Popular Science Monthly/Volume 87/August 1915/The Constitution of Matter and the Evolution of the Elements
|THE CONSTITUTION OF MATTER AND THE EVOLUTION OF THE ELEMENTS|
By Professor Sir ERNEST RUTHERFORD, F.R.S.
UNIVERSITY OF MANCHESTER
SPECULATIONS as to the constitution of matter have occupied an important place in the development of scientific knowledge. The idea that all matter was composed of minute particles called atoms was put forward long ago by the Greek philosophers, and was advanced again with varying degrees of confidence by philosophic men at the dawn of the scientific age. For example, Newton suggested that matter was composed of atoms which were likened to "hard massy balls" while Robert Boyle regarded a gas to consist of atoms which were in brisk motion. The first definite formulation of the atomic theory as a scientific hypothesis was given by Dalton of Manchester in 1803 in order to explain the combination of atoms in multiple proportion. The necessity of distinguishing between the chemical atom and the chemical molecule was soon recognized, while the famous hypothesis of Avogadro that equal volumes of all gases at the same temperature and pressure contain equal numbers of molecules still further extended the usefulness of the theory. The whole superstructure of modern chemistry has been largely reared on the foundations of the atomic theory. The labors of the chemist have revealed to us the presence of more than eighty distinct types of elements, each of which has a characteristic atomic weight, and in most cases sufficiently distinct physical and chemical properties to allow of its separation from any other element by the application of suitable methods.
It has been generally assumed that all the atoms of one element are identical in shape and weight, and until a few years ago were supposed to be permanent and indestructible. The close study of the variation of chemical properties of the elements with atomic weight led Frankland and Mendelief to put forward the famous "periodic law," in which it was shown that there was a periodic variation in the chemical properties of elements when arranged in order of increasing atomic weight. This empirical generalization has exercised a wide influence on the development of chemistry, and the periodic law has been considered by many to indicate that all the atoms are composed of some elementary substance or protyle. It is only within the last few years that our knowledge of atoms has reached a stage to offer a reasonable explanation of this remarkable periodicity.
Time does not allow me to more than refer in passing to the important contributions of Le Bel and van' t Hoff to the structure of complex molecules, and the arrangements of the atoms in space, which has exercised such a wide and important influence on the development of organic chemistry.
While the chemist was busy disentangling the elements, determining their relative atomic weights and studying their possible combinations, the physicist had not been idle. The idea that a gas consisted of a large number of molecules in swift but irregular movement had been tentatively advanced at various times to explain some of the properties of gases. These conceptions were independently revived and developed in great detail by the genius of Clausius and Clerk Maxwell about the middle of the last century. On their theory, now known as the kinetic or dynamical theory of gases, the molecules of a gas are supposed to be in continuous agitation, colliding with each other and with the walls of the containing vessel. Their velocity of agitation is supposed to increase with temperature, and the pressure is due to the impact of the molecules of the gas on the walls of the enclosure. This theory was found to explain in a simple and obvious way the fundamental properties of gases, and has proved of great importance in molecular theory. The idea that atoms must be in brisk and turbulent motion is strongly supported by the well-known property of the inter-diffusion of gases and also of liquids, and in recent years has received practically a direct and concrete proof from the study of a very interesting phenomenon included under the name "Brownian Motion." The English botanist, Brown, in 1827 discovered that small vegetable spores immersed in a liquid appeared to be in continuous motion when viewed with a high power microscope. This motion of small particles in liquids was at first supposed to be a result of temperature disturbances, but at the close of the last century the Brownian movement was shown to be a fundamental property of small particles in liquids. The whole question has been investigated in recent years with great ability and skill by Perrin. He examined in detail the state of equilibrium and of motion of minute particles in suspension in liquids. The excursions due to the Brownian movements depend mainly on the size of the particles, although influenced to some extent by the nature of the liquid. Small spheres of the size required can be produced by a variety of methods. One of the simplest used by Perrin is to allow a solution of pure water to pour slowly out of a funnel under an alcoholic solution of gamboge or mastic. An emulsion is formed where the layers meet which consists of a great number of minute spheres. When these particles are viewed in a strong light with a high power microscope, they all exhibit the characteristic Brownian movement, i. e., the particles dart to and fro in irregular and tumultuous fashion, and never appear to be at rest for more than a moment. The motions of these small particles under a microscope irresistibly convey the impression that they are hurled to and fro by the action of mysterious forces resident in the solution. Such a result is to be anticipated if the molecules of the liquid are themselves in rapid though invisible tumultuous motion of the kind outlined on the kinetic theory. The particle is very large compared with the molecule, and it is bombarded on all sides by great numbers of molecules. Occasionally the pressure due to the bombardment is for a moment greater on one side of the particle than on the other, and the particle is urged forward, until a new distribution of impacts hurls it in another direction. In fact, the movement of these particles has been found to conform exactly with that predicted by the molecular theory.
It would take too long to discuss the remarkable conclusions that Perrin has reached from a study of the distribution and motion of small particles. The particle which may be an agglomeration of many millions of molecules, behaves in many respects like the much smaller molecule. A great number of particles in a liquid do not distribute themselves uniformly under gravity, but the numbers decrease with height according to the same law as the gases in our atmosphere.
On the kinetic theory, we thus have strong evidence for believing that the atoms of matter, whether in the solid, liquid or gaseous form, are in continuous agitation and irregular motion. The velocity of agitation decreases with lowering of temperature, and at the lowest attainable temperature the motion has either ceased or become very small. It is well known that under suitable conditions, the same type of matter can exist in three distinct forms, solid, liquid and gas. If we take the ordinary air of the room, it can be turned into a clear liquid under certain conditions of temperature and pressure, and this liquid can be frozen solid by still further lowering of the temperature. The most refractory gas of all, helium, has only recently been shown to conform with the behavior of all other gases, and to pass into a liquid at a temperature only a few degrees removed from absolute zero. The remarkable changes in appearance and physical qualities of an element in passing from one state to another is a matter of common knowledge—but it is not for that reason very easy of explanation. These changes are believed to be connected with the average distance which separates one atom or molecule from the other and their rapidity of motion. In the gas or vapor form, the molecules are, on an average, so far apart that their mutual attractions are relatively unimportant. With lowering of temperature, the distance and rapidity of motion of the molecules diminish until under certain conditions, the attraction of the molecules for one another predominates, resulting in a much closer packing, and the appearance of the liquid form. The molecules, however, still retain a certain freedom of motion, but this is diminished with lowering of the temperature until at a certain stage the molecules form a tighter grouping, corresponding to the solid state where the freedom of motion of the individual molecules is much restricted. In order to account for the resistance of solids to compression or extension, it has been supposed that the force between molecules is attractive at large distances but repulsive at small distances. While we are able to offer a general explanation of the passage of an element from one state to another, a complete explanation of such phenomena will only be possible when we know the detailed structure of the atoms and the nature and magnitude of the forces between them.
While the kinetic theory of gases has proved very successful in explaining the fundamental properties of gases, its strength, and at the same time its weakness, lies in the fact that in most cases it is unnecessary for the explanation to know anything of the structure of the atom or molecule, or of the forces between them. In some investigations, in order to explain some of the more recondite properties of gases, assumptions have been made of definite laws of force between the molecules, but no very definite or certain results have so far been achieved in this direction. It should, however, be pointed out that the kinetic theory afforded us for the first time with a satisfactory method of estimating approximately the dimensions of molecules and the actual number in a given weight of matter. As the recent development of science has provided us with more certain methods of estimation of these important quantities, we shall not enter further into the question at present.
There is another very striking form that matter sometimes assumes, which has always attracted much attention and which has recently emerged into much prominence. It is well known that the majority of substances under suitable conditions form crystals of definite geometrical form, which is characteristic of the particular atoms or groups of atoms. The great variety of crystal forms that are known have all been classified as belonging to one or more of the 230 forms of point symmetry which are theoretically possible. While considerations of symmetry are a sufficient guide to the classification of crystals, they offer no explanation of the definite architecture of the crystal nor of the nature of the forces that cause the atoms or molecules to arrange themselves in such definite geometric patterns. We are inevitably led to the conclusion that the atoms of the crystal are arranged according to a definite system, which is characteristic of the particular crystalline form, and the unit of structure is repeated indefinitely with continued growth of the crystal. In fact, if we had no other evidence, the crystalline form of matter would itself point to the necessity of an atomic structure of matter. While many attempts have been made to explain the grouping of the atoms in a crystal, there has been on the whole little success with the exception, possibly, of Pope and Barlow's theory that the atoms take up the positions of closest packing, the dimensions assigned to the atom depending on a quantity connected with its chemical valency. It is only within the last year that a new and powerful method of attack of this problem has been developed, largely through the experiments of Professor Bragg and his son, W. L. Bragg. On account of the definite ordering of the atoms in a crystal, it acts like an almost perfect optical grating, only in three dimensions, where the grating space is exceedingly small—in most cases about one hundred millionth of a centimeter. Laue showed that when Röntgen rays passed through a crystal, definite interference patterns were observed. This result was of great importance, as it showed that Röntgen rays must consist of very short transverse waves akin to those of light. Bragg showed that the reflection, or rather diffraction, of Röntgen rays incident on the face of a crystal, afforded a very simple method of determining the wave length of the bright lines generally present in an X-ray spectrum. By a study of the position and intensity of the spectra
Fig. 1. Arrangement of Atoms in a Rock Salt (NaCl) Crystal, White Circles represent Sodium Atoms, Black Chlorine.
in different orders thrown by the crystal, it was possible to examine in detail the structure of the crystal, and to deduce the grating space, i.e., the distance between successive planes of atoms. The subject is so large and the discovery of this method so recent, that so far only a few of the typical crystals have been examined, but in these cases we are able to obtain most positive evidence of the grouping of the atoms in the crystal. The results indicate that the atom and not the molecule is the unit of the crystal structure. Consider the structure of the simple cubic crystal of rock salt (sodium chloride). The structure of the crystal deduced by Bragg is shown in Fig. 1. The sodium atoms are marked by black spheres, the chlorine atoms by white spheres. The simplicity of the crystal architecture is obvious, for all the atoms are equi-distant. The structure of the diamond is more complicated but it is one of great interest, for all the atoms in these cases are of one kind, carbon. The structure found by Bragg is seen in Fig. 2 A.
