and the Theory of Atomic Structure. 1071 



groups of energy-levels whose existence may be inferred 

 from the X-ray spectrum are usually denoted as K, L, M, N. . . 

 levels. As pointed out by Kossel, these levels may be assumed 

 to be connected with the different groups of electrons in the 

 atom, in such a way that the various energy terms correspond 

 to the work required for the removal of an electron from one 

 of these groups. The various groups of electrons of the atom 

 are in consequence often lermed the K-, L-, M-, N- ... 

 shells. This interpretation of the levels affords an explana- 

 tion of the laws governing the absorption in the X-ray 

 region : it is well known that this absorption is not connected 

 with the single lines in the spectrum but extends over 

 spectral regions which are sharply limited by the so-called 

 absorption edges, the frequencies of which correspond to the 

 energies of the levels concerned. Corresponding to the 

 different ways in which the removal of an election from a 

 shell may take place we obtain several levels for each shell. 



As has been pointed out by Sommerfeld, part at any rate 

 of this complexity in the groups of levels may be connected 

 with the complexity of the ensemble of the stationary states 

 of the hydrogen atom. According to Sommerfeld's theory 

 of the fine structure of the hydrogen lines, the stationary 

 states of an atom containing one electron are characterized 

 by two quantum numbers. One of these numbers, which 

 we shall term the total quantum number n, is the same as 

 that occurring in Bohr's interpretation of the simple formula 

 for the hydrogen spectrum. The other number is the so- 

 called "azimuthal" quantum number, which determines the 

 value of the angular momentum of the electron round the 

 nucleus, and which we shall denote by k. 



§2. In his recent publications* Bohr has developed a 

 theory of atomic structure which contains certain essentially 

 new features, and which seems to give a natural interpreta- 

 tion of the periodic system and at the same time to offer an 

 explanation of the results of Kossel and Sommerfeld on the 

 X-ray spectrum. According to Bohr's theory, the 'orbits of 

 the electrons in the different groups of the atom are charac- 

 terized by different total quantum numbers, this number 

 being equal to 1 for the innermost group (K-shell), 2 for the 

 next group (L-shell), and so on, every time increasing by one 

 unit until the surface of the atom is reached. Within each 

 group the electrons are again divided into sub-groups, corre- 

 sponding to different types of orbits and characterized by 

 different values of k. A survey of the gradual development 



* ' Nature,' March 1921 and October 1921 . See also for a fuller account, 

 Zeitschrift f. Physik, ix. p. 1 (1922). 



