658 BELL SYSTEM TECHNICAL JOURNAL 



Thus in helium, since its two electrons can have oppositely directed 

 spins, each fills one of the \s states; we say the " electron configuration " 

 of helium is Is^ (read as "one ess squared"). For lithium, Z = Z, the 

 third electron, which cannot go to the completely filled Is states, 

 goes to the next highest, 25, giving Xs^ 2s. In going from helium to 

 lithium, all the states move to lower energies but not so much lower 

 as to make 25 for lithium as low as Is for helium. For this reason 

 lithium can be relatively easily ionized, as is seen in Fig. 3. 



Before continuing the discussion of particular atoms, we must point 

 out that two changes accompany each advance from one element to the 

 next in the periodic table. In each step the nuclear charge increases 

 by one plus unit and at the same time an electron is added to the 

 atom and the combined effects produce the results of Fig. 2. Quite 

 different results are obtained if one electron alone is added to the 

 atom. Then instead of the general falling of the levels which accom- 

 panies the double change, there is a general rising of all the levels. 

 This is due to the unbalanced negative charge on the added elec- 

 tron, whose presence on the atom raises the potential energy of all 

 the electrons and therefore raises their energy levels. For some atoms, 

 the raising of the energy levels produced by an unbalanced electron 

 may be so great that the electron is not bound at all or at least only 

 very slightly, and for these atoms negative ions do not form. On the 

 other hand, when an electron is removed from an atom all the remain- 

 ing electrons become more tightly bound and the energy levels are 

 lowered. 



Exchange Energy 



In Fig. 4 we show the electron configurations for the elements from 

 lithium to neon. The decrease in ionization potential in going from 

 beryllium to boron is due to the completed filling of the 25 states and 

 the consequent start of filling of the 2p states. The decrease in going 

 from nitrogen to oxygen suggests that not only do the 25 and 2p states 

 lie at different levels but that the 2p states themselves lie at two 

 different levels. This is true but in a rather special sense : the difference 

 in energy between the two sets of 2p states depends upon how they are 

 occupied. This difference is an "exchange energy." We shall discuss 

 the origin of the exchange effect in the next paragraph but one; how- 

 ever, the aspect of it needed for this paper is illustrated in Fig. 4. 

 We there imagine that the quantum states are represented by little 

 trays upon which are placed weights to represent occupancy by elec- 

 trons. The exchange effect corresponds to hanging the trays on 

 springs; in this way we see that as the electrons fill up the 2p states 