Fig. 2a. Arrangement of Carbon Atoms in a Diamond.
The atoms are all equi-distant, but the general arrangement differs markedly from that of rock salt. It is seen that each carbon atom is linked with four neighbors in a perfectly symmetrical way, while the linking of six carbon atoms in a ring is also obvious from the figure. The distance between the planes containing atoms is seen to alternate in the ratio 1:3. This variation of the grating space is brought out clearly
Fig. 2b. Cubical Arrangement of Carbon Atoms in a Diamond.
from the study of the spectra, and is an essential feature of the structure of the diamond. The cubical arrangement is shown by turning the model so that the lines joining the atoms are vertical and horizontal (see Fig. 2B).
Now that we have a method of determining the arrangement and distances apart of the atoms in a crystal, the next step will be to examine the intensity and type of forces which are brought into play to keep the atoms in equilibrium and relatively fixed in their places. It is to be expected that the atoms are able to move to and fro about their position of equilibrium, and this is indicated by the effect of lowering the temperature of the crystal; for the intensity of the diffraction spectra increases as the amplitude of motion of the atom diminishes. The sharpness of the diffraction spectra suggests that the atoms are not only arranged at definite distances from one another but that each atom is orientated in a definite position with regard to its neighbor.
While varieties of crystals are known of all degrees of hardness, the work of Lehmann has brought to light the unexpected existence of crystalline arrangement in some liquids. These liquid crystals are best shown in certain complex organic substances at a temperature slightly above their melting point, and they are only observable in the liquid by the patterns and colors developed when polarized light passes through them. These crystals are mobile like a drop of oil in a solution and can be squeezed into a variety of patterns. Such results would indicate that the molecules of the liquid have a tendency to arrange themselves in ordered patterns, although it is difficult to understand how the freedom of relative motion that is supposed to characterize a liquid can contemporaneously exist with an ordered arrangement of some of the constituent molecules.
We will now direct our attention to another type of phenomenon which ultimately promises to throw much light on the detailed structure of the atom. When the light from an incandescent vapor or gas is passed through a prism or reflected from a grating, it is resolved and gives a characteristic spectrum consisting of a number of bright lines. By suitable methods, the wave-length of these radiations can be determined with great accuracy. Each of these lines represents a definite and characteristic mode of vibration of the atom, and from the exceeding complexity of the spectra of many of the heavy elements, we are forced to conclude that an atom can vibrate in a great variety of ways. When the meaning of the dark lines in the solar spectrum was correctly interpreted, we were enabled at one stride to extend our methods of observation to the sun and the furthest fixed stars. It was soon recognized that atoms of the same element always vibrated the same way under all conditions. It was found, for example, that hydrogen atoms in the earth vibrated in exactly the same way as the same atoms in a distant star. The important bearing of this result on the structure of atoms was pointed out by Clerk Maxwell in his well-known address on "Atoms and Molecules" before the British Association at Bradford in 1873, from which it is interesting to quote the following.
Each molecule therefore throughout the universe bears impressed upon it the stamp of a metric system as distinctly as does the metre of the Archives at Paris, or the double royal cubit of the temple of Karnac.
No theory of evolution can be formed to account for the similarity of molecules, for evolution necessarily implies continuous change, and the molecule is incapable of growth or decay, of generation or destruction.
None of the processes of nature, since the time when nature began, have produced the slightest difference in the properties of any molecule. We are therefore unable to ascribe either the existence of the molecules or the identity of their properties to any of the causes which we call natural.
On the other hand, the exact equality of each molecule to all others of the same kind gives it, as Sir John Herschel has well said, the essential character of a manufactured article, and precludes the idea of its being eternal and self-existent.
While there is no doubt that an atom of an element in the earth or in a star vibrates in identical fashion under the same physical conditions, it is now known that the frequency of vibration of an element is not the exact constant that was at first supposed. It is altered to a slight extent by motion of the source, by change of pressure, and by the application of magnetic and electric fields. The apparent change of frequency of vibration with the motion of the source relative to the observer has proved an invaluable method for studying the motion of stars in the line of sight, while the displacement of the lines of hydrogen in the sun has in the hands of Professor Hale and his assistants proved of great power in throwing light on some of the physical conditions that exist in that distant body. It has been found that there is order and system in the great complex of modes of vibration of an atom, and that many of the lines can be arranged in definite series whose rates of vibration are connected by simple and definite laws. It is only within the last year or two that we have been able to form some idea of the origin of these spectra and the meaning of a spectral series. The fact that the lightest and presumably the simplest atom known, viz., hydrogen, gives a very complicated light spectrum was at first, and quite naturally, believed to indicate that the hydrogen atom must be a very complex structure. We shall see later, however, that the hydrogen atom is believed to have an exceedingly simple structure, and that the complexity of the spectrum is to be ascribed rather to a complexity in the laws of radiation.
We have seen that the study of the spectrum led Maxwell to conclude not only that the atoms were identical in weight and form but that they were the only permanent and indestructible units in this changing world. The apparent identity of the spectrum under all conditions certainly strongly supported such a view at that time. It was believed that if some of the atoms were changing, it would be shown by a gradual alteration of their modes of vibration, i.e., of the spectrum. It was left to the beginning of this century to show the fallacy in this deduction, and to bring undoubted evidence that some elements at least are undergoing spontaneous transformation with the appearance of new types of matter giving a new and characteristic spectrum. This question will be discussed later in some detail.
Before, however, considering the bearing of radioactive phenomena on the structure of the atom, I must refer to a discovery which has exercised a most profound influence on the development of physics in general and on our ideas of the structure of atoms. Sir William Crookes long ago found that when an electric discharge was passed through a vacuum tube at very low pressures, a peculiar type of radiation appeared, known as the "cathode rays." This radiation appeared to be projected from the cathode in straight lines, and, unlike light, was deflected by a magnet. These rays excited strong phosphorescence in many substances in which they fell, and also produced marked heating effects. Crookes concluded that the cathode rays consisted of a stream of negatively charged particles moving at high speed. The general properties of this radiation appeared so remarkable that Crookes concluded that the material constituting the cathode stream corresponded to a "new or fourth state of matter." After a controversy extending over twenty years, the true nature of these rays was finally independently shown in 1897 by the experiments of Weichert and Sir J. J. Thomson. They proved, as Crookes had surmised, that the rays consisted of a stream of negatively charged particles travelling with enormous velocities from 10,000 to 100,000 miles a second, depending on the potential applied to the vacuum tube. In addition, it was found that the mass of the particle was exceedingly small, about 1/1800 of the mass of the hydrogen atom—the lightest atom known to science. These results were soon confirmed and widely extended. These corpuscles, or electrons, as they are now termed, were found to be liberated from matter not only in an electric discharge but by a variety of other agencies; for example, from a metal on which ultra-violet light falls, and also in enormous numbers from an incandescent body. Radium and other radioactive substances were found to emit them spontaneously at much greater speeds than those observed in a vacuum tube. It thus appeared that the electrons must be a constituent of the atoms of matter and could be released from the atom by a variety of agencies. This idea was much widened and strengthened by the investigations of Zeeman and Lorentz, who showed that the radiation of light must be mainly ascribed to the movements of electrons of the same small mass within the atom.
It does not fall within the scope of my address to outline the very important consequences that followed in many directions from this fundamental discovery of the independent existence of the electron and its connection with matter. It was found by Kaufmann that the mass of the electron was not a constant but increased with its speed, and from this result it was deduced that the electron was an atom of disembodied or condensed electricity occupying an exceedingly small volume, whose mass was entirely electrical in origin.
