PRACTICALLY simultaneously with the first announcement of these results for thorium lead, a series of investigations was published on the atomic weight of lead from uranium minerals by T. W. Richards and collaborators at Harvard, Maurice Curie in Paris, and Honigschmid and collaborators in Vienna, which show that the atomic weight is lower than that of ordinary lead. The lowest result hitherto obtained is 200.046, by Honigschmid and Mlle. Horovitz, for the lead from the very pure crystallized pitch-blende from Morogoro (German East Africa), whilst Richards and Wadsworth obtained 206.085 for a carefully selected specimen of Norwegian deveite. Numerous other results have been obtained, as, for example, 200.405 for lead from Joach- imsthal pitch-blende, 206.82 for lead from Ceylon thorianite, 207.08 for lead from monazite, the two latter being mixed uranium and thorium minerals. But the essential proportion between the two elements has not, unfortunately, been determined. Richards and Wads- worth have also examined the density of their uranium lead, and in every case they have been able to confirm the conclusion that the atomic volume of isotopes is constant, the uranium lead being as much lighter as its atomic weight is smaller than common lead. Many careful investigations of the spectra of these varieties of lead show that the spectrum is absolutely the same so far as can be seen. thorium and ionium A second quite independent case of a difference In atomic weight between isotopes has been established. It concerns the isotopes thorium and ionium, and it is connected in an important way with the researches of which, on two previous occasions, I have given an account here, the researches on the growth of radium from uranium which have been in progress now for fourteen years. It is the intervention of ionium with its very long period of life which has made the experimental proof of the production of radium from uranium such a long piece of work. Previously only negative results w ere available. One could only say from the smallness of the expected growth of radium that the period of average life of ionium must be at least 100,000 years, forty times longer than that of radium, and, therefore, that there must be at least forty times as much ionium by weight as radium in uranium minerals, or at least 13.6 grams per 1,000 kilos of uranium. Since then further measurements carried out by Miss Hitchins last year have shown definitely for the first time a clear growth of radium from uranium in the largest preparation, containing 3 kilos of uranium, and this growth, as theory requires, is proceeding according to the square of the time. In three years it amounted to 2 x 10—” grams of radium, and in six years to just four times this quantity. From this result it was concluded that the previous estimate of 100,000 years for the period of ionium, though still of the nature of a minimum rather than a maximum, was very near to the actual period. Joachimsthal pitch-blende, the Austrian source of radium, contains only an infinitesimal proportion of thorium. An ionium preparation, separated by Auer von Welsbach from 30 tons of this mineral, since no thorium was added during the process, was an extremely concentrated ionium preparation. The atomic weight of ionium—calculated by adding to the atomic weight of its product, radium, four for the a particle expelled in the change—is 230, whereas that of thorium, its isotope, is slightly above 232. The question was whether the ionium-thorium preparation would contain enough ionium to show the difference. Honigschmid and Mlle. Horovitz have made a special examination of the point, first re-determining as accurately as possible the atomic weight of thorium, and then that of the thorium-ionium preparation from pitch-blende. They found 232.12 for the atomic weight of thorium, and by the same method 231.51 for that of the ionium-thorium. A very careful and complete examination of the spectra of the two materials showed for both absolutely the same spectrum and a complete absence of impurities. If the atomic weight of ionium is 230, the ionlum- thorium preparation must, from its atomic weight, contain 30 per cent. of ionium and 70 per cent. of thorium by weight. Prof. Meyer has made a comparison of the number of a particles given per second by this preparation with that given by pure radium, and found it to be in the ratio of 1 to 200. If 30 per cent. is ionium, the activity of pure ionium would be one-sixtieth of that of pure radium, and its period some sixty times greater, or 150,000 years. This confirms in a very satisfactory manner our direct estimate of 100,000 years as a minimum, and incidentally raises rather an interesting question. My direct estimate involves directly the period of uranium itself, and if the value accepted for this is too high, that for the ionium will be correspondingly too low. Now, on May 11 Prof. Joly was bringing before you, I believe, some of his exceedingly interesting work on pleochroic halos, from which he has grounds for the conclusion that the accepted period of uranium may be too long. But since I obtained, for the period of ionium, a minimum value two-thirds of that estimated by Meyer from the atomic weight, it is difficult to believe that the accepted period of uranium can have been overestimated by more than 50 per cent. of the real period. The matter could be pushed to a further conclusion if it were found possible to estimate the percentage of thorium in the thorium-ionium preparation, a piece of work that