570 WYCKOFF: FORCES BETWEEN ATOMS IN SOLIDS 



able information concerning the outermost ones. As we pass along 

 the periodic table horizontally from left to right, each atom differs 

 from the one preceding it in carrying a nuclear charge greater 

 by one and hence in possessing one more electron.^" It is seen 

 that with simple atoms (short series of the periodic table) the. 

 general characteristics of an atom repeat themselves after the 

 addition of eight electrons: lithium and sodium are similar. 

 With the long periods eighteen electrons (thirty-two in the last 

 complete series) must be added before there is a complete repeti- 

 tion of properties. Three elements, those in the eighth group, 

 exhibit a similar valence, so that there are really only twice 

 eight kinds of valence (valence groups). There is, however, a 

 partial repetition in the long periods after eight valence groups 

 have been passed over. The sub-group elements show a valence 

 similar to that of the primary elements: copper, silver and gold 

 resemble the alkalies. 



Radio-active phenomena and more especially the whole sub- 

 ject of electrochemistry emphasize the intimate connection 

 between valence and the electron. The alkalies ionize losing 

 one electron, the alkaline earths lose two; the elements of the 

 oxygen group tend to acquire two, the halogens one; and so on. 

 In 1904 Abegg'*^ pointed out that the sum of what we choose to 

 call the maximum positive and negative valences of an element 

 is always equal to eight. Eight is the number involved in the 

 recurrence of properties. 



All of this seems to point clearly to some sort of repetition of 

 the configuration of the outside electrons characteristic of the 

 atom after the addition of eight outside electrons. ^^ The repeti- 

 tion of properties is so striking that one is forced to the conclu- 

 sion that when a certain definite number of electrons has been 

 added to the outside of the atom, the force fields about the 

 atom, except those resulting from the unneutralized charges 

 upon the nucleus, become practically negligible. If hydrogen 

 is the simplest element, this definite number of electrons in the 

 case of the very simple elements (first short period of the table) 

 is two. For the other simple elements it is eight. The heavier 



^' R. Abegg. Zeits. anorg. Chem. 39: 330. 1904. 

 " See references 2, 3, and 6, on page 565. 