Unit of Electricity
I should mention here one important consequence that has followed from these discoveries. From the laws which control the passage of electricity in conducting solutions, Faraday recognized that there must be a close connection between the atom of matter and its electrical charge. Maxwell and Helmholtz suggested that the results were simply explained by supposing that electricity was atomic in nature. This conclusion is now definitely established, and the positive charge carried by the hydrogen atoms in the electrolysis of water is believed to be the fundamental unit of electrical charge. This charge is equal to and opposite to the charge carried by the electron. Any charge of electricity, however small or large, must be expressed by an integral multiple of this fundamental unit of electricity. The actual value of this unit charge has been measured by a great variety of methods and with concordant results. One of the most detailed and accurate investigations of this important constant has been made by Professor Millikan, of the University of Chicago.
Objections to the Atomic Theory
We have so far implicitly assumed that the great majority of scientific men now regard the atomic theory not only as a working hypothesis of great value but as affording a correct description of one stage of the sub-division of matter. While this is undoubtedly the case to-day, it is of interest to recall that less than twenty years ago there was a revolt by a limited number of scientific men against the domination of the atomic theory in chemistry. The followers of this school considered that the atomic theory should be regarded as a hypothesis, which was of necessity unverifiable by direct experiment and should, therefore, not be employed as a basis of explanation of chemistry. This point of view was much strengthened by the recognition of the power of thermodynamics in affording a quantitative explanation of the changes of energy in chemical reactions without the assumption of any definite theory of the constitution of matter. This tendency advanced so far that text-hooks of chemistry were written in which the word atom or molecule was taboo, and chemistry was based instead on the law of combination in multiple proportion. At that time, it did undoubtedly appear that there was little, if any, hope of finding a concrete proof of the validity of the atomic hypothesis, or of detecting by its effects a single atom of matter or a single electron, for it was known that the smallest fragment of matter visible under a high power microscope must still contain many millions, or even billions, of atoms.
The march of science has, however, been so rapid in this direction that we have been able in recent years to show in a definite and concrete way the independent existence of atoms and also of electrons in rapid motion.
Counting Atoms and Electrons
We shall first of all consider the method devised by Rutherford and Geiger for detecting and recording the effects of single alpha particles from radium. At this stage, it is unnecessary to enter into details of the nature of the transformations occurring in radioactive matter. It suffices to say here that the atoms of a radioactive substance are unstable and occasionally break up with explosive violence. In many cases, the explosion is accompanied by the ejection of a charged body, called the alpha particle, with a velocity of about 10,000 miles a second. These alpha particles are known from other investigations to consist of charged atoms of the rare gas helium. The presence of these rays is simply shown by the marked phosphorescence they set up in certain substances. I have here a fine glass tube which was filled about a week ago in Manchester with purified emanation released from about one fifth of a gram of pure radium. In the interval of its journey across the Atlantic, the activity of the emanation has decayed to about one quarter of its original value. The glass walls of the tube are made so thin—about 1/100 millimeter—that the alpha rays are able to escape freely into the surrounding air. They produce a small phosphorescence in the walls of the glass tube which is just visible in the darkened room. On bringing near, however, a screen covered with zinc sulphide, a brilliant phosphorescence is observed which increases in intensity as we approach the tube. Similar effects are seen to be produced in this crystal of willemite, while the crystal of kunzite is seen to be translucent and emit a ruddy light. This phosphorescence of zinc sulphide and willemite is due mainly to the alpha rays, and from the present emanation tube about 5,000,000,000 of these particles are projected each second.
In their passage through air or other gas, the alpha particles produce from the neutral molecules a large number of negatively charged particles called ions. The ionization due to the alpha particles can be readily measured by electrical methods, and it can be shown that the effect to be expected from a single alpha particle is much too small to detect except by very refined methods. In order to overcome this difficulty, Rutherford and Geiger employed a method of magnifying automatically several thousand times the electric effect due to an alpha
particle. The general arrangement of the original apparatus is seen in Fig. 3.
A few of the alpha rays from a radioactive substance passed along an exhausted tube E through an opening D covered with thin mica into the detecting tube AB. The latter contained a central insulated electrode B connected with an electrometer, and the pressure of the gas inside was adjusted to a few centimeters of mercury. The tube B was connected with the negative pole of a battery of about 1,500 volts, the other pole being earthen. The potential was adjusted so that a spark was on the point of passing between A and B. Under such conditions, the ionization due to an alpha particle passing along the detecting vessel is magnified several thousand times by collision of the negative and positive ions with the neutral molecules.
The entrance of an alpha particle into the detecting vessel is then signified by a sudden ballistic throw of the electrometer needle, and the number of particles entering the vessel in a given time can be counted by observing the throws. The amount of active matter and its distance from the opening were adjusted so that three to five alpha particles entered the opening per minute. The following table illustrates the results obtained:
|Number of Throws||Magnitude of Successive|
Throws, Scale Divisions
|1st minute||4||11, 12, 10, 11|
|2d minute||3||10, 11, 8|
|3d minute||5||10, 9, 13, 8, 12|
|4th minute||4||18*, 8, 12|
|5th minute||3||10, 6, 10|
|6th minute||4||9, 10, 12, 11|
|7th minute||2||10, 11|
|8th minute||3||11, 13, 8|
|9th minute||4||8, 20|
|10th minute||3||8, 12, 14, 6|
|Average per minute, 3.5.||Average throw, 10 divisions.|
It will be seen that the number of throws varies from minute to minute. This is to be expected since the chance of an alpha particle entering the opening is governed by the ordinary laws of probability. It will be seen that two throws, marked by asterisks, are much larger than the others. These were due to the passage of two alpha particles through the opening within a short interval. This was readily seen from the motion of the spot of light reflected from the electrometer needle. As the needle was moving slowly near the end of its swing caused by one alpha particle, a second impulse due to the entrance of another was communicated to it.
By this method, the number of alpha particles expelled from one gram of radium per second was determined. Of course only a minute fraction of the alpha particles was actually counted, but the total number was deduced on the assumption, verified by experiment, that the alpha particles on an average were expelled equally in all directions. In this way, one gram of radium in equilibrium was found to expel the enormous number of alpha particles each second.
Another interesting result followed from these experiments. It has long been known that the alpha particles produce a marked phosphorescence in crystalline zinc sulphide. When examined by a lens, the light is found not to be uniform but exhibits a very beautiful scintillating effect. By counting the number of scintillations due to the alpha particles, it was found that each scintillation was produced by the impact of a single alpha particle. It is thus seen that two distinct methods, one electrical and the other optical, are available for detecting and counting single alpha particles, i. e., single atoms of matter. This is only possible because the atoms are in swift motion and expend their great energy of motion in ionizing the gas or in producing luminosity in zinc sulphide.
Still another simple method was devised later. Kinoshita first showed that a single alpha particle produced a detectable effect on a photographic plate which was observable under a microscope. A number of experiments have been made by Reinganum, Makower, and Kinoshita to examine the effect of single alpha particles on a photographic plate. If a fine needle point coated with a trace of radioactive matter rests on the surface of the film, the plate on development shows a number of distinct trails radiating from the active point. Each of these trails results from the action of a single alpha particle. A beautiful photograph of this kind (magnification about 300) obtained by Kinoshita is shown in Fig. 4. It appears that each alpha particle makes a certain number of the grains, through which it passes, capable of development.
The use of an ordinary electrometer is not very suitable for counting alpha particles by the electric method, since the time of swing of the electrometer needle is fairly long, and accurate counting can be made when only a few alpha particles enter the detecting vessel per minute. This difficulty can be got over by the use of a string electrometer in
Fig. 4. Photographic Effect due to Alpha Particles from a Central Point.
which the moving system consists of a fine silvered quartz fiber suspended between two charged parallel plates and viewed with a high power microscope. The entrance of an alpha particle is shown by a sudden movement of the fiber, and if the current is allowed to leak away through a suitable resistance, the fiber returns to the position of rest in a small fraction of a second. The movement of the fiber can be recorded
Fig. 5. Photographic Record on String Electrometer of Entrance of Alpha Particles into the Detecting Vessel.
photographically on a moving film, and it is possible in this way to count accurately the number of particles, even if several thousand enter the detecting vessel per minute.
Examples of such photographic records, obtained by Rutherford and Geiger, are shown in Fig. 5. The vertical movements of the fiber from the horizontal line are due to the entrance of alpha particles, and it is seen how clearly the detailed movements of the fiber are registered. In some cases, one alpha particle follows another so rapidly that the fiber has not time to come to rest in between, and this is shown by the saw-like appearance of some of the peaks in the photograph. It will be noticed also that while the heights of most of the deflections are nearly the same, in a few cases the deflections are nearly twice as great as the normal. This is due to the nearly simultaneous entrance of two alpha particles into the vessel. Although the photographic film moved at a constant rate, it is seen that the throws due to the alpha particles are distributed very irregularly along it. A close examination of such records shows that variations of this kind are in accord with the ordinary laws of probability.
