120 BELL SYSTEM TECHNICAL JOURNAL 



elements, the intrusion of which Into the Periodic Table brings about 

 a disruption of the simplicity of its first three rows. There are other 

 such intrusive groups of elements, notably the celebrated groups of the 

 rare earths. It is supposed that wherever such a group commences, 

 there the residue begins to vary from one atom to the next. The spec- 

 troscopic evidence is lacking; it is awaited with extreme interest. 



The reader will very probably have seen one or more tables of the 

 distribution of electrons in atoms; tables in which it is stated, for 

 instance, that the atom of sodium contains two electrons moving in 

 li orbits, four in 2i orbits, four in 22 orbits, and one in a 3i orbit; 

 or more succinctly that it contains "two li, four 2i, four 2-2 and one 3i 

 electron." Such tables are built by piecing together bits of evidence, 

 some of which are such as I have described in this section, while 

 others are inferences from X-ray spectra, magnetic properties, or 

 observations of still other kinds. That they are still highly speculative 

 is confirmed by the fact that they are continually being remodeled. 

 If we could produce the spectra corresponding to all the stages of 

 formation of an atom, we should be able to set up a tabulation more 

 reliable than any yet put together. Even then, however, we should 

 be confronted with the question whether the addition of a new electron 

 to a kernel fundamentally alters the distribution of those already there. 



Having considered the facts at such length in this section, we are 

 entitled to consider the theory. In the coupled cases of hydrogen and 

 ionized helium it was shown by experiment, and rendered plausible 

 by theory, that the vStationary States of the element with one electron 

 and a double charge on its nucleus correspond exactly to those of the 

 element with one electron and a single charge on its nucleus, and are 

 endowed with fourfold the energy of these latter. This conclusion 

 can be extended to cover the case of a valence-electron circulating in 

 an orbit at a great distance from a kernel composed of (Z — b) electrons 

 and a nucleus bearing the charge -\-Ze. The field due to the kernel 

 will at great distances approximate the field due to a solitary nucleus 

 bearing the charge be. We have seen already that when b = l (so 

 that the total charge on the nucleus balances the total charge of the 

 electrons, valence-electron included) the Stationary States corres- 

 ponding to orbits for which n and k are large coincide with Stationary 

 States of h\-drogen. It follows equally that when b = 2, the Stationary 

 States for which n and k are large have approximately fourfold the 

 energy of stationary states of hydrogen, and coincide approximately 

 with Stationary States of ionized helium. This is verified by experi- 

 ment, and so are the corresponding conclusions for the cases b = 3 and 

 6 = 4. 



