"Constitution Of Matter 17.891). - In the decade 1910-20 many important advances were made which gave much more definiteness and precision to our knowledge of the constitution of matter. The atomic theory of matter, which for long appeared to be of necessity unverifiable by direct experiment on account of the minuteness of the atom, received almost direct proof in a number of ways. Methods have been developed, for example, to detect the electrical effect of a single a particle from radium, and a single swift electron (see Gases, Electrical Prop Erties Of).
The a particle has been shown to be a charged atom of helium projected with high velocity; the number of a particles from a given quantity of radium have been counted, and the volume of helium which they produce has been measured. In this direct way it has been shown that about 2.7 X Io 19 particles or atoms of helium are required to form one cubic centimetre of helium gas at normal pressure and temperature. Not only is it feasible to detect the effect of a single atom of matter in special circumstances but also to show the path of a swift a particle or electron through a gas. This has been made possible by the discovery of C. T. R. Wilson that under suitable conditions the charged ions produced in gases by a or s rays become centres for the condensation of water vapours, and are thus rendered visible as the nuclei of visible drops of water. The photographs of these droplets show in a most striking way the track of the particle through the gas, and illustrate with extraordinary detail the main effects produced by the passage of ionizing radiations through gases.
The essential correctness of the kinetic theory of matter, which assumes that the molecules of matter are in vigorous but irregular motion, has been clearly demonstrated by the experiments of Perrin and others on the motion and equilibrium of small spheres of matter in suspension in fluids which show the Brownian movement. At the same time the atomic or discrete nature of electricity, which had been implicitly assumed in many theories, has received complete experimental verification, and the magnitude of this fundamental unit of charge has been measured with precision. The most accurate experiments on this subject have been made by Millikan by measuring the electric field required to support a small, charged droplet of oil or mercury. The charge on the drop was varied by.ionizing the gas in its neighbourhood. In this way he has been able to show that the charge always varies by integral multiples of a fundamental unit. The charge given to a drop by friction or any other method is always an integral multiple of this unit charge. This fundamental unit is the same both for positive and negative electricity, and is numerically equal to the charge carried by the negative electron, the positive and negative ions produced in a gas by X rays, and also to the positive charge carried by the hydrogen atom in the electrolysis of water. The magnitude of this unit charge, combined with electrochemical data, gives a most reliable method of measuring a number of important and molecular magnitudes. The value of the fundamental unit of charge and thus the mass of the individual atoms of matter are now known with an accuracy of certainly within one per cent and possibly within one-tenth of one per cent. The data found by Millikan are given in the following table: Fundamental unit of charge. e= 4.774. X toi ° electrostatic units The Avogadro Constant, i.e. the number of molecules in one gramme molecule.. N= 6062 X Iona The number of molecules per c.c.
of any gas at o° C. and 760 mms. n =2.705 X 1019 Mass of hydrogen atom in grammes m =1.662 X Io-24 From these data the number of atoms in one gramme of any element can be determined. While the average distance apart of the atoms or molecules can at once be deduced, the actual dimensions of the molecules or sphere of action of the molecules can only be approximately estimated with the aid of other and much less precise data.