During this year, Dr. Geiger has found a still more sensitive detector for counting alpha particles. The arrangement, which is very simple, is shown in Fig. 6. A fine sharply pointed needle ends about
Fig. 6. Geiger's Detector of Individual Alpha and Beta Particles.
one centimeter from the opening O, where the alpha particles enter. If the outer brass tube be charged positively to about 1,000 volts, and the needle connected with a string electrometer, it is found that the entrance of an alpha particle produces a very great deflection of the fiber. So sensitive is this method, that Geiger has found that individual beta particles can easily be detected and counted by its aid. This is very remarkable when it is remembered that the ionization effect
Fig. 7. Record with String Electrometer, Upper Record for Beta Particles. Lower for Alpha Particles.
due to a beta particle is on the average not more than 1/100 of that due to an alpha particle.
A photographic record of the entrance of beta particles into the detecting vessel is shown in Fig. 7. The upper record is for beta particles and the lower for alpha particles. I am indebted to Dr. Geiger for this photograph. It is seen that the effect of a beta particle is just as marked and as definite as for an alpha particle with the old form of detector. We are thus in a position not only to count single atoms of matter but also to detect the presence of a single electron in swift motion, although the mass of the latter is exceedingly small compared with that of the lightest atom.
I would now very briefly direct your attention to some results, which to my mind not only completely prove the hypothesis of the atomic structure of matter but allow us at once to calculate the number of atoms in a given weight of matter with the minimum amount of assumption. We have seen that by direct counting it has been found that alpha particles are expelled per second from one gram of radium in equilibrium with its rapidly changing products. Now it has been definitely shown, by methods I need not discuss here, that each alpha particle consists of a helium atom carrying two unit positive charges. Since the alpha particle, when it has lost its charge, becomes a neutral helium atom, we should expect to find that helium would be produced by radium at a definite rate. This is found to be the case, and it is not difficult to determine by actual measurement the volume of helium formed by a known quantity of radium in a given time. It has been found that one gram of radium in equilibrium produces each year 156 cubic millimeters of helium at standard pressure and temperature. Now the number of alpha particles expelled per year per gram is , giving rise to 156 cubic millimeters of helium; each of these alpha particles is an atom of helium, and consequently the number of atoms of helium in one cubic centimeter of that gas at normal pressure and temperature is .
It appears to me that no more direct and convincing proof could be obtained of the atomic structure of matter or of the number of atoms forming a given weight or volume of helium; for the number of separate constituents are counted and the volume of the resulting gas is measured. The value so obtained is in good accord with measurements based on entirely different data of various kinds.
It is somewhat remarkable that while the study of radioactive phenomena has clearly indicated that the atom is not always permanent and indestructible, it has at the same time supplied the most convincing proof of the actual reality of atoms, and has provided some of the most direct methods of determining the values of atomic magnitudes.
Tracks of Swift Atoms and Electrons
We have seen how it is possible to detect single alpha and beta particles and to count their number. We will next consider a most remarkable experimental method not only for detecting such particles but of following in detail the effects produced by them in their passage through a gas. C. T. R. Wilson showed many years ago that the positively and negatively charged ions produced in a gas by the passage of alpha and beta and X rays possessed a remarkable property. When air, for example, saturated with water vapor is suddenly expanded, the air is rapidly cooled and the water tends to deposit on any nuclei present. C. T. R. Wilson showed that in dust-free air, the ions produced by external radiations become nuclei for the condensation of water upon them when the cooling by expansion was sufficiently great. Under such conditions, each ion becomes the center of a visible globule of water, and the number of drops formed is equal to the number of ions present.
C. T. R. Wilson later perfected this method to show the trail of a single alpha or beta particle in passing through the gas; for each of the
Fig. 8. Tracks of Alpha Particles from Central Points (C. T. R. Wilson's Method).
ions produced by the flying particle becomes a visible drop of water by the sudden expansion. By suitable arrangements, the trails of the individual particle can be photographed, and the pictures obtained show with remarkable fidelity and detail the ionizing effects produced in the passage of alpha and beta particles or X rays through gases.
Fig. 8 shows the tracks of the alpha particles shot out from a small fragment of radium. The number of ions produced per centimeter in the gas by the alpha particle is so great that the trail of drops shows as a continuous line. The alpha particles are seen to radiate in straight lines from the active point, and have a definite range in air—a characteristic property discovered by Bragg many years ago. The next photograph (Fig. 9) shows a magnified image of these trails. It is seen that the tracks are generally quite straight, but in a few cases there is a sudden bend near the end. The significance and causes of these sudden deviations in the rectilinear paths of the alpha particles will be discussed later.
A radioactive substance like radium emits not only alpha particles but beta particles which are electrons in very swift motion. These beta particles are generally far more penetrating than the alpha rays, but produce a much smaller number of ions per centimeter of their path
Fig. 9. Magnified Track of Alpha Particles (Wilson).
through a gas. In Fig. 10 is seen the track of a swift beta particle crossing the expansion chamber. It will be observed that the path is not straight but tortuous, due to the marked scattering of the particle by collisions with the atoms of matter in its path. Although the trail
is clearly defined, the density corresponding to the number of drops per centimeter is much smaller than for the alpha particle. In fact by magnifying still further small portions of the track, the individual ions, or rather the drop formed round each ion produced by the beta particle, are clearly visible. In this way, it is obviously possible to count directly the number of ions produced in any length of the path.
These beautiful photographs thus not only bring out clearly that alpha and beta particles are definite entities but show with great perfection the actual path of the particles in traversing matter. The next photograph (Fig. 11) shows the effect of passing a pencil of
Röntgen rays through the expansion chamber. It is believed that these rays do not ionize the gas directly but indirectly through the slow speed electrons which are liberated by some of the atoms acted on by the radiation. These electrons are not nearly so swift as some of those emitted by radium, for they are only able to transverse a few millimeters of air before being stopped. The photograph brings out clearly these effects, and shows the tortuous path of a beta particle resulting from collisions with the atoms. Such scattering effects become more marked the slower the velocity of ejection of the beta particle.
Transformation of Matter
While the discovery of the independent existence of the electron as a constituent of the structure of atoms gave a great impetus to the study of atomic structure, it was soon found that the removal or addition of an electron from an atom did not appear to cause a permanent transformation of the atom; for no evidence has yet been obtained that the passage of an electric current through a gas or metal is accompanied by a permanent alteration of the atoms of matter through which the current passes, although there is little doubt the current is carried in part at least by the electrons liberated from the atoms.
The first definite evidence of the transformation of matter was obtained from a study of the processes occurring in radioactive substances. The writer and Mr. Soddy in 1903 put forward the theory that the radiations from active matter accompanied a veritable transformation of the atoms themselves. The correctness of this theory as an explanation of radioactive phenomena is now generally accepted. As an illustration of these processes, consider the transformation of the radioactive element uranium. The series of substances which arise from the transformation of uranium are shown clearly in the diagram (Fig. 12). The best known of these elements is radium, which will be
taken as a typical example of a radioactive substance. Radium differs from an ordinary element in its power of spontaneously expelling alpha particles with very great speed. This property is ascribed to an inherent instability which is not manifest in the atoms of ordinary elements. A small fraction of the radium atoms—about one in 100,000 million—break up each second with explosive violence expelling a fragment of the atom—the alpha particle—with very great speed. The residue of the atom is lighter than before and becomes the atom of an entirely new substance, which is called the radium emanation. The atoms of the latter are far more unstable than those of radium, for half of them break up in 3.85 days, while half of the radium atoms break up in about 2,000 years. After the loss of an alpha particle, an atom of the emanation changes into an atom of a new substance radium A, which behaves as a solid. Radium A is very unstable, half of it breaking up in 3 minutes with the emission of an alpha particle, and gives rise to radium B. The latter differs from the substances already mentioned in the nature of its radiation, for it emits only beta rays but no alpha rays. Notwithstanding this fact, it is transformed according to the same law as an alpha ray substance, and gives rise to an entirely distinct element, radium C. In the transformation of the latter, not only are swift alpha rays emitted but also beta rays of great speed. There is some evidence, however, that the substance called radium C is complex, and that the alpha and beta rays arise from two distinct substances.
The successive substances arising from radium C are radium D, radium E and radium F. The two former, like radium B, emit only beta rays; the latter, known generally as polonium, emits only alpha rays. It is believed that the sequence of changes ends with the transformation of radium F, which is supposed to change into the well-known non-radioactive element lead.
According to the transformation theory radium, like all other radioactive products, must be regarded as a changing element, but one whose rate of transformation is very slow compared with its successive products. Boltwood showed experimentally that radium is half transformed in about 2,000 years, and a quantity of radium would practically have disappeared as such in 100,000 years. In order to account for the continued existence of radium in the earth, it is necessary to suppose that it is steadily produced from some other element. Boltwood showed that the parent substance is a radioactive element called ionium, which is itself derived from the transformation of uranium. A quantity of ionium, entirely freed from radium, will grow radium at a slow but constant rate. The primary element of the ionium-radium series is uranium, which we can calculate should be half transformed in 5,000 million years—a period probably long compared with the age of many of the minerals in which uranium is found.
