1104: Mr. D. Coster on the Spectra of X-rays 



observe the same thing for Ta. It will be very difficult to 

 investigate j3 7 in the region Ba-Rb, as it must coincide for 

 several elements with /5 n or /3 U (see Part II. § 8). According 

 to Bohr's theory we must expect that this line may perhaps 

 occur as early as Rb, as for this element 5i electrons appear 

 for the first time. 



§ 8. It is a very important feature of Bohr's theory of 

 atomic structure, that within the region of the rare-earth 

 metals the N-shell is developed from a shell containing 3 

 subgroups of 6 electrons each with the quantum symbols 4 b 

 4 2 , 4 3 , into a shell containing 4 subgroups of 8 electrons each 

 with the quantum symbols 4 1? 4 2 , 4 3 , and 4 4 respectively. 

 In favour of this view the following experimental facts may 

 be considered. As is seen from the diagrams the energy 

 difference between the N 3 level and the region outside the 

 atom is given by the frequency difference of the L/3 2 line 

 and the L L discontinuity. This difference corresponds to a 

 volt difference of 420"for Bi (83), 170 for W (74), 90 for 

 Ba (56), and less than 10 for Ag (47). This means that in 

 the region from Ba to W the binding of the 4 3 elections 

 increases much more slowly than in the regions from Ag to 

 Ba and from W to Bi. This is just what we should expect 

 from theoretical considerations. Within the region of the 

 rare earths not only do the three subgroups of 4 1? 4 2 , and 4 3 

 electrons increase by two electrons each, but also a new 

 subgroup of eight 4 4 electrons is built up. As part of the 

 orbits of the 4 3 electrons lies outside the orbits of the 4 4 

 electrons, and as a 4 3 electron during its revolution spends 

 most of its time in this part wnere its velocity is smallest, 

 the 4 4 electrons must be assumed to contribute appreciably 

 to the total screening effect on the 4 3 electrons. 



New measurements of the absorption discontinuities of 

 the elements Ba-Sb which recently have been made by 

 Mr. Lindsay in this laboratory give also a strong support 

 to Bohr's theory. From the values of the L 3 -discontinuities 

 of these elements which Mr. Lindsay has kindly put at my 

 disposal and my values of the wave-lengths of the line Ly 4 

 for the same elements, we are able to calculate the energy 

 difference between the O r level and the outside of the atom. 

 This difference, the value of which most probably corresponds 

 to 30-40 volts for W(74), is still 30-25 volts for Ba (56) 

 and Cs (55), but decreases to less than 10 volts for Te (52) 

 and less than 6 volts for Sb (51). From this it appears 

 very clearly that the outmost shells of the atom (O- and 

 P-shell) do not change very much for the elements from W 



