1072 



Mr. D. Coster on the Spectra of X-rays 



of these groups and sub-groups with increasing atomic 

 number is illustrated by the following table, which indic;ites 

 the proposed constitution of the atoms of the inert gases. 



Element. 



Number of electrons in n^ orbits. 



2 







2 2 



h 



3 



3 3 



k 



4 ? 



4a 



4 4 



_ 

 5i 



5 2 



5 3 



5 4 



5 5 



6i 



6 2 



6 3 



64 



65 6 6 



Helium 2 ... 



Neon 10 ... 



2 



4 



4 





































Argon 18 ... 



2 



4 



4 



4 



4 



- 































Krypton 36 ... 



2 



4 



4 



6 



6 



6 



* 



4 



- 



- 























Xenon 54 ... 



2 



4 



4 



6 



6 



6 



6 



6 



6 



- 



4 



4 



- 



- 



- 













Niton 86 ... 



2 



4 



4 



6 



6 



6 



8 



8 



8 



8 



6 



1 



6 



6 



— 



- 



4 



4 



— 



— 





§ 3. In previous papers * I have been able to show that 

 nearly all the lines of the X-ray spectra of the heavier 

 elements can be arranged in a simple scheme, involving the 

 existence of one K-level, three L-levels, five M -levels, and 

 seven N-levels t> and that in continuation of the work of 

 Sommerfeld it is possible to characterize every level in a 

 definite way by means of two quantum numbers n and k as 

 denned above. Further, the appearance of the observed 

 X-ray lines was found to be governed by two simple " rules 

 of selection/' According to the first rule only those com- 

 binations between two levels will appear, for which the 

 quantum number k remains unaltered or changes by one 

 unit. The second rule t states that the levels may be divided 



* Zeitschrift f. Physik, v. p. 139 (1921), denoted in the following- by I., 

 and vi. p. 185 (1921), (denoted by II). Compare also : A. Smekal, 

 Zeitschr.f. Phys. v. p. 91 (1921), and v. p. 121 (1921) ; A. Dauvillier, 

 C. P. clxxii. p. 1350 (1921), C. P. clxxiii. p. 35 (1921), C. P. clxxiii. 

 p. 647 (1921) ; G. Wentzel, Zeitschr. f. Phys. vi. p. 84 (1921) ; A. Som- 

 merfeld and Gr. Wentzel, Zeitschr.f. Phys. vii. p. 86 (1921). 



t This also suggests that we must expect the existence of one K-, three 

 L-, five JVI-, and seven N-absorption edges. These absorption edges have 

 actually been found in the K- and L-series,by several authors for different 

 elements, and recently I have been able to establish the existence of five 

 absorption edges in the M-series for U and Th. (See Phys. Eev. II. 

 xix. p. 20, 1922.) 



\ This rule of selection has been stated independently by Sommerfeld 

 and Wentzel in another way. They introduce a third quantum-number, 

 " Grundquantenzahl." This number is equal to or one unit larger than 

 the azimuthal quantum number. The transitions are subject to the 

 condition that this " Grundquantenzahl " must change by one unit. As 

 we do not yet know the physical meaning of this u Grundquantenzahl," 

 it seemed to me better to state this rule of selection in the same way as 

 it was first suggested to me by Mr. H. A. Kramers. 