The complete sequence of changes in the uranium-radium series is shown in the diagram. The nature of the radiation and the half period of transformation are added for each element. In addition to uranium, there are two other radioactive elements, thorium and actinium, which are transformed with the appearance of a number of new substances. The time at my disposal, however, is too short to discuss these changes in detail. Thorium is known to be a primary element whose radioactive life is even longer than uranium, but actinium is believed to be a branch descendant from some point of the uranium series, and is thus to be regarded as a product of that element. In all, thirty-four of these radioactive substances have been discovered, and the position of each in the three main radioactive series has been determined.
Each of these new substances is to be regarded as a distinct chemical element in the ordinary sense, but differs from ordinary stable elements in the spontaneous emission of special radiations which accompanies the disintegration of the atoms. The radioactive substances are thus transition elements which have a limited life and which carry within themselves the seeds of their own destruction. Not only are these transition elements distinguished by their types of radiation but also by their distinct physical and chemical properties. The extraordinary differences in properties which sometimes exist between a product and its parent substance are well illustrated by the comparison of radium and its product, the emanation. Radium is a solid element of atomic weight 226, which has chemical properties allied to barium but is capable of separation from it. The emanation is a heavy monatomic gas of atomic weight 222, which by its absence of chemical properties is allied to the well known group of rare gases, helium, argon, neon, xenon and krypton. In some cases, the elements show almost identical physical and chemical properties with those of known elements, although they differ from them in their atomic weight and radioactivity. For example, radium B appears to be identical in ordinary chemical and physical properties with lead although its atomic weight, 214, is quite distinct from lead, 207. The probable explanation of this, at first sight, remarkable identity will be discussed later.
It is of interest to note that in the majority of cases a radioactive element breaks up in only one way which is characteristic for all the atoms of that element, and gives rise to only one new product. The work of Fajans and Marsden, however, has clearly shown that in the case of radium and the corresponding products in the thorium and actinium series, the atoms break up in two distinct ways and give rise to two distinct radioactive elements. It has already been pointed out that actinium is in reality one of these side or branch products. It is supposed that uranium breaks up in two distinct ways, the smaller fraction giving rise to actinium. The evidence, however, on this point, is not yet complete.
The radioactive elements are in some respects more interesting and important than stable elements, for, in addition to the ordinary physical and chemical properties, they possess the radioactive property which allows us to study the mode and rate of transformation of their atoms.
It may be asked what is the essential difference between radioactive changes and ordinary chemical changes. In the radioactive changes we are not dealing with the dissociation of molecules into atoms but an actual disruption of the chemical atom. The disintegration of any given element appears to be a spontaneous and uncontrollable process which, unlike ordinary chemical changes, is quite unaffected by the most drastic changes in temperature or by any other known physical or chemical agency.
The radioactive changes differ entirely from chemical changes not only in the peculiar character of the emitted radiations but also in the enormous emission of energy. It can be simply shown that the energy emitted from a radioactive substance which expels alpha particles is several million times greater than the energy emitted from an equal weight of matter in any known chemical reaction. This emission of energy is mainly to be ascribed to the conversion of the energy of motion of the swift alpha and beta particles into heat, and is thus in a sense a secondary effect of the radiations. The enormous emission of energy is most simply illustrated by considering the case of the radium emanation together with its swiftly changing products, radium , radium and radium . The heating effect of a given volume or weight of this gas has been accurately determined. From the data, it can be calculated that one pound weight of the emanation would emit heat energy initially at the rate of 23,000 horse power. The rate of emission decreases with the time, falling successively to half value after intervals of 3.85 days. During the life of the emanation the total energy emitted corresponds to an engine working at 128,000 horse power for one day. Such a quantity of emanation would be an enormously concentrated source of power, for the total energy emitted is many million times greater than for an equal weight of the most powerful known explosive.
The emission of energy from radioactive substances does not controvert the law of the conservation of energy; for the energy is derived from the atom itself where it exists in kinetic or potential form. We shall see later that the atom is believed to consist of a large number of positively and negatively charged particles which are collected in a very small volume and held together by intense electrical forces. Such an idea of atomic structure involves the necessity of a large store of energy resident in the individual atom. The great emission of energy from a radioactive substance like the emanation illustrates in a striking way the enormous reservoir of energy that must exist in the atoms themselves; for there is every reason to believe that an equivalent amount of energy is present in the atoms of the common heavy elements. This store of energy ordinarily does not manifest itself and is not available for use. It is only when there is a drastic rearrangement of the atom resulting from an atomic explosion that part of this store of energy is liberated.
It must be borne in mind that the processes occurring in radioactive matter are spontaneous and uncontrollable. There is at present no evidence to indicate that we shall be able in any way to influence radioactive changes. We are at present only able to watch and investigate this remarkable phenomenon of nature without any power of controlling it. In a recent book, H. G. Wells has discussed in an interesting way some of the future possibilities if this great reservoir of energy resident in the atoms were made available for the use of man. This will only be possible on a large scale if we are able in some way to alter the rate of radioactive change and to cause a substance like uranium, or thorium, to give out its energy in the course of a few hours or days instead of over a period of many thousands or millions of years. The possibility, however, of altering the rate of transformation of radioactive matter, or of inducing similar effects in ordinary matter, does not at present seem at all promising.
We have seen that in recent years a number of methods have been devised for determining with precision the actual weight of any atom of matter. If it be assumed that in the solid state the atoms, or molecules, of matter are in close contact, it is a simple matter to deduce the diameter of the atom. This varies slightly for different atoms, but on an average comes out to be about one hundred-millionth of a centimeter. It is necessary, however, to be cautious in speaking of the diameter of the atom. The term "diameter of the sphere of action" of the atom is preferable, for it is not at all certain that the actual atomic structure is nearly so extensive as the region through which the atomic forces are appreciable.
Even before the discovery of the electron, the general idea had been suggested that the atom was an electrical structure composed of negatively and positively charged particles held in equilibrium by electrical forces. Such ideas had been proposed and developed by Larmor and Lorentz in order to explain the electrical and optical properties of the atom. The proof that the negative electron was an independent unit of the structure of the atom gave a great impetus to the formation of more concrete ideas on atomic structure. There was one important difficulty, however, that arose at the outset. While negative electricity had been shown to exist in independent units of very small apparent mass, the corresponding unit of positive electricity was never found associated with a mass less than the atom of hydrogen. All attempts to show the existence of a positive electron of small mass, which is a counterpart of the negative electron, have resulted in failure, and it seems doubtful whether such a positive electron exists. The rôle played by positive electricity in the atom was thus a matter of conjecture. In a paper called "Æpinus Atomized," the late Lord Kelvin considered an atom to consist of a uniform sphere of positive electrification, throughout which negative electricity was distributed in the form of discrete electrons. In order to make such an atom electrically neutral, it is, of course, necessary that the positive charge should be equal and opposite to the charge carried by the electrons. This idea of the structure of the atom was taken up and developed with great mathematical skill by Sir J. J. Thomson. He investigated the constitution of atoms containing different numbers of electrons, and showed that such model atoms possessed properties very similar to those shown by the actual atoms. The Thomson atom proved for many years very useful in giving a concrete idea of the possible structure of the atom, and had the great advantage of being amenable to calculation.
The rapid advance of science in the last decade has provided us with new and powerful methods of attack on this problem, and has allowed us to distinguish to some extent between various theories of atomic structure. One of these methods depends on the study of the deflection of swiftly moving bodies like alpha and beta particles in their passage through matter. It is found that these rays are always scattered in their passage through matter, i. e., a narrow pencil of rays opens out into a diffuse or scattered beam. The alpha and beta particles move so swiftly that they are actually able to pass through the structure of the atom and are deflected by the intense forces within the atom. Geiger first drew attention to a very unexpected effect with alpha particles. When a pencil of alpha rays falls on a thin film of gold, for example, the great majority of the particles pass through with little absorption. A few, however, are found to be so scattered that they are turned back through an angle of more than a right angle. Taking into consideration the great energy of motion of the alpha particle, such a result is as surprising as it would be to a gunner if an occasional shot at a light target was deflected back towards the gun. It was found that these large deflections must result from an encounter with a single atom. The occasional sudden deflection of an alpha particle is well illustrated in one of the later photographs of the trail of an alpha particle obtained by Mr. C. T. R. Wilson, and shown in Fig. 13. It is
seen that the rectilinear path of the particle suffers two sharp bends, no doubt resulting in each case from a single close encounter with an atom. In the sharp bend near the end a slight spur is seen, indicating that the atom was set in such swift motion by the encounter with the alpha particle that it was able to ionize gas at a short distance. If the forces causing the deflection were electrical, it was at once evident that the electric field within the atom must be exceedingly intense. The distribution of positive electricity assumed in the Thomson atom was much too diffuse to produce the intense fields required. To overcome this difficulty, the writer inverted the rôle of positive electricity. Instead of being distributed through a sphere comparable in size with the sphere of action of the atom, the positive electricity is supposed to be concentrated in a very minute volume or nucleus, and the greater part of the mass of the atom is supposed to be resident in this nucleus. The latter is supposed to be surrounded by a distribution of negative electrons extending over a distance comparable with the diameter of the atom as ordinarily understood. On this point of view, the alpha particle is the minute nucleus of the helium atom, which has lost its two external electrons. In this type of atom, the large deviations of the alpha particle take place when it passes through the intense electric field close to the nucleus of the colliding atom. The nearer it passes to the nucleus, the greater the deflection of the particle. Assuming that the forces between the alpha particle and the nucleus of the colliding atom are mainly electrical and vary according to an inverse square law, the alpha particle describes a hyperbolic orbit round the nucleus, and the relative number of alpha particles deflected through different angles can be simply calculated.