ought not to be beyond the resources of radio-chemical analysis, This would then constitute a check on the period of uranium as well as on that of ionium. Such a direct check would be of considerable importance in the determination of geological ages. The period of ionium enables us to calculate the ratio between the weights of ionium and uranium in pitchblende as 17.4 to 10”, and the doctrine of the non- separability of isotopes leads directly to the ratio between the thorium and uranium in the mineral as 41.7 to 100. This quantity of thorium is unfortunately too small for direct estimation. Otherwise it would be possible to devise a very strict test of the degree of non- separability. As it is, the work is sufficiently convincing. Thirty tons of a mineral containing a majority of the known elements in detectable amount. in the hands of one whose researches in the most difficult field of chemical separation are world-renowned, yield a preparation of the order of one-millionth of the weight of the mineral. which cannot be distinguished from pure thorium in its chemical character. Anyone could tell In the dark that it was not pure thorium, for its a activity is 30,000 times greater than that of thorium. This is then submitted to that particular series of purifications designed to give the purest possible thorium for an atomic weight determination, and it emerges without any separation of the ionium, but with a spectrum identical with that of a control specimen of thorium similarly purified. The complete absence of Impurities in the spectrum shows that the chemical work has been very effectively done, and the atomic weight shows that it must contain 30 per cent. by weight of the isotope ionium. a result which agrees with its a activity and the now known period of the latter. DETERMINATION OF ATOMIC WEIGHTS The results enumerated thus prove that the atomic weight can no longer be regarded as a natural constant, or the chemically pure element as a homogeneous type of matter. The latter may be, and doubtless often is, a mixture of isotopes varying in atomic weight over a small number of units, and the former then has no exact physical significance, being a mean value in which the proportions of the mixture as well as the separate atomic weights are both unknown. New Ideals emerge and old ones are resuscitated by this development. There may be, after all, a very simple numerical relation between the true atomic weights. The view that seems most probably true at present is that while hydrogen and helium may be the ultimate constituents of matter in the Proutian sense, and the atomic weights therefore approximate multiples of that of hydrogen. small deviations, such as exist between the atomic weights of these two constituent elements themselves, may be due to the manner in which the atom is constituted, in accordance with the principle of mutual electromagnetic mass, developed by Silberstein and others. The electro-magnetic mass of two charges in juxtaposition would not be the exact sum of the masses when the charges are separated. The atomic weight of hydrogen is 1.0078 in terms of that of helium as 3.99, and that the latter is not exactly four times the former may be the expression of this effect. Harkins and Wilson have recently gone into the question with some thoroughness, and the conclusion of most interest in the present connection which appears to emerge is in favor of regarding most of the effect to occur in the formation of helium from hydrogen, and very little In subsequent aggregations of the helium. In the region of the radio-elements, where we have abundant examples of the expulsion of helium atoms as a particles, it seems as if we could almost safely neglect this effect altogether. Thus radium has the atomic weight almost exactly 226, and the ultimate product almost exactly 206, showing that in five a- and four (?-ray changes the mean effect is nil, and the atomic weights are, moreover, integers in terms of oxygen as 16, or helium 4. It is true that the atomic weights of both thorium and uranium are between 0.1 and 0.2 greater than exact integers, but it is difficult to be sure that this difference is real. When, among the light elements, we come across a clear case of large departure from an integral value, such as magnesium, 24.32, and chlorine, 35.46, we may reasonably suspect the elements to be a mixture of isotopes. If this is true for chlorine, it suggests a most undesirable feature in the modern practice of determining atomic weights. More and more the one method has come to be relied upon, the preparation of the chloride of the element and the comparison of its weight with that of the silver necessary to combine with the chlorine, and with the weight of the silver chloride formed. Almost the only practical method, and that a very laborious and imperfect one, which may be expected to resolve a mixture of isotopes is by long-continued fractional gaseous diffusion, which is likely to be the more effective the lower the atomic weight. Assume, for example, that chlorine were a mixture of isotopes of separate atomic weights 34 and 36, or 35 and 36. The 34 isotope would diffuse some 3 per cent. faster than the 36, and the 35 some 1.5 per cent. faster. The determination of the atomic weight of chlorine in terms of that of silver has reached now such a pitch ef refinement that it should be able to detect a difference in the end fractions of the a^^i'C weight of chlorine, if chlorine or hydrogen chloride were systematically subjected to diffusion. It is