PRESENT STATUS OF FERROMAGNETIC THEORY 67 



but the maximum number in each shell is not always reached before 

 the next shell begins to be formed. For example, when formation 

 of the fourth shell begins, the third shell contains only eight electrons 

 instead of eighteen; it is the subsequent building up of this third 

 shell that is intimately connected with ferromagnetism. In this shell 

 some electrons will be spinning in one direction and others in the 

 opposite, and these two senses of the spins may be conveniently 

 referred to as positive and negative. The numbers inserted in Fig. 2 

 show how many electrons are present in each shell which have positive 

 and negative spins, and it may be noticed that in the iron atom all 

 of the shells except the third contain as many electrons spinning in 

 one direction as in the opposite. The magnetic moments of the 

 electrons in each of these shells mutually compensate one another so 

 that the shell is magnetically neutral and cannot have magnetic 

 polarization. In the third shell, however, which is not yet filled to 

 this extent, there are five electrons with a positive spin and one with 

 a negative so that four electron spins are (unbalanced or) uncom- 

 pensated and there is a resultant polarization of the atom as a whole. 

 If one more positive charge and its associated mass (a proton) be added 

 to the nucleus and one more electron to an outer shell, the iron is trans- 

 formed into cobalt; and by repeating the process, the cobalt is trans- 

 formed into nickel. In iron these additional electrons and their spins 

 are so oriented that there is what may be called an excess spin of four 

 units in iron, three in cobalt, and two in nickel. In manganese, the 

 element just preceding iron in the periodic table, there is an excess of 

 five spins. Only in incomplete shells such as this, shells that are being 

 filled as new and heavier atoms are made, is there such excess spin. 

 The completed shells are magnetically neutral because the spins 

 mutually compensate one another. 



The outermost electrons are those responsible for the ordinary 

 chemical properties, and they are influenced by chemical combination. 

 They do not contribute to ferromagnetism for reasons that will appear 

 later. 



Exchange Forces 



Only in certain parts of the periodic table are there found electrons 

 being added to inner shells, and one of these places is in the iron 

 group; but since there are other parts, notably those occupied by the 

 palladium, platinum, and rare earth metals, where these inner groups 

 are being filled, there arises the further question: Why are not these 

 other elements also ferromagnetic? 



For an element to be ferromagnetic, it is necessary not only that 

 there be uncompensated spin in the electron orbits, but also that the 



