662 



BELL SYSTEM TECHNICAL JOURNAL 



The Transition Elements 



Actually argon does not correspond to a complete system of closed 

 shells, since for it none of the M states in the M shell is occupied. For 

 the elements below copper, Z = 29, these 2>d levels lie above the 45 

 and below the 4/>. They are filled up progressively in the series of 

 elements scandium, titanium, vanadium, chromium, manganese, iron, 

 cobalt, and nickel, which are known as the transition elements of the 

 first long period of the periodic table. The first two elements after 

 argon are potassium (an alkali) and calcium (an alkaline earth); these 

 are similar to sodium and magnesium in having respectively one and 

 two 5 electrons. The first transition element, scandium, however, is 

 not like beryllium or aluminum, for with it the filling of the M states 

 begins. The electron configurations for several of the transition ele- 

 ments are shown in Fig. 6. An interesting case occurs at chromium; 



SCANDIUM 3d 4S2 



III''' 



ill ± 



CHROMIUM 3d^4S 



JL. 



NICKEL 3d®4s2 



AiLL ±, ^ 



COPPER 3d'°4S 



Fig. 6 — Electron configurations for transition elements. 



for it the exchange effect is so great that the 3>d levels drop below the 

 4^ and one 45 electron is transferred. Since there is an exchange 

 effect between all electrons of the same spin, the remaining occupied 

 45 state in chromium has the same spin as the occupied M states. A 

 similar transfer of a 45 electron occurs at copper, which is then left 

 with one 45 electron and tends therefore to be monovalent (in the 

 divalent copper ion the 45 electron and one M electron are removed). 

 These transition elements are of particular interest because three of 

 them, iron, nickel, and cobalt, are ferromagnetic in the solid. The 

 atoms themselves are magnetic, as may readily be seen for chromium, 

 for example; in it all the electrons have their spins parallel and hence 

 their magnetic moments add to give a free chromium atom a magnetic 



