105 



necessary to take into account, that for an electron moving in a cenlrnl licM Ihc 

 stationary states corresponding to same /) bul différent % will nol be a-[)ri()ri ('(iiially 

 probable (Compare Part I p. 27). 



§ 2. Series spectra of individual elements. 



By means of a comparison with the spectrum to be expected from a system 

 consisting of an electron moving in a central field of force we may thus explain 

 certain general features of the series spectra of elements of higher atomic number. 

 We cannot expect, however, that it is possible in this way to account in detail for 

 the spectra of the elements, as is indicated already by the complex structure (doub- 

 lets, triplets, satellites etc.), shown by the lines of many spectra. In a detailed dis- 

 cussion of these spectra it seems necessary to take the mutual perturbing eflect of 

 the orbits of the inner electrons and of the outer electron into account. In general 

 this constitutes a very intricate problem due to the fact that already when the outer 

 electron is absent, the system of inner electrons will in general be unstable for 

 small displacements, if the effect of such displacements is calculated by means of 

 ordinary mechanics. In case of helium, however, where there are only two electrons 

 in the neutral atom, this is different since the motion of the inner electron will be 

 mechanically stable for any shape or position of its orbit, if the outer electron is 

 removed to infinite distance from the nucleus. Just in this property of the helium 

 atom an explanation may be sought for the fact that helium, besides its simple 

 spark spectrum, mentioned in Part II § 1, possesses two complete series spectra of 

 the first order, the so called orthohelium and parhelium spectrum, for which no 

 mutual combination lines are observed. This, which is seen to be in striking contrast 

 to what should be expected for the spectrum of a simple central system, must be 

 ascribed to the existence of two different sets of stationary states of the neutral 

 helium atom corresponding to two different types of motion of the inner electron. 

 This problem will be discussed in a later paper based on a detailed study of the 

 mutual perturbations of the orbits of the two electrons in the helium atom performed 

 in collaboration with Mr. H. A. Kramers, and it will be shown that it seems possible 

 on the assumption that both electrons move in the same plane to obtain an inter 

 pretation of both helium spectra by means of considerations of the same kind 

 as those applied in the preceding Parts. 



In case of lithium, where the neutral atom contains three electrons, only one 

 series spectrum of the first order is observed. In this case we may assume, that 

 when one of the electrons is removed the two other electrons will move in the 



same circular orbit round the nucleus each with angular momentum , as in the 



normal state of the helium atom '). Looking apart from the mechanical instability 



') See N. Bohr, Phil. Mag. XXVI, p. 489, 491 (1913J. 



