THE PRESENT CONCEPTION OF MATTER 85 



eight. Professor Langmuir, who assumes as a working 

 hypothesis that the electrons are in static equihbrium and not 

 in continual orