286 BELL SYSTEM TECHNICAL JOURNAL 



that the concept of "spin " is a valuable addition to atom- models. This 

 feature it is which is responsible for calling the nucleus a "quantized" 

 vector. But I need not further justify at this point my suggestion that 

 spin is a quality both strange and remote from immediate experience, 

 for this will be only too evident in what is to follow. 



It is desirable to make a longish excursion through some of those 

 early-known and more-or-less familiar features of spectra, which are 

 explained solely by calling into account the orbital or circum-nuclear 

 electrons of the atom, without taking notice of any property of the 

 nucleus excepting its charge which holds those electrons together. 

 This excursion will have the incidental advantage of making us aware 

 of the magnetic moment and the spin of the electron, though we cannot 

 pause long enough to get an adequate appreciation of the scope and 

 value of this concept of the "spinning electron." It is best to simplify 

 these preliminary steps as much as is conveniently possible, and 

 therefore I will speak of a single spectrum only; yet not the spectrum 

 of hydrogen, for this would carry simplicity rather too far. By a 

 queer paradox of which we are destined to see altogether too much, 

 it often happens in quantum mechanics that the seemingly-simplest 

 of all cases — the ones to which one would go by choice for a start — ^are 

 liable to a singularly-confusing complexity quaintly known as a 

 "degeneracy." Let us therefore take sodium for our example. 



The sodium atom has a nuclear charge -+- lie, and eleven orbital 

 electrons, of which the eleventh or "valence electron" is sharply 

 contrasted with the other ten. Employing the original atom-model 

 of Bohr, one visualizes the ten as describing a network of interlacing 

 close-packed orbits enclosing the nucleus as in a sort of cage, while the 

 eleventh darts about in some sort of a far-flung orbit which at one 

 end may enter the cage, while the other end reaches far out into space 

 ("penetrating orbits") or alternatively an entire orbit may encircle 

 the cage completely, not entering it at all ("non-penetrating orbit"). 



We now consider the major feature of the sodium spectrum — which 

 indeed is the only feature of the absorption-spectrum of tranquil 

 sodium vapor — a beautiful converging series of lines. All of the 

 lines correspond to transitions between one and the same state, viz, the 

 normal state of the sodium atom, and the various members of a 

 sequence of abnormal or excited states. These are distinguished by an 

 index n to which consecutive integer values are attached; and all 

 together they are known as the P sequence, to distinguish them from 

 other sequences of states which a fuller study of the sodium spectrum 

 (not confined to the absorption-spectrum of the tranquil vapor) dis- 

 closes. The terminology, of course, is a detail. What is essential is. 