extremely desirable that such a test of the homogeneity of this gas should be made in this way. Clearly a change must come in this class of work. It is not of much use starting with stuff out of a bottle labelled “purissimum,” or “garantirt,” and determining to the highest possible degree of accuracy the atomic weight of an element of unknown origin. The great pioneers in the subject, like Berzelius, were masters of 'the whole domain of inorganic chemistry, and knew the sources of the elements in Nature firsthand. Their successors must revert to their practice and go direct to Nature for their materials, must select them carefully with due regard to what geology teaches as to their age and history, and, before carrying out a single determination, they must analyze their actual raw materials completely, and know exactly what it is they are dealing with. Much of the work on the atomic weight of lead from mixed minerals is useless from failure to do this. Workers must rely more on the agreement, or disagreement, of a great variety of results by methods and for materials as different as possible than on the result of a single method pushed to the limit of refinement for an element provisionally purified by a dealer from quite unknown materials. The preconceived notion that the results must necessarily agree if the work is well done must be replaced by a system of co-operation between the workers of the world checking each other's results for the same material. A year ago anyone bold enough to publish atomic weight determinations which were not up to the modern standards of agreement among themselves would have been regarded as having mistaken his vocation. If these wider ideals are pursued, all the labor that has been lavished in this field, and which now seems to have been so largely wasted, may possibly bear fruit, and where the newer methods fail, far beyond the narrow belt of elements which it is possible to watch changing, the atomic weight worker may be able to pick up the threads of the great story. No doubt it is writ in full in the natural records preserved by rock and mineral, and the evidence of the atomic weights may be able to carry to a triumphant conclusion the course of elementary evolution, of which as yet only an isolated chapter has been deciphered. the structure OF the atom The third line of recent advance which does much to explain the meaning of isotopes and the periodic law starts from Sir Ernest Rutherford's nuclear theory of the atom, which is an attempt to determine the nature of atomic structure, which again is the necessary preliminary to the understanding of the third aspect in which the elements are, or may be, complex. That uranium and thorium are built up of different isotopes of lead, helium and electrons is now an experimental fact, since they have been proved to change into these constituents. But the questions how they are built up, and what is the nature of the non-radio-active elements, which do not undergo changes, remain unsolved. Prof. Bragg showed in 1905 that the a particles can traverse the atoms of matter in their path almost as though they were not there. So far as he could tell—and the statement is still true of the vast majority of a particles colliding with the atoms of matter—the a particle ploughs its way straight through, pursuing a practically rectilinear course, losing slightly in kinetic energy at each encounter with an atom until its velocity is reduced to the point at which it can no longer be detected. From that time the a particle became, as it were, a messenger that could penetrate the atom, traverse regions which hitherto had been bolted and barred from human curiosity, and on re-emerging could be questioned, as it was questioned effectively by Rutherford, with regard to what was inside. Sir J. J. Thomson, using the electron as the messenger, had obtained valuable information as to the number of electrons in the atom, but the massive material a particle alone can disclose the material atom. It was found that though the vast majority of a particles re-emerge from their encounters with the atoms practically in the same direction as they started, suffering only slight hither and thither scattering due to their collisions with the electrons in the atom, a minute proportion of them suffer very large and abrupt changes of direction. Some are swung around, emerging in the opposite to their original direction. The vast majority, that get through all but undefiected, have met nothing in their passage save electrons, 8,000 times lighter than themselves. The few that are violently swung out of their course must have been in collision with an exceedingly massive nucleus in the atom, occupying only ail insignificant fraction of its total volume. The atomic volume is the total volume swept out by systems of electrons in orbits of revolution round the nucleus, and beyond these rings or shells guarding the nucleus it is ordinarily impossible to penetrate. The nucleus is regarded by Rutherford as carrying a single concentrated positive charge, equal and opposite to that of the sum of the electrons. Chemical phenomena deal almost certainly with the outermost system of detachable or valency electrons alone, the loss or gain of which conditions chemical combining power. Light spectra originate probably in the same region, though possibly more systems of electrons than the outermost may contribute, while the X-rays and 'Y rays seem to take their rise in a deep- seated ring or shell around the nucleus. But mass phenomena, all but