PAPERS PRESENTED AT GENERAL SESSIONS 51 



and two, then, are to be placed in a series by themselves 

 at the beginning of the periodic table — the hydrogen 

 atom anxious to acquire two electrons by chemical com- 

 bination and the helium atom, having two electrons al- 

 ready, refusing to combine chemically with anything. 

 Helium gas is absolutely unreactive chemically. 



The chemical jDroperties of any atom, then, are to be 

 explained by the number of valence electrons it possesses 

 and the pull upon these valence electrons exerted by the 

 positive nucleus. If the pull exerted by the nucleus is 

 very strong, as in the case of chlorine, which not only 

 retains its valence electron with great force but strives 

 to acquire additional ones, we say that the element is 

 electronegative. If the force upon the valence electrons 

 is weak, as in the case of sodium, we say that the element 

 is electropositive, meaning, of course, only slightly elec- 

 tronegative since all atoms must be regarded as exerting 

 some pull on their valence electrons and hence being 

 electronegative to some degree. Xow the concept of 

 atomic number would lead us to expect that the atoms 

 would increase continuously in electronegativety from 

 hydrogen, with an atomic number of one, up to uranium 

 with an atomic number of ninety-two, since the force 

 upon the valence electron should be proportional to the 

 atomic number by the ordinary laws of electrostatics. 

 As a matter of fact, the electronegativety is a periodic 

 function of the atomic number. The atoms may be ar- 

 ranged in the well known periodic table in periods of 

 two, eight, eight, eighteen, eighteen, thirty-two and 

 thirty-two. The electronegativety rises to a maximum 

 at the end of each period and drops almost to zero at the 

 beginning of the next period, and the greatest electro- 

 negativety in the whole periodic system is reached in the 

 element fluorine^ with an atomic number of nine. It was 

 to explain this periodicity of the elements that Lewis 

 and Kossel introduced the concept of shells, concentric 

 about the atomic nucleus. Each period ends in an inert 

 gas where presumably a shell of electrons is completely 



^ Actually, neon may be supposed to be more electronegative than 

 fluorine, but since it does not combine with any other element it is not 

 ordinarily so considered. 