Structure of the Atom. - Since the proof that the negative electron of small mass is a constituent of all atoms of matter, there has been a vigorous attack on the fundamental problem of the structure of the atom. After passing through a number of phases the general ideas on this subject have crystallized into a fairly definite form, and it is now generally believed that the atom is composed of a massive positively charged nucleus of minute dimensions surrounded at a distance by a compensating distribution of negative electricity in the form of negative electrons. Since electricity is atomic the resultant positive charge on the nucleus must be an integral multiple N of the fundamental unit of charge e and is given by Ne. In order for the atom to be electrically neutral it must be surrounded by a distribution of N negative electrons. The value of N for each of the atoms is a fundamental constant, for on it depends the magnitude of the electric field surrounding the nucleus and the arrangement of the external electrons which in turn determine the main physical and chemical properties of the atom. The idea of the nuclear structure of atoms arose initially from a study of the scattering of a particles in their passage through matter. On account of its great energy of motion the charged a particle penetrates the structure of some of the atoms and comes under the influence of the intense repulsive field of the nucleus. Assuming that the law of force is that of the inverse square the a particle describes a hyperbolic path, and the angle of deflexion depends on the nearness of approach to the nucleus. From a close study of the scattering of a rays by Geiger and Marsden it was concluded that the number of a particles scattered through different angles was in close accord with the idea of the nucleus atom, while the actual number scattered through a given angle gave information on the magnitude of the charge carried by the nucleus. The preliminary experiments indicated that for the heavier atoms the value of N was about half the atomic weight in terms of hydrogen. A notable advance was made by the fundamental experiments of Moseley on the X-ray spectra of the elements. He found that the X-ray spectrum was similar for all elements, and that the frequency of vibration of corresponding lines in the spectrum was proportional to the square of a number which varied by unity in passing from one element to the next. He concluded that the nuclear charge in fundamental units was equal to the atomic or ordinal number of the elements when arranged in increasing order of their atomic weights. On this view the' lightest element, hydrogen, has a nuclear charge r, helium 2, lithium 3, and so on up to the heaviest element, uranium, of ordinal number 92. This is a generalization of great importance and simplicity which has guided all subsequent work on the structure of atoms. The essential correctness of Moseley's conclusion has been directly verified in the case of a few representative elements by Chadwick by accurate measurement of the nuclear charge based on the scatteringof a rays. Moseley showed that with few exceptions all values of the nuclear charge between I and 92 were represented by known elements. The missing elements were of ordinal numbers 43, 61 and 75, corresponding to positions in the Periodic Table where the existence of additional elements had been suspected. Moreover, when the atomic weight of the element in Mendeleef's classification was replaced by its ordinal number certain irregularities were removed. For example, the positions of argon and potassium, cobalt and nickel, iodine and tellurium were interchanged - a result in complete accord with their chemical properties (see Chemistry).
It thus follows that the main physical and chemical properties of an element are defined by a whole number which represents both its nuclear charge in fundamental units and the number of external electrons. The atomic weight of an element is in a sense a secondary property, for, as we shall see, elements can exist of the same nuclear charge but of different atomic weights. The number and position of the external electrons, on which the ordinary chemical and physical properties of an atom depend, are defined by the nuclear charge. The mass of the atom which resides mainly in the nucleus exercises a subordinate effect on the external arrangement of the electrons.
Isotopes
On Moseley's classification only 92 elements of ordinal numbers r to 92 are possible, assuming that uranium (92) is the last of the elements. We shall now briefly discuss some recent advances which clearly show that in some cases several elements can exist with the same nuclear charge but of different atomic masses. Information on this point was first obtained from a study of the radioactive bodies. It was early observed that a number of products which showed different radioactive properties were inseparable from one another by ordinary physical and chemical methods. For example, ionium and thorium, radium and mesothorium, radium D and lead cannot be separated from each other, and appear to be identical in chemical properties. Elements so closely alike in chemical properties were called" isotopes "by Soddy, since they appeared to occupy the same position in the periodic arrangement of the elements. Viewed from the standpoint of the nuclear theory isotopes are elements of the same nuclear charge but of different atomic masses. As we have seen, the nuclear charge controls the ordinary physical and chemical properties of the atom, and the mass which resides almost entirely in the nucleus has only a second-order effect. On the other hand, the property of radioactivity depends on the structure and stability of the nucleus, which may be very different for atoms of the same resultant nuclear charge.
In the article on Radioactivity attention is drawn to the remarkably simple relation which exists between the chemical properties and radiations of the series of radioactive elements. With the aid of this relation we can at once write down the ordinal numbers and masses of the long series of elements which arise from the transformation of uranium, thorium and actinium, and can follow the origin of the numerous isotopes which arise. One of the most striking results of this generalization was the prediction that the end product of the uranium and thorium series should be an element of the same ordinal number as lead but of atomic masses 206 and 208 respectively, instead of the mass 207 found for ordinary lead. This result has been directly confirmed by atomic weight determinations of uranium-lead and thoriumlead, and was the first definite proof of the existence of isotopes of a non-radioactive element.