an insignificant fraction, originate in the nucleus. In the original electrical theory of matter the whole mass of the atom was attributed to electrons, of which there would have been required nearly 2,000 times the atomic weight in terms of hydrogen as unity. With the more definite determination of this number and the realization that there were only about half as many as the number representing the atomic weight, it was clear that all but an insignificant fraction of the mass of the atom was accounted for. In the nuclear hypothesis this mass is concentrated in the exceedingly minute nucleus. The electromagnetic theory of inertia accounts for the greater mass if t1]e positive charges that make up the nucleus are very much more concentrated than the negative charges which constitute the separate electrons. The experiments on scattering clearly indicated the existence of such a concentrated central positive charge, or nucleus. The mathematical consideration of the results of a-ray scattering, obtained for a large number of different elements, and for different velocities of a ray, gave further evidence that the number of electrons, and therefore the + charge on the nucleus, is about half the number representing the atomic weight. But van der Broek, reviving an isolated suggestion from a former paper full of suggestions on the periodic law, which were, I think, in every other respect at fault, pointed out that closer agreement with the theory would be obtained if the number of electrons in the atom, or the nuclear charge, was the number of the place the element occupied in the periodic table. This is now called the atomic number, that of hydrogen being taken as 1, helium 2, lithium 3, and so on to the end of the table, uranium 92, as we now know. For the light elements it is practically half the atomic weight, for the heavy elements rather less than half. I pointed out that this accorded well with the law of radio-active change that had been established to hold over the last thirteen places in the periodic table. This law might be expressed as follows:—The expulsion of the a particle carrying two positive charges lowers the atomic number by two, while the expulsion of the (3 particle, carrying a single negative charge, increases it by one. In ignorance of van der Broek's original suggestion, I had, in representing the generalization, shown the last thirteen places as differing unit by unit in the number of electrons in the atom. Then followed Moseley's all-embracing advance, showing how from the wave-lengths of the X-rays, characteristic of the elements, this conception explained the whole periodic table. The square roots of the frequency of the characteristic X-rays are proportional to the atomic numbers. The total number of elements existing between uranium and hydrogen could thus be determined, and it was found to be ninety-two, only five of the places being vacant. The “exceptions” to the periodic law, such as argon and potassium, nickel and cobalt, tellurium and iodine, in which an element with higher atomic weight precedes instead of succeeding one with lower, were confirmed by the determination of the atomic numbers in every case. From now oii, this number, which represents the + charge on the nucleus I'ather than the atomic weight, becomes the natural constant which determines chemical character, light and X-ray spectra, and, in fact, all the properties of matter except those that depend directly on the nucleus—mass and weight on one hand, and radio-active properties on the other. What, then, were the isotopes on this scheme? Obviously they were elements with the same atomic number, the same net charge on the nucleus, but with a differently constituted nucleus. Take the very ordinary sequence in the disintegration series, one a and two (3 rays being successively expelled in any order. Two + and two — charges have been expelled, the net charge of the nucleus remains the same, the chemical character and spectrum the same as those of the first parent, but the mass is reduced four units because a helium atom, or rather nucleus, has been expelled as an a particle. The mass depends on the gross number of + charges in the nucleus, chemical properties on the difference between the gross numbers of + and — charges. But the radio-active properties depend not only on the gross number of charges, but on the constitution of the nucleus. We can have isotopes with identity of atomic weight, as well as of chemical character, which are different in their stability and mode of breaking up. Hence we can infer that this finer degree of isotopy may also exist among the stable elements, in which case it would be completely beyond our present means to detect. But when transmutation becomes possible such a difference would be at once revealed. The case is not one entirely of academic interest, because it is probable that the reconciliation of the conflicting views of the geologists and chemists who concluded tliat lead was not the ultimate product of thorium, and those who by atomic weight determinations on the lead have shown that it is, depends probably on this point. As has long been thorium-C, an isotope of bismuth, disintegrates dually. For 35 per cent. of the atoms disintegrating, an a ray is expelled, followed by a (3 ray. For the remaining 65 per cent. the fI ray is first expelled, and is followed by the a ray. The two products are both isotopes of lead, and both the same atomic weight, but they are not the same. More energy is expelled in the changes of the ((5 per cent. fraction