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BELL SYSTEM TECHNICAL JOURNAL 



of electrons, as contrasted with the 4^ shell which extends farther from 

 the nucleus, so far that in the solid the shells of neighboring atoms over- 

 lap considerably. In {h) the number of electrons having energy be- 

 tween E and E -\- dE is plotted against the energy E; this representa- 

 tion is similar to that of Fig. 10 but now the squares and rectangles 

 are replaced by the more appropriate curved surfaces. If {b) is turned 

 90° relative to (a) the two pairs of curves bear some resemblance to 

 each other. This is so because the energy of binding is generally less 

 at greater distances from the nucleus. The M-\- level is represented 

 as lower in energy than the ?>d— since one of these bands is preferred. 



ifl 6 



S) 2 



2 



12 3 4 5 



DISTANCE FROM NUCLEUS IN ANGSTROMS 



(a) 



NUMBER OF ELECTRONS PER UNIT ENERGY 



(b) 



Fig. 11 — The filling of electron positions in iron, and some elements near it in the 

 periodic table. Electron positions for closed shells, containing 18 electrons, are not 

 shown. 



The area enclosed by each Zd curve corresponds to 5 electrons while 

 that enclosed by the ^s corresponds to 2. 



The line "Fe" in Fig. 11(6) represents the limit to which the 2>d and 

 45 shells are filled in iron ; neither M-\- nor Zd — is completely full. The 

 lowest energy levels are filled first, and the picture is drawn so that the 

 analogy with the filling of connected vessels with water is apparent. 

 In cobalt and nickel the extra one and two electrons completely fill M + 

 but not M — , as indicated by the line "Ni" for nickel. Since the 

 range of energy in the M "bands" is much greater than in the 45 

 bands the additional electrons do not alter greatly the number in 45, 

 and from the saturation intensity of nickel we estimate this number 

 as 0.6. In copper the additional electron is sufficient to fill both 3d 

 shells with one electron to spare, and this electron must go into the 



