, but attached to the atom of a new element which has twice the mass of a hydrogen atom. The other possibility supposes that we are dealing with a helium instead of with a hydrogen atom. Now the atomic weight of helium is 4, i. e., an atom of helium has four times the mass of an atom of hydrogen. If, therefore, we assume that an α-particle is an atom of helium, we must suppose it to be carrying two unit positive charges. To distinguish between these possibilities, it obviously becomes necessary to measure the actual charge carried by an α-particle. This was done by direct experiment. The number of α-particles emitted from a source can be counted directly, and the total charge they carry can also be measured. The charge carried by a single particle can thus be determined. Its value proved to be twice the value of the unit charge. The hypothesis, therefore, that an α-particle consists of a helium atom carrying two units of positive charge is justified by experiment. Thus we see that radioactive elements can emit positively charged helium atoms. This conclusion was directly confirmed by Rutherford and Royds, who collected α-particles in an evacuated space, and, on causing an electric discharge to pass, obtained the spectrum of helium.
The α-particles are easily absorbed in their passage through matter. They can be stopped by an ordinary sheet of writing-paper. The velocity of the α-particles varies with the nature of the radioactive substance which emits them, but, speaking approximately, we may say that their velocity is about 2 × 109 centimetres per second. This is much less than the velocity of light, which is about 3 × 1010 centimetres per second.
The β-particles, on the other hand, sometimes have a velocity which is within one per cent. of that of light itself. It is evident that the radioactive process, whatever it may be, must be tremendously energetic to produce these high velocities. But although, on the average, the velocity of a β-particle is ten times that of an α-particle, the latter, owing to its greater mass, has greater momentum and energy. The β-particle, in its passage through matter, is readily deflected, and is sometimes deflected through a very considerable angle. It may, in fact, be turned so much out of its path as to emerge again on the same side that it entered. For this reason, it is difficult to say just what penetrative power the β-radiations have, but we may say, roughly, that they are about 100 times as penetrating as α-rays.
The γ-rays, as we have said, do not consist of electrified particles at all. They have the character of extremely minute light-waves, although they do not, of course, cause visibility. They always accompany the emission of β-particles from radioactive substances and their penetrative powers are considerable, being about 100 times greater than those of the β-rays. We shall learn more of their properties in the section discussing X-rays.
Now what are we to suppose is happening during these radioactive processes, attended, as they are, by so great an expenditure of energy? The theory now universally accepted is that the atoms of a substance manifesting radioactivity are actually disintegrating. The atoms of such substances are unstable and are breaking up. The electrified particles, the α- and the β-rays, are shot out by the atom in its process of disruption. This process of disruption cannot be hastened or retarded by any artificial means. It takes place, for a given substance, at the same rate whether the temperature be that of liquid air or of red-hot iron. The atom, on breaking up, becomes transformed into a different atom, having a different atomic weight. The second atom may be, in its turn, unstable, and disintegrate into yet another atom. In this way, before a disrupting atom settles down into a stable condition, it may pass through quite a long series of states—transforming itself from one substance into another. Thus uranium, with an atomic weight of 238, passes through a long series of changes to reach stability finally as lead, with an atomic weight of 206. This fact has led to a method of determining the ages of some uranium minerals. The amount of lead produced by a known weight of uranium in a given time can be determined, and the examination of the amount of lead present in a uranium mineral enables a maximum age for the mineral to be calculated. The assumption is that the lead present in the mineral has resulted from the transformations of the uranium. In this way a mineral of the Carboniferous period has been found to have an age of 340 million years, and a pre-Cambrian mineral to have an age of 1640 million years. We have seen, also, that the α-particles expelled during some radioactive processes are really helium atoms. Now helium is only found in large quantities in old minerals rich in uranium or thorium (another radioactive substance), and if the helium be supposed to have resulted from the disintegration of these substances the age of the mineral can be calculated. But this value will be a minimum value of the age of the mineral, since we must suppose that some of the gas has been lost. In this way figures have been obtained for different geological strata ranging from 8 million to 700 million years.
§ 3. X-rays
It was in 1895 that Röntgen discovered that invisible radiations of some kind passed through the cathode tube and that these radiations had great penetrative power.
He discovered that many substances, opaque to ordinary light, are transparent to these Xrays, as they were called. The rays arose at the point where the stream of electrons within the tube struck the glass walls, and radiated from these points of impact in all directions. Ordinary deflection experiments showed that the X-rays did not consist of electrified particles, but were a form of wave motion. Now there are two forms of wave motion, longitudinal and transversal. If a rope, held by the hand at one end and permanently fastened at the other, be shaken, a wave motion is propagated along it. The peculiarity of this motion is that each point of the rope, as the disturbance reaches it, moves in a direction at right angles to the direction of propagation of the wave. Such a wave motion is said to be transversal. The wave motion which constitutes ordinary light is known to be of this character. But there is another form of wave motion where each point of the medium set in motion moves to and fro in the direction of propagation of the wave. Sound consists of waves of this type. Such waves are called longitudinal. The question arose whether the waves constituting X-rays were longitudinal or transverse. It was not till ten years later that this point was definitely settled, and it was shown that X-rays, like ordinary light, consist of transverse waves, but waves which are, compared with light-waves, of exceedingly small wave-length. The waves constituting X-rays are about 10,000 times smaller than those constituting ordinary light. It is this extraordinarily small wave-length that gives them their great penetrative power. Not all X-rays have the same wave-length; their wave-length depends on the manner in which they are generated. The shorter the wave-length the greater the penetrative power or “hardness” of the rays.
We have said that X-rays are produced by the sudden stoppage of the electrons on striking the wall of a cathode tube. The sudden alteration in velocity creates the wave disturbance called X-rays, and the greater the velocity of the electrons the greater is the “hardness” or penetrative power of the resultant waves. In modern cathode tubes, it is usual, instead of allowing the stream of electrons to strike the glass tube, to direct the stream on to a piece of metal having a high melting point, such as platinum. This piece of metal, which receives the impact of the electrons, is called the anti-cathode. Now the very important discovery was made that the X-rays which result from the bombardment of the anti-cathode are of two kinds. The first kind is due merely to the stoppage of the electrons, as we have seen. But besides these, the anti-cathode, under the influence of the bombardment, sends out X-rays of its own. This second group of X-rays is of particular wave-lengths, the same for the same substance, but different for different substances. The X-rays so emitted are, in fact, entirely characteristic of the substance that emits them. For a given element these X-rays remain the same whether the element is isolated or whether it is in chemical combination with others. It is evident, therefore, that these X-rays manifest some property which belongs to the atoms of the element. If we compare the X-rays characteristic of elements of different atomic weights we find that the heavier the atom the shorter the wave-lengths of the characteristic X-rays. The “hardness” of the X-rays proper to an element increases as the atomic weight of the element increases. We shall find that this group of X-rays, those proper to the substance itself, throws much light on the structure of the atom.
The γ-rays, emitted by radioactive substances, resemble X-rays in being waves of very small wave-length and consequently great penetrative power. They are much smaller even than X-rays, for γ-rays can be obtained about twenty times smaller even than the hardest X-rays. But that they are essentially similar to X-rays there can be no doubt, and it must be supposed that they have a similar origin. We have seen that X-rays are produced by sudden alterations in the velocity of a moving electron. We have also seen that the β-rays of radioactive substances are electrons moving with very high velocities, and we have further noted that γ-rays always attend the expulsion of β-rays. It is very reasonable to suppose, therefore, that the γ-rays are produced by the β-rays in their escape from the atom. But we cannot go into this matter more closely until we know more about the constitution of the atom.
Chapter IV: The Structure of the Atom