than in those of the 35 per cent. Unless they are both completely stable a difference of period of change is to be anticipated. The same thing is true for radium-C, though here all but a very minute proportion of the atoms disintegrating follow the mode followed by the 65 per cent. in the case of thorium-C. The product in this case, ra- dium-D, which. of course, is also an isotope of lead, with atomic weight 210, is not permanently stable, though it has a fairly long period, twenty-four years. The other product is not known to change further. but then, even if it did, it is in such small quantity that it is doubtful whether the change would have been detected. But, so far as is known, it forms a stable isotope of lead of atomic weight 210, formed in the proportion of only 0.03 per cent. of the whole. Now the atomic weight evidence merely shows that one of the two isotopes of lead formed from thorium is stable enough to accumulate over geological epochs, and it does not necessarily follow that both are. Dr. Arthur Holmes has pointed out to me that the analysis I gave of the Ceylon thorite leads to a curiously anomalous value for the age of the mineral. The quantity of thorium lead per gram of thorium is 0.0062, and this, divided by the rate at which the lead is being produced, 4.72 x l^—11 grams of lead per gram of thorium per year, gives the age as ]31 million years. But a Ceylon pitch-blende, with uranium 72.88 per cent.. and lead 4.65 per cent., and ratio of lead to uranium as 0.064, gives the age as 512 million years. Dr. Holmes regards the two minerals as likely to be of the same age, and the pitch-blende to be, of all the Ceylon results, the one most truthworthy for age measurement. If we suppose that, as in the case of radium-D, the 05 per cent. isotope of lead derived from thorium is not stable, and that only the 35 per cent. isotope accumulates, the age of the mineral would be 375 million years, which the geologists are likely to consider much nearer the truth. But the most interesting point is that, if we take the atomic weight of the lead isotope derived from uranium as 206.0, and that derived from thorium as 208.0, and calculate the atomic weight of the lead in Ceylon thorite, assuming it to consist entirely of uranium lead and of only the 35 per cent. isotope from thorium, we get the value 207.74, which is exactly what I found from the density, and what Prof. Honigschmid determined (207.77) (compare.vaturc, May 24, p. 244). The question remains: If this is what occurs, what does this unstable lead change iato? If an a particle were expelled mercury would result, or if a (3 particle bismuth, two elements of which I could find no trace in the lead group separated from the whole 20 kilos of mineral. But if an a and a (3 particle were both expelled, the product wonld be thallium, which is P'l'es- ent in amount small, but sufficient for chemical as well as spectroscopic characterization. If the process of disintegration does proceed as suggested, it should be possible to trace it, for this particular lead should give a feeble specific a or (3 radiation, in addition, of course, to that due to other lead isotopes. So far it has not been possible to test this. In the meantime, the explanation offered is put forward provisionally as being consistent with all the known evidence. Looking for a moment, in conclusion, at the broader aspects of the new ideas of atomic structure, it seems that though a sound basis for further development has been roughed out, almost all the detail remains to be supplied. We have got to know the nucleus, but, beyond the fact that it is constituted, in heavy atoms, of nuclei of helium and electrons, nothing is known; whilst, as regards the separate shells or rings of electrons Which neu.tralize itfl charge and are supposed to surround it like the shells of an onion, we really know nothing yet at all. The original explanation, in terms of the electron, of the periodicity of properties displayed by the elements still remains all that has been attempted. We may suppose that as we pass through the successive elements in the table one more electron is added to the outermost ring for each unit increase in the charge on the nucleus, or atomic number, and that when a certain number, 8 in the early part of the table, and 18 in the later, has been added, a complete new shell or ring forms, which no longer participates directly in the chemical activities of the atom. Thanks, however, to Moseley's work, this, now, is not sufficiently precise. For we know the exact number of the elements and the various atomic numbers at which the remarkable changes, in the nature of the periodicity displayed, occur. Any real knowledge in this field will account not only for the two short initial periods, but also for the curious double periodicity later on, in which the abrupt changes of properties in the neighborhood of the zero family alternate with the gradual changes in the neighborhood of the eighth groups. The extraordinary exception to the principle of the whole scheme presented by the rare-earth elements remains a complete enigma, none the less impressive because, beyond them in the table, the normal course is again resumed and continues to the end. This latter, highly significant, feature of the periodic table is one of the definite conclusions following from the chemical characterizations of the numerous radio-elements.
This article was originally published with the title "The Complexity of the Chemical Elements—II" in s , , 94-96 (December 2012)