It was thus possible to test this theory of atomic structure by actual experiment. This was undertaken by Geiger and Marsden in a very important but difficult investigation. They examined the relative number of alpha particles scattered through various angles by their passage through thin films of matter, e. g., aluminium, silver and gold, by actually counting the alpha particles by means of the scintillations on a zinc sulphide screen. The experimental results were found to be in very good accord with the theory, while Darwin, in addition, showed that any other law of force except the inverse square was incompatible with the observations.
From these results, it is a simple matter to show that the radium of the nucleus of the gold atom can not be greater than cm.—an exceedingly small distance and only about one ten-thousandth part of the diameter of the atom. While the results thus indicated that the nucleus of a heavy atom was of minute dimensions, it was of interest to see whether a still lower limit could be obtained for lighter atoms. On the theory, the helium atom has a nucleus of two unit positive charges, and the lighter atom, hydrogen, should have a nucleus of only one unit. When an alpha particle passes through hydrogen gas, there should be occasional very close encounters between the particle and nucleus of the hydrogen atom. Since the mass of the hydrogen atom is only one quarter of that of helium, it is to be anticipated that the former should be set in very swift motion by a close collision with an alpha particle, and in special cases should be given a velocity 1.6 times greater than that of the colliding alpha particle, and should travel four times as far. Such swiftly moving hydrogen nuclei were actually observed by Marsden with the scintillation method when a pencil of alpha rays passed through hydrogen, and they were found to travel, as the theory predicted, about four times further than the alpha particle itself. Since the energy gained by the hydrogen nucleus depends on the closeness of its approach to the alpha particle, it can be simply calculated that the centers of the nuclei must have passed within cm. of each other. This is an extraordinarily small distance, even smaller than the diameter of the electron itself. It is thus clear that the nuclei of hydrogen and of helium must be exceedingly minute. It should be borne in mind that such observations only give a maximum limit to the size of the nucleus, and there is no experimental evidence against the view that the nucleus of the hydrogen atom may not actually prove to be minute in volume compared even with the negative electron. If this be the case, it appears probable that the hydrogen nucleus is the positive electron and that its great mass compared with the negative electron is due to the greater concentration of its charge. According to modern theory, the electrical mass of a charged particle varies inversely as its radius. The greater mass of the positive than of the negative electron would thus be explained if its radius were only 1/1800 of that of the negative electron, viz., about cm.
There is no evidence to contradict this point of view, and its simplicity has much to commend it. In viewing the essential differences exhibited by positive and negative electricity in connection with matter and the obvious asymmetry of the distribution of the two electricities in the atom, one is driven to the conclusion that there is a fundamental distinction between positive and negative electricity. Since the unit of positive charge is identical in magnitude with the unit of negative charge, the only possible difference is the mass of the two units, and this on modern views is mainly dependent on the dimensions or degree of concentration of the electricity in these fundamental entities.
If we take the view that the hydrogen nucleus is the positive electron, it is to be anticipated that the nuclei of all atoms are built up of positive and negative electrons, the positive electricity being always in excess, so that the nucleus shows a resultant positive charge. The mass of the atom will depend mainly on the number of the massive positive electrons in the nucleus, although it will be affected to a slight extent by the number of the lighter negative electrons involved in the structure of the whole atom. The mass of the atom will no doubt be influenced also by the distribution of the positive and negative electrons in the nucleus, for these must be packed so closely together that their field must interact. As Lorentz has shown, the mass of a number of closely packed electrons is not necessarily the same as the sum of individual masses of the component electrons. Taking such factors into account, we should not necessarily expect the mass of all atoms to be nearly an integral multiple of the mass of the hydrogen atom, although it is known that in a number of cases such a relation appears to hold fairly closely.
The appearance of a helium atom in such a fundamental process as the transformation of radioactive atoms indicates that helium is one of the units, possibly secondary, of which the nuclei of the heavy atoms are built up. In course of its successive transformations, a uranium atom loses eight helium atoms, a thorium atom six, and an atom of actinium five. The probability that helium is one of the units of atomic structure not only in the case of radioactive atoms but for ordinary atoms is strengthened by the fact that the atomic weights of a number of elements differ by about four units.
The fact that the helium nucleus survives the intense disturbance resulting in its violent ejection from a radioactive atom suggests that it is a very stable configuration. On the views discussed, it is natural to suppose that the helium nucleus of atomic weight about four is made up of four positive electrons united with two negative electrons. No doubt it is difficult to understand why such a system should hold together, but it must be remembered that we have no information as to the nature of magnitude of the forces existing at such minute distances as are involved in the structure of the nucleus.
We have so far assumed without proof that while the nucleus of an atom carries a resultant positive charge, negative electrons are also present. The main evidence on this point comes from a study of the radioactive elements. A substance which breaks up with the emission of swift electrons (beta rays), but no alpha particles, suffers disintegration according to the same laws and gives rise to a new element in the same way as when an alpha particle is lost. It seems necessary to suppose from a number of lines of evidence that a transformation which is accompanied by the emission of primary beta particles must have its origin in the ejection of a negative electron from the nucleus itself, or from a point very close to the nucleus.
There are no means at present of deciding definitely the relative number of positive and negative units composing the nucleus, except possibly from a consideration of the atomic weight of the atom in terms of hydrogen. It is, however, premature to discuss such questions until more information is obtained as to the structure of the nucleus and the effect of concentration and distribution of the component electrical charges on its apparent mass.
Charge Carried by the Nucleus
We are now in a position to consider a very important question, viz., the magnitude of the positive charge carried by the atomic nucleus. Since an atom is electrically neutral, the negative charge carried by the exterior distribution of electrons in the structure of the atom must be equal and opposite to the resultant positive charge carried by the nucleus. The electrical charge is most conveniently expressed in terms of the number of the fundamental units of charge in the nucleus. Since the charge carried by the electron is one unit, the charge on the nucleus of the atom may be expressed numerically by the number of electrons exterior to the nucleus. Several methods of attack on this problem have been suggested. Sir J. J. Thomson showed that the scattering of Röntgen rays in passing through the atoms of matter must depend on the number of electrons composing the atom. By assuming that each electron scattered is an independent unit, an expression for the scattering was found in terms of the number of electrons in the atom. By comparison of the theory with experiment, Barkla deduced that for many elements the number of electrons in an atom was approximately proportional to its atomic weight and numerically equal to about one half of the atomic weight in terms of hydrogen.
The charge in the nucleus can also be directly determined from the experiments on scattering of alpha rays, to which attention has previously been drawn. Geiger and Marsden found that the large angle scattering of alpha rays in passing through different substances was proportional per atom to the square of its atomic weight. This showed that the positive charge on the nucleus was approximately proportional to the atomic weight at any rate for elements of atomic weight varying between aluminium and gold. By measuring the fraction of the total number of alpha particles which were deflected through a definite angle in passing through a known thickness of matter, the charge on the nucleus was deduced directly. The number of positive units of charge on the nucleus, which is equal to the number of external negative electrons, was found to be expressed by about one half of the atomic weight in terms of hydrogen. The results obtained by two entirely distinct methods of attack are thus in good accord and give approximately the magnitude of this important atomic constant.
It is obvious, however, that the deduction that the number of units of charge on the nucleus is half the atomic weight, must be only a first approximation to the truth even in the case of the heavier atoms. It has already been pointed out that the nucleus of the helium atom of atomic mass four must carry two unit charges, for it is difficult to believe that any of the exterior electrons of helium can remain attached after its violent expulsion from the atom and its subsequent passage through matter. If this be the case, the nucleus of the hydrogen atom of atomic mass one, must carry one unit charge. Van den Broek and Bohr have suggested that the charge on the nucleus might be equal to the actual number of the element when all the known elements are arranged in order of increasing atomic weight. This is in excellent accord with the experiments of scattering and removes a difficulty in regard to the lighter atoms. Taking this view, the nucleus charge is for hydrogen 1, helium 2, lithium 3, carbon 6, oxygen 8, etc. The simplicity of this conception has much to commend it.