It seemed probable that in a similar way many of the ordinary elements might consist of a mixture of isotopes, i.e. elements with the same nuclear charge but different atomic masses. This has been confirmed in a number of cases chiefly by the work of Aston. The masses of the positively charged atoms present in the electric discharge in a vacuum tube are examined by bending the rays in a combined magnetic and electric field. In this way it was found that neon consisted of two isotopes of masses 20 and 22 and chlorine of isotopes of masses 35 and 37. The relative proportions of the two isotopes in chlorine was in good accord with that to be expected from the ordinary atomic weight of the mixture of isotopes, viz. 35.45.
This new method of analysis had, up to 1921, been employed only for a small number of the elements, but had yielded results of great interest. Some of the elements, like carbon, nitrogen and oxygen, give no isotopes, and are thus to be regarded as" pure "elements where the atoms have all the same mass and nuclear charge. Others, like chlorine, argon, krypton, and mercury, are composed of a mixture of two or more isotopes. In cases like krypton and mercury as many as six well-defined isotopes have been detected. As far as observation has gone the masses of all the isotopes are expressed by a whole number in terms of 0= 16 with an accuracy of about i in 1,000. For example, the isotopes of neon are 20.00 and 22.00. This important conclusion, which has been verified in a number of cases, affords a strong indication that the masses of the parts composing the nucleus have a mass either of one or a multiple of one, and are not direct multiples of the mass of the hydrogen atom which is 1.008 where O = 16. The reason of this will be discussed later.
While the ordinary physical and chemical properties of isotopes are closely similar, it is to be expected that they should differ in all qualities which involve directly the mass of the atom, e.g. the coefficients of diffusion and specific heats. In a similar way second-order effect is to be expected in the rate of vibration of the external electrons, i.e. in the light spectrum of the element, and a small effect has been observed in several cases. The most obvious method of partial separation of isotopes is by the process of diffusion or evaporation. In this way a partial separation into light and heavy fractions has been shown in the case of neon, mercury, and chlorine. No evidence of the separation of isotopes in nature has been so far observed except in the case of uraniumlead and thorium-lead already referred to. It will be of great interest to test, for example, whether chlorine obtained from widely different sources shows any difference in the relative proportions of its component isotopes.
Distribution of Electrons
We have seen that the atom is to be regarded as an electrical structure in which a positively charged nucleus is surrounded by a number of electrons. The magnitude of the nuclear charge and the number of the external electrons are known for each of the elements. In considering the distribution of the external electrons round the nucleus, we are at the outset faced by the great difficulty that no possible arrangement can be permanently stable on the basis of the classical dynamics. For example, an electron in motion round the nucleus must on the classical theory radiate energy and fall into the nucleus. To overcome this fundamental difficulty Bohr has introduced a conception based on the quantum theory, in which radiation only occurs in definite quanta. In this way it is possible to postulate the position of the electrons in the simpler atoms and to calculate their frequency of vibration. The theory of Bohr developed by Sommerfeld and others has achieved remarkable success in explaining many of the details of the spectra of hydrogen and helium both in electric and magnetic fields. Owing, however, to the great complexity of the possible modes of motion when three or more electrons are present, it is difficult to calculate the distribution of the electrons and their modes of vibration in the case of the more complex atoms.
A number of suggestions have been made as to the grouping of the electrons in the atom, notably by Kossel, Lewis, Langmuir and Sir J. J. Thomson, which have had a certain measure of success in offering an explanation of the periodic variation in the properties of the elements with atomic number and the methods of combination to form molecules. These theories, however, are for the most part descriptive and not quantitative in character. The whole problem of the distribution and motion of the electrons in a complex atom is a very difficult one. While definite progress had been made by 1921, much still remained to be done before we could hope to define with any certainty the position, motion and modes of vibration of the electrons for even the lighter and less complex elements.
Structure of the Nucleus
While it is difficult to estimate the dimensions of atomic nuclei, the general evidence indicates that the nucleus of a heavy atom like uranium, if assumed spherical, has a radius of less than 1011 cm. or less than 1/100o of the radius of the external atom. No doubt the dimensions of a nucleus depend on its complexity and are much smaller for the lighter atoms. From experiments on the passage of a particles through hydrogen, it has been calculated that the dimensions of the helium nucleus of mass 4 is of the order 1 0 -" cm.
