Supplement to ''Nature,'' September 15, 1923 



415 



of highest frequency arises when the electron is removed 

 from the K group. 



It is possibly too soon to express a final opinion on 

 the accuracy of this theory which defines the outer 

 structure of the atom, but there can be no doubt that 

 Kt constitutes a great advance. Not only does it offer 

 a general explanation of the optical and X-ray spectra 

 of the atom, but it accounts in detail for many of the 

 most characteristic features of the periodic law of 

 Mendeleeff. It gives us for the first time a clear idea 

 of the reason for the appearance in the family of 

 elements of groups of consecutive elements with similar 

 chemical properties, such as the groups analogous to 

 the iron group and the unique group of rare earths. 

 The theory of Bohr, like all living theories, has not 

 only correlated a multitude of isolated facts known 

 about the atom, but has shown its power to predict 

 new relations which can be verified by experiment. 

 For example, the theory predicted the relations which 

 must subsist between the Rydberg constants of the 

 arc and spark spectra, and generally between all the 

 successive optical spectra of an element, a prediction 

 so strikingly confirmed by Paschen's work on the 

 spectrum of doubly ionised aluminium and Fowler's 

 worik on the spectrum of trebly ionised silicon. Finally, 

 it predicted with such great confidence the chemical 

 properties of the missing element, number 72, that it 

 gave the necessary incentive for its recent discovery. 



While the progress of our knowledge of the outer 

 structure of atoms has been much more rapid than 

 could have been anticipated, we clearly see that only 

 a beginning has been made on this great problem, and 

 that an enormous amount of work is still required 

 before we can hope to form anything like a complete 

 picture even of the outer structure of the atom. We 

 may be confident that the main features of the structure 

 are clear, but in a problem of such great complexity 

 progress in detail must of necessity be difficult and 

 slow. 



We have not so far referred to the very difficult 

 question of the explanation on this theory of the 

 chemical combination of atoms. In fact, as yet the 

 theory has scarcely concerned itself with molecular 

 structure. On the chemical side, however, certain 

 advances have already been made, notably by G. N. 

 Lewis, Kossel, and Langmuir, in the interpretation of 

 the chemical evidence by the idea of shared electrons, 

 which play a part in the electronic structure of two 

 combined atoms. There can be little doubt that the 

 next decade will see an intensified attack by physicists 

 und chemists on this very important but undoubtedly 

 very complicated question. 



Before leaving this subject, it may be of interest to 

 refer to certain points in Bohr's theory of a more 

 philosophical nature. It is seen that the orbits and 

 energies of the various groups of electrons can be 

 specified by certain quantum numbers, and the nature 

 if the radiation associated with a change of orbit can 

 l)e defined. But at the same time we cannot explain 

 why these orbits are alone permissible under normal 

 conditions, or understand the mechanism by which 

 radiation is emitted. It may be quite possible to 

 formulate accurately the energy relation of the electrons 

 in the atom on a simple theory, and to explain in 

 considerable detail all the properties of an atom, 



without any clear understanding of the underlying 

 processes which lead to these results. It is natural to 

 hope that with advance of knowledge we may be able 

 to grasp the details of the process which leads to the 

 emission of radiation, and to understand why the orbits 

 of the electrons in the atom are defined by the quantum 

 relations. Some, however, are inclined to take the 

 view that in the present state of knowledge it may be 

 quite impossible in the nature of things to form that 

 detailed picture in space and time of successive events 

 that we have been accustomed to consider as so im- 

 portant a part of a complete theory. The atom is 

 naturally the most fundamental structure presented 

 to us. Its properties must explain the properties of 

 all more complicated structures, including matter in 

 bulk, but we may not, therefore, be justified in expect- 

 ing that its processes can be explained in terms of 

 concepts derived entirely from a study of molar 

 properties. The atomic processes involved may be so 

 fundamental that a complete understanding may be 

 denied us. It is early yet to be pessimistic on this 

 question, for we may hope that our difficulties may any 

 day be resolved by further discoveries. 



We must now turn our attention to that new and 

 comparatively unexplored territory, the nucleus of 

 the atom. In a discussion on the structure of the 

 atom ten years ago, in answer to a question on the 

 structure of the nucleus, I was rash enough to say 

 that it was a problem that might well be left to the 

 next generation, for at that time there seemed to be 

 few obvious methods of attack to throw light on its 

 constitution. While much more progress has been 

 made than appeared possible at that time, the problem 

 of the structure of the nucleus is inherently more 

 difficult than the allied problem already considered of 

 the structure of the outer atom, where we have a 

 wealth of information obtained from the study of 

 light and X-ray spectra and from the chemical pro- 

 perties to test the accuracy of our theories. 



In the case of the nucleus, we know its resultant 

 charge, fixed by Moseley's law, and its mass, which is 

 very nearly equal to the mass of the whole atom, 

 since the mass of the planetary electrons is relatively 

 very small and may for most purposes be neglected. 

 We know that the nucleus is of size minute compared 

 with that of the whole atom, and can with some con- 

 fidence set a maximum limit to its size. The study 

 of radioactive bodies has provided us with very valu- 

 able information on the structure of the nucleus, for 

 we know that the a- and /^-particles must be expelled 

 from it, and there is strong evidence that the very 

 penetrating y-rays represent modes of vibration of 

 the electrons contained in its structure. In the long 

 series of transformations which occur in the uranium 

 atom, eight o-particles are emitted and six electrons, 

 and it seems clear that the nucleus of a heavy atom 

 is built up, in part at least, of helium nuclei and 

 electrons. It is natural to suppose that many of the 

 ordinary stable atoms are constituted in a similar way. 

 It is a matter of remark that no indication has been 

 obtained that the lightest nucleus, namely, that of 

 hydrogen, is liberated in these transformations, where the 

 processes occurring are of so fundamental a character. 

 At the same time, it is evident that the hydrogen 

 nucleus must be a unit in the structure of some atoms, 