During the last year a new and powerful method of attack on this fundamental problem has been developed by Moseley by the study of X-ray spectra. In 1912, Laue found that X rays showed obvious interference or diffraction effects in their passage through crystals, thus proving definitely that the X rays consist of very short waves analogous to those of light. W. H. Bragg and W. L. Bragg and Moseley and Darwin found that the reflection of the X rays from crystals provided a very simple method of measuring the wave length of the X rays when the spacing of the atoms in the crystal is known. If the X rays give a spectrum containing some bright lines, the wave-lengths of the latter can be simply determined. The work of Barkla has shown us that an X radiation, characteristic of each element, is excited under certain conditions when X rays fall upon it. The penetrating power of this characteristic radiation increases rapidly with the atomic weight of the radiator. In heavy elements, another type of characteristic radiation makes its appearance. These two types of characteristic radiation have been called by Barkla the "K" and "L" radiations respectively. These radiations can be excited either by X rays of suitable penetrating power or by direct bombardment of the element by cathode rays in a vacuum tube. Moseley made a systematic examination of the X-ray spectra of a great majority of the elements. For this purpose,
copper and zinc.
the elements examined were bombarded by cathode rays, and the spectrum of the radiation examined by reflection from a suitable crystal. He found that the spectra of the "K" radiation from elements varying in atomic weight from aluminium to silver were all similar in type, consisting mainly of two strong lines. An example of the spectrum obtained for a number of successive elements is shown in Fig. 14. It is seen that with increasing atomic weight, the wave-lengths of the corresponding lines diminish, not irregularly but by definite and well marked steps. Moseley found that for the K radiation the frequency of the radiation was proportional to where N was a whole number which varied by unity in passing from one element to the next of higher atomic weight and a constant about unity. From silver to gold, the spectra given by the L radiations of elements were compared. These spectra consist of about five lines, of which two are relatively very strong. It was found again that the spectra were similar in type and that the frequency of a given line diminished by definite steps in passing from one element to another. The frequency of the radiation in this case was proportional to where b was a constant and N a whole number. Moseley concluded that the value of N in these expressions was the atomic number, i.e., the number of the element arranged in order of increasing atomic weight. Taking aluminium as the 13th element, he found that succeeding elements were expressed by the value of N 14, 15, 16, 17, etc., up to 77 for gold.
There appears to be little doubt that the X-ray spectrum of an element arises from the vibrations of the rings of electrons deep in the atomic structure outside the nucleus. Quite apart from the very interesting question of the mode of origin of these very high frequency spectra, it is seen that the fundamental modes of vibration of the distribution of electrons are simply connected with the square of a number, which varies by unity in passing from one element to the next. There appears to be no doubt that the atomic number represents the number of units of positive charge carried by the nucleus, which on account of the atomic nature of electricity can only vary by whole numbers and not by fractions.
It is obvious that the study of X-ray spectra reveals at once whether any atomic number is missing, and also affords a remarkably simple method of settling the number of elements possible in the rare earth group about which there has been so much difference of opinion. Moseley concluded that from aluminium to gold, only three possible elements were missing which should have atomic numbers 43, 61, 75, and only one element of number 61 appears to be missing in the rare earth group. The frequencies of the X-ray spectra of these missing elements can be calculated with certainty, and these data should prove an invaluable aid in a search for these missing elements. It has long been known that nickel and cobalt occupy an anomalous position in the periodic table when arranged according to atomic weights. This difficulty is now removed, for Moseley found that when arranged in order of nucleus charge, both elements fall into the position to be expected from their chemical properties.
Nucleus Charge and Chemical Properties
It is established by the work of Moseley that the elements can be defined by their nucleus charge, and that probably elements exist which have all the nucleus charges from 1 for hydrogen to 92 for lead. There is, however, another very important consequence that follows from this conception of the atom. Disregarding for a moment the atomic weight which depends mainly on the structure of the nucleus, the main physical and chemical properties of the atom are determined by the nucleus charge and not by the atomic mass. This must obviously be the case, for the number and distribution of electrons round the nucleus is determined by the electric forces between the electrons and the nucleus, and this is dependent on the magnitude of the nucleus charge which may be regarded as a point charge. Without entering into the difficult question of the actual distribution of the exterior electrons in any atom, it is obvious that the number and position of the outlying electrons of the atomic structure, which probably mainly influence the chemical and physical properties of the atom, are determined by the charge on the nucleus. No doubt if the electrons are in motion, their positions relative to the nucleus and possibly also their rates of vibration will be slightly influenced by the mass of the nucleus as well as its charge, but the general evidence indicates that this effect must be very small.
We thus see that there is in the structure of every atom a quantity which is more fundamental and important than its atomic weight, viz., its nuclear charge. It is known that the variation of the atomic weights of the elements with atomic number, while showing certain well-marked relationships, shows no definite regularity. From the point of view of the nucleus theory, the atomic weight of an element, while in some cases approximately proportional to its atomic number, is in reality a complicated function of the actual structure of the nucleus. The question why the atomic mass should not necessarily be proportional to the atomic number has already been discussed. While the main properties of an atom are controlled by its nuclear charge, the property of gravitation and also that of radioactivity are to be ascribed mainly, if not entirely, to the nucleus.
Radioactive Elements and the Periodic Series
Since the nucleus charge of an atom determines the main physical and chemical properties of an atom, it is possible that elements may exist of equal nuclear charges but different atomic weights. For example, if it were possible to add a helium nucleus to the nucleus of another atom, it would increase the nuclear charge by two and the mass by about four; if instead of the helium nucleus two hydrogen nuclei were added, the charge would be the same but the mass of the resulting atom two units less than with helium. In such a case, two atoms would be possible of identical nuclear charge but different atomic weights. In a similar way, it may be possible for elements to exist of the same atomic mass but different nuclear charges. This would be brought about by the loss or gain of one or more negative electrons in the nucleus.
The study of radioactive elements has in the last year thrown a flood of light not only on this problem but on the underlying meaning of the periodic law of the elements. Russell, Fajans and Soddy independently put forward a remarkable and important generalization in regard to the change of chemical properties of the successive products of transformation of the primary radioactive elements. This generalization can be very simply expressed in terms of the usual arrangement of the elements in groups according to the periodic law. It is found that after a transformation in which alpha particles are expelled, the resulting element has chemical properties which shift its place two groups lower in the direction of diminishing mass. On the other hand, the element resulting from a beta ray transformation shifts one place in the opposite direction. For example, radium, which is in group II., changes after loss of an alpha particle into the emanation into group 0, which included all the inert gases of the helium-argon type. The emanation after loss of another particle becomes radium A, which belongs to group VI., and this in turn becomes radium B belonging to group IV. Since radium B is transformed by the loss of a beta particle, the resulting element radium C takes up a position in group V. By this simple rule, it has been found possible to define the essential chemical properties of all known radioactive elements. It was found that on this theory one element was missing in the general scheme. This element was discovered a few weeks later by Fajans and Göhring, and found to have the general chemical properties predicted for it.
This generalization is capable of a very simple explanation on the nucleus theory. The loss of an alpha particle of charge 2 lowers the nuclear charge of the resulting elements two units; the loss of a beta particle, which carries a unit negative charge, raises the nuclear charge by one unit. In other words, the atomic number of an element shifts two units lower after loss of an alpha particle and shifts one unit higher after loss of a beta particle.
The atomic numbers of the elements in the uranium-radium series can be simply deduced from this rule if the atomic number of one element is known. We shall see later that the atomic number of radium B is 82 and identical with that of lead. The actual atomic numbers of the various elements are given in the circles representing the atoms in Fig. 12. It is seen that uranium, the heaviest known element, has an atomic number 92, while the elements radium B, radium D and the end product, which is believed to be lead, have the same atomic number, viz., 82. The evidence of the correctness of this striking conclusion will now be discussed.
As a result of a careful examination of the radioactive substances, it has been found that in a number of cases elements, which are of different atomic weight and exhibit different radioactive properties, yet show identical general physical and chemical behavior. For example, the elements radium B, radium D and lead, of atomic weights 214, 210, and 207, respectively, are so closely allied in chemical and physical properties that all attempts to separate a mixture of any two of them have failed completely. This would be explained if the nuclear charges were identical for those elements, as the generalization, already referred to, indicates. If this be the case, they should give identical spectra under similar conditions. Unfortunately the elements radium B and radium D are in too small quantity to determine their ordinary light spectra, but we can compare the X-ray spectrum of lead with that given by radium B under the excitation of its own beta rays. Experiments of this kind were recently made by Dr. Andrade and the writer, and the two spectra were found to be identical within the limits of experimental error. It is to be anticipated that their light spectra would also prove to be identical, or nearly so, for, as previously pointed out, the effect of the mass of the nucleus on the spectrum is probably very small.
The fact that the atoms of these three elements are not identical as regards mass or radioactive properties, shows that the structure of the nucleus is different in each case.
There is another important deduction that should be mentioned. The end product of the uranium-radium series is an inactive element which has long been considered to be lead, but it has been difficult to verify this conclusion by direct experiment. We have seen that the end product has the same atomic number as lead, but should have an atomic weight about 206 instead of 207 as found for ordinary lead. In a similar way, it has been concluded by Soddy and Fajans that the end product of thorium has the same atomic number as lead, but should have an atomic weight about 208.5. In order to test these remarkable conclusions, experiments are now in progress by a number of investigators in different countries to examine whether the lead always found in radioactive minerals and which presumably has partly, if not wholly, a radioactive origin, shows the same atomic weight as ordinary lead. Soddy has already found evidence that there is a distinct difference in the atomic weights in the direction predicted by the theory.