The most direct evidence on the constitution of the nucleus is derived from the study of the radioactive transformations. The disintegration of an atom is accompanied either by the expulsion of an a particle, i.e. in helium nucleus, or the release of a swift electron from the nucleus. This shows that the nucleus of the radioactive atoms contains both positively charged masses and negative electrons, and that the nuclear charge represents the resultant charge. It is natural to conclude that the helium nucleus of mass 4 is one of the secondary units which make up the structure of a complex nucleus. This is supported by the observation that the atomic mass of many atoms is expressed by 4 n where n is a whole number. It is clear, however, from the work of Aston on isotopes that, in addition to the helium nucleus, an element of mass 1 or integral multiple of 1 enters into the structure of all nuclei. This fundamental unit of structure has been named "proton," and its atomic mass is 1 or very nearly in terms of 0= 16. On this view the nuclei of all elements are made up of positively charged protons and electrons. The mass of the atom measures the number of protons in the nucleus. This is in a sense a return to the famous hypothesis of Prout in which all the atoms are supposed to be built up of hydrogen as the fundamental unit.
It seems clear that if a proton could be removed from an atomic nucleus it would prove to be the hydrogen nucleus carrying a unit positive charge. In fact, Rutherford and Chadwick have shown that the hydrogen nucleus can be liberated from certain atoms like nitrogen and aluminium by bombardment with swift a particles. It remains, however, to explain why the proton in a nucleus has a different mass from the free hydrogen nucleus. The latter has a mass 1.008 in terms of O = 16 while the proton in the nucleus has a mass unity, or nearly unity.
While the negative unit of electricity exists in the form of the electron of very small mass, no evidence has been obtained that its counterpart, the positive electron of very small mass, exists. The unit of positive electricity has never been found to be associated with a mass less than that of the hydrogen atom. This has led to the view that the hydrogen nucleus is the positive electron, and that its mass is about 1,845 times that of the negative electron. This difference in mass between the units of positive and negative electricity appears to be fundamental, and offers an explanation of the asymmetrical distribution of positive and negative electricity in the structure of atoms.
Since the helium nucleus has a mass 4 and charge 2, it should be composed of four hydrogen nuclei and two electrons. Its mass, however, is less than that of four free hydrogen nuclei. Such a change of mass in the very close combinations of positive and negative nuclei is to be expected. According to the theory of relativity energy has mass, and the loss of mass m of a system is numerically given by E=rnc 2 where E is the energy liberated and c the velocity of light. On this view the combination of the positive and negative electrons to form the helium nucleus is accompanied by a large release of energy. From the difference between the mass of the helium nucleus and that of four hydrogen nuclei, it can readily be calculated that the helium nucleus is such a stable combination that an amount of energy corresponding to four or five a particles from radium would be required to dissociate it. The difference between the masses of the protons in the nucleus and free hydrogen nuclei is thus to be ascribed in general to the close packing of the positive and negative units composing the nucleus.
On the views outlined. above the number of electrons in any nucleus can at once be calculated. For example, oxygen of nuclear charge 8 should be made up of 16 positive units and 8 electrons. For such a nucleus to hold together it seems clear that the forces between the charged units at such small distances must be different from that of the inverse square. While it has been experimentally shown that the law of the inverse square holds at any rate approximately close to the nucleus of a heavy atom like gold, this law breaks down in very close collisions of light atoms where the nuclei approach very close to each other. For example, it has been found that the number of hydrogen atoms which are set in swift motion when a particles pass through hydrogen is very different from that to be expected if the nuclei behave as point charges repelling each other according to the law of the inverse square. The experimental information at present available is too indefinite to hazard more than a guess as to the nature and magnitude of the forces that come into play when nuclei approach very close to one another, as they must do in the structure of the nucleus of a heavy atom.
