Table 606 

 FIRST IONIZATION POTENTIALS OF THE ELEMENTS 



(Russell, Astrophys. Journ., 70, 1929; Mt. Wilson Contr. 383.) 



499 



In discussing the relative strength of the arc and enhanced lines a knowledge of ionization 

 potentials is necessary. These are implicitly contained in Tables 604 and 605. Generally 

 the energy difference between the normal states of the neutral atom and ion is required; 

 complications arise in the Fe group. Table 604 gives the energy difference between the 4s 

 state of the ion (d n_2 s) and the various states of the neutral atom, of which we must evidently 

 choose the lowest, whether it be 4s(d n ' 2 s 2 ) or 3d(d n_1 s). Sometimes the 3d(d n -*) state of the 

 ion is the lowest, and the values of Table 604 must be corrected. For the 2nd ionizations no 

 such complication arises. Both tables include heavier elements for which data are at hand. 

 Each is divided into two sections, corresponding to the "building on" of shells of s and d, 

 or of p electrons. He, Be, Mg are put on line with Zn, Cd, Hg, because their spectra resemble 

 those of the latter much more closely than those for Ca, Sr, Ba. The line below La marks 

 the position of the rare earths, where 14 4f electrons are added which are listed separately. 

 While each shell of outer electrons is being completed, the ionization potential increases 

 (minor irregularities occur for space quantization or interchange of s and d configurations. 

 Maximum occurs when a shell is filled). The drop after the filling of an s shell in Be and Mg 

 is between 1 and 2 volts; after completion of the combined s and d shells in Zn, Cd, Hg, 

 it is 3 or 4 volts, while that following the completion of a p shell in the inert gases is much 

 greater — 8 to 16 volts. For the 2nd ionization, these discontinuities are greater in absolute 

 value, but are a smaller fraction of the potentials themselves. (Continued on next page.) 



First Ionization Potentials of the Elements 



Smithsonian Tables 