The question naturally arises whether some of the ordinary elements may not prove to be a mixture of two, or even more, of these "isotopes," as they have been termed. Unless the component isotopes are present in different proportion in different natural sources of the element, it will be difficult to settle this problem by ordinary chemical methods. There is one element, however, besides lead, about which interesting evidence has been obtained on this point. Sir J. J. Thomson found by examining the deflection of the positively charged particles produced by an electric discharge through the rare gas, neon, that two elements were present of atomic weights about 20 (neon) and 22. Aston was able by diffusion experiments to separate partially the two components of neon and to show that they differed in density, but failed in attempts to separate them by fractional distillation in charcoal cooled by liquid air. Such results are to be anticipated if neon is a mixture of two isotopes, i.e., elements of identical nuclear charges but different atomic weights.
It is obvious that this new point of view will result in a systematic examination of the elements to test for the possible presence of isotopes, and thus give an additional reason for the accurate determination of atomic weights for elements obtained from widely different sources.
Distribution of Electrons in the Atom
It is seen that the nucleus theory of the atom offers a simple explanation of many important facts which have been brought to light in recent years, and for this purpose it has not been necessary to make any special assumptions as to the actual structure of the nucleus, or of the way in which the external electrons are distributed. The investigation of the latter problem is beset with many difficulties; for an electron is attracted towards the nucleus, and even if it is in orbital motion, it must on the electromagnetic theory lose energy by radiation and ultimately fall into the nucleus. It appears likely that this difficulty is in reality due to our ignorance of the conditions under which an electron radiates energy. According to the views outlined in this lecture, the hydrogen atom has the simplest possible structure, for it consists of a nucleus of one unit charge and one negative electron. The question naturally arises how such a simple structure can give rise to the complex spectrum observed for hydrogen. This problem has been attacked in a series of remarkable papers by Bohr, who concludes that the complexity of the spectrum is not due to the complexity of the atomic structure but to the variety of modes in which an electron can emit radiation. Suppose, for example, that a hydrogen atom has lost its negative electron. Bohr supposes that an electron falling towards the positively charged nucleus may occupy temporarily any one of a number of stationary positions fixed relatively to the nucleus. In falling from one stationary state to another, radiation is emitted of a definite frequency which is connected with the difference of potential energy E of the electron in the two stationary states by E·h where h is Planck's fundamental constant. On this hypothesis, he has been able to account for the series spectra of hydrogen and to deduce directly from the theory the value of Balmer's constant which plays such an important part in the spectra of all atoms. In a similar way, the helium atom is supposed to consist of a nucleus of two unit charges surrounded by two electrons. On the theory, the spectrum of helium is connected in a very simple way with that of hydrogen. Bohr also pointed out that the Pickering series of spectral lines observed in certain stars which were originally attributed to hydrogen must be ascribed to helium. This conclusion has since been strongly supported by the direct experiments of Fowler and Evans, In a similar way, Bohr described the possible distribution of electrons in several of the lighter atoms and also discussed the structure of the hydrogen molecule, which is composed of two hydrogen nuclei and two electrons. The heat of combination deduced for the theoretical molecule is in fair accord with experiment. He found that two helium atoms were unable to unite to form a molecule—in agreement with a well-known property of this gas.
While there is room for much difference of opinion as to the interpretation of the rather revolutionary assumptions made by Bohr to explain the structure of the simple atoms and molecules, there can be no doubt of the great interest and importance of this first attempt to deduce the structure of the simple atoms and to explain the origin of their spectra. The agreement of the properties of such theoretical structures with the actual atoms is in several cases so remarkable that it is difficult to believe that the theory is not in some way an expression of the actual facts. While much work will be necessary before we can hope to understand the structure of any but the simplest atoms, a promising beginning has been made in the attack on this most difficult and fundamental of problems.
There seems to be little doubt that the more marked physical and chemical properties of an atom are to be attributed to a few outlying electrons in the atomic structure. The position and number of these "valency" electrons, as they have been termed by Stark, are defined by the magnitude of the nucleus charge. It has previously been pointed out that the loss of an alpha particle from a radioactive atom changes the position of the element two groups lower in the periodic table, while the loss of a beta particle raises it one group higher. Consequently it follows that the loss or gain of a unit charge from the nucleus of an atom causes it to change its position from one group to the next. If, for example, we follow the chemical properties of successive elements when the nucleus charge increases by unity, we soon reach an element which belongs to the same group as the first, although of much higher atomic weight. We must consequently conclude that the number and position of the outlying electrons in the structure of the atom passes through successive changes which are regularly repeated with increasing atomic weight. Quite apart from any detailed knowledge of the electronic distribution of atoms, the regular recurrence of elements of similar chemical properties with increasing atomic weight is to be anticipated on the general theory that an atom is an electrical structure.
Evolution of the Elements
It has long been thought probable that the elements are all built up of some fundamental substance, and Prout's well-known hypothesis that all atoms are composed of hydrogen is one of the best known examples of this idea. The evidence of radioactivity certainly indicates that the heavy radioactive elements are in part composed of helium, for an atom of the latter appears as a result of many of the radioactive transformations. No definite evidence, however, has been obtained that hydrogen appears as a result of such transformations; but as previously pointed out, helium may prove to be an important secondary unit in the structure of heavy atoms. While we have thus undoubted evidence of the breaking up of heavy atoms, no indication has yet been observed that the radioactive processes are reversible under ordinary conditions. Many investigations have been made to test whether new elements appear in strong electric discharges in vacuum tubes. While some of the results obtained are difficult of interpretation, no reliable evidence has yet been adduced that one element can be transformed into another under such conditions.
The question of the evolution of the elements has been attacked from another side. Sir Norman Lockyer and others have suggested that the elements composing the star are in a state of inorganic evolution. In the hottest stars the spectra of hydrogen and helium predominate, but with decreasing temperature, the spectra becomes more complicated and the lines of heavier elements appear. On this view, it is supposed that the light elements combine with decreasing temperature to form the heavier elements.
There is no doubt that it will prove a very difficult task to bring about the transmutation of matter under ordinary terrestrial conditions. The enormous evolution of energy which accompanies the transformation of radioactive matter affords some indication of the great intensity of the forces that will be required to build up lighter into heavier atoms. On the point of view outlined in these lectures, the building up of a new atom will require the addition to the atomic nucleus of either the nucleus of hydrogen or of helium, or a combination of these nuclei. On present data, this is only possible if the hydrogen or helium atom is shot into the atom with such great speed that it passes close to the nucleus. In any case, it presumes there are forces close to the nucleus which are equivalent to forces of attraction for positively charged masses. It is possible that the nucleus of an atom may be altered either by direct collision of the nucleus with very swift electrons or atoms of helium such as are ejected from radioactive matter. There is no doubt that under favorable conditions, these particles must pass very close to the nucleus and may either lead to a disruption of the nucleus or to a combination with it. Unfortunately, the chance of such a disruption or combination is so small under experimental conditions that the amount of new matter which is possible of formation within a reasonable time would be exceedingly small, and so very difficult of detection by direct methods. Very penetrating X rays or gamma rays may for similar reasons prove to be possible agencies for changing atoms. Although it is difficult to obtain direct evidence, I personally am inclined to believe that all atoms are built up of positive electrons—hydrogen nuclei—and negative electrons, and that atoms are purely electrical structures.
There can be little doubt that conditions have existed in the past in which these electrons have combined to form the atoms of the elements, and it may be quite possible under the very intense electrical disturbances which may exist in hot stars that the process of combination and dissociation of atoms still continues.
In these lectures, I have tried to give an idea of some modern views of the structure of the atoms and of the great variety of new and powerful methods which have been applied to the attack of this problem in recent years. We have seen that a heavy atom is undoubtedly a complex electrical system consisting of positively and negatively charged particles in rapid motion. The general evidence indicates that each atom contains at its center a massive charged nucleus or core of very small dimensions surrounded by a cluster of electrons probably in rapid motion which extend for distances from the center very great compared with the diameter of the nucleus. Such a view affords a reasonable and simple explanation of many important facts obtained in recent years, but so far only a beginning has been made in the attack on the detailed structure of atoms—that fundamental problem which lies at the basis of physics and chemistry.
- First course of lectures on the William Ellery Hale Foundation, National Academy of Sciences, delivered at the Washington Meeting, April, 1914.
- Maxwell used the term "molecule" where we now use the term "atom."
- In later work Rawlinson and Bragg have found that each of these lines is in reality a very close double.
- Since the delivery of this lecture, similar conclusions have been reached by the experiments of Richards in Cambridge and Hönigschmid in Vienna. There still, however, remains some doubt as to the actual difference in atomic weight of uranium lead, thorium lead and ordinary lead. A very promising beginning has thus been made on the attack of this most important and fundamental problem.