Supplement to '' Nature'' J^ily 7, 1923 



43 



until now it has not been possible, however, to give 

 any theoretical basis for such a development of an 

 inner group, we see that our extension of the quantum 

 theory provides us with an unforced explanation. 

 Indeed, it is scarcely an exaggeration to say that if 

 the existence of the rare-earths had not been established 

 by direct chemical investigation, the occurrence of 

 a family of elements of this character within the 

 6th period of the natural system of the elements 

 might have been theoretically predicted. 



When we proceed to the 7th period of the system, 

 we meet for the first time with 7-quantum orbits, 

 and we shall expect to find within this period features 

 that are essentially similar to those in the 6th period, 

 in that besides the first stage in the development of 

 the 7-quantum orbits, we must expect to encounter 

 further stages in the development of the group with 

 6- or 5-quantum orbits. However, it has not been 

 possible directly to confirm this expectation, because 

 only a few elements are known in the beginning of 

 the 7th period. The latter circumstance may be 

 supposed to be intimately connected with the instability 

 of atomic nuclei with large charges, which is expressed 

 in the prevalent radioactivity among elements with 

 high atomic number. 



X-RAY Spectra and Atomic Constitution. 



In the discussion of the conceptions of atomic 

 structure we have hitherto placed the emphasis on 

 the formation of the atom by successive capture of 

 electrons. Our picture would, however, be incomplete 

 without some reference to the confirmation of the 

 theory afforded by the study of X-ray spectra. Since 

 the interruption of Moseley's fundamental researches 

 by his untimely death, the study of these spectra has 

 been continued in a most admirable way by Prof. 

 Siegbahn in Lund. On the basis of the large amount 

 of experimental evidence adduced by him and his 

 collaborators, it has been possible recently to give a 

 classification of X-ray spectra that allows an immediate 

 interpretation on the quantum theory. In the first 

 place it has been possible, just as in the case of the 

 optical spectra, to represent the frequency of each 

 of the X-ray lines as the difference between two out 

 of a manifold of spectral terms characteristic of the 

 element in question. Next, a direct connexion with 

 the atomic theory is obtained by the assumption 

 that each of these spectral terms multiplied by Planck's 

 constant is equal to the work which must be done 

 on the atom to remove one of its inner electrons. 

 Tn fact, the removal of one of the inner electrons 

 from the completed atom may, in accordance with 

 the above considerations on the formation of atoms by 

 capture of electrons, give rise to transition processes 

 by which the place of the electron removed is taken 

 by an electron belonging to one of the more loosely 

 bound electron groups of the atom, with the result 

 that after the transition an electron will be lacking 

 in this latter group. 



The X-ray lines may thus be considered as giving 

 evidence of stages in a process by which the atom 

 undergoes a reorganisation after a disturbance in its 

 interior. According to our views on the stability of 

 the electronic configuration such a disturbance must 

 consist in the total removal of electrons from the atom. 



or at any rate in their transference from normal orbits 

 to orbits of higher quantum numbers than those 

 belonging to completed groups ; a circumstance which 

 is clearly illustrated in the characteristic difference 

 between selective absorption in the X-ray region, 

 and that exhibited in the optical region. 



The classification of the X-ray spectra, to the achieve- 

 ment of which the above-mentioned work of Sommer- 

 feld and Kossel has contributed materially, has 

 recently made it possible, by means of a closer examina- 

 tion of the manner in which the terms occurring in 

 the X-ray spectra vary with the atomic number, to 

 obtain a very direct test of a number of the theoretical 





/ftomic //umber.s 

 Fig. 10. 



conclusions as regards the structure of the atom. 

 In Fig. 10 the abscissae are the atomic numbers and 

 the ordinates are proportional to the square roots 

 of the spectral terms, while the symbols K, L, M, N, 0, 

 for the individual terms refer to the characteristic 

 discontinuities in the selective absorption of the 

 elements for X-rays ; these were originally found by 

 Barkla before the discovery of the interference of 

 X-rays in crystals had provided a means for the 

 closer investigation of X-ray spectra. Although the 

 curves generally run very uniformly, they exhibit 

 a number of deviations from uniformity which have 

 been especially brought to light by the recent investiga- 

 tion of Coster, who has for some years worked in 

 Siegbahn's laboratory. 



These deviations, the existence of which was not 

 discovered until after the publication of the theory 