Stability of Atoms
Apart from the heavy radioactive elements which belong to a class by themselves, and two other elements - potassium and rubidium - which spontaneously emit swift electrons, the atoms of the ordinary elements appear to be very stable structures which cannot be broken up by ordinary chemical and physical agencies. Some experiments have suggested that possibly helium and hydrogen may be liberated by the passage of an electric discharge through gases, but on account of the presence of these elements in many materials it is difficult to prove definitely that they arise from artificial transformation. In considering the possibility of the disintegration of elements it should be borne in mind that the loss of one or more electrons from the outer electronic system has no permanent effect on the atom, for other electrons ultimately fall into the atom to fill their place. In order to produce a permanent transformation of the atom it appears necessary to remove a positively charged particle or an electron from the nucleus of the atom. This can only be effected by agencies which are able to penetrate the nucleus or to pass very close to its structure.
The a particle expelled from radium is one of the most concentrated sources of energy known to us, and on account of its speed should be able to penetrate the structure of the nuclei of many of the lighter atoms and still retain sufficient energy to disrupt the bonds that hold the parts of the nucleus together. In the case of an atom of high nuclear charge the a particle may lose so much of its energy in approaching the nucleus that it may be unable to effect its disintegration. It has been found that when a particles pass through hydrogen or any material containing combined hydrogen some of the particles pass so close to the hydrogen nucleus that they set it in swift motion. These swift hydrogen atoms can be detected by the scintillations they produce on a zinc-sulphide screen. This is purely a case of collisions of atomic 'nuclei, and the speed of the "H" atom set in motion can be calculated by the ordinary laws of mechanics. The maximum range or distance of penetration of such a particle is about four times that of the incident a particle.
In a similar way other nuclei must be set in swift motion by their collision with a particles, but it can be calculated that in most cases such nuclei are unable to travel as far as the a particle, and thus remain undetected amid the great number of incident a particles.
When a strong beam of a rays passes through oxygen or carbon dioxide only a few H atoms are observed, and these appear to come from the radioactive source. When, however, the rays pass through dry nitrogen a much greater number of penetrating particles is observed. Rutherford has shown by the action of a magnetic field that these particles are not atoms of nitrogen but probably charged atoms of hydrogen. Rutherford and Chadwick have tested a number of elements in this way and have found that, in addition to nitrogen, boron, fluorine, sodium and phosphorus show a similar property. As far as observation has gone it seems probable that these expelled particles are H atoms which are released by the disintegration of the nucleus. The velocity of expulsion of such H atoms is greater than that of an H atom in a direct collision with an a particle. For example, using a particles of range 7o cm. in air, ordinary H atoms travel 29 cm. in air while the atoms from nitrogen go 40 cm., and those from aluminium not less than 80 cm. It thus seems clear that the effects observed in nitrogen and aluminium cannot be ascribed to ordinary hydrogen as an impurity. It is of interest to note that if the particle from aluminium is an H atom it is released with more energy than that of the incident a particle. Elements like carbon, oxygen, and sulphur, whose atomic mass is given by 4n where n is a whole number, do not give rise to H atoms, but only those elements whose mass is given by 4n+ 2 or 4n+3. It thus seems clear that a disintegration of certain atoms can be produced by the intense collisions with the a particle in which an H atom is released with great velocity. General evidence indicates that not only H atoms but possibly also atoms of mass 3 or 4 may be liberated in a similar way, but the experimental evidence was in 1921 too indefinite for any certain conclusion.
It should be borne in mind that the disintegration observed in this way is on an exceedingly small scale. Not more than one particle in a million gets sufficiently close to a nucleus to release an H atom. It seems clear, however, that while the ordinary atom is undoubtedly very stable, its disintegration can be brought about by the aid of sufficiently powerful agencies which are able to penetrate its structure. As already pointed out, there are strong reasons for believing that the helium nucleus is a very stable structure which cannot be broken up even by the swiftest a particle at our disposal.
While it is reasonable to suppose that 'all the elements have been built up by combinations of protons and electrons, there was in 1921 no experimental evidence to throw light on the conditions necessary to lead to the formation of complex nuclei. No doubt, however, this process of aggregation has gone on in the past, and may still be in progress under favourable conditions, if not on this earth at any rate on some of the stars. (E. Ru.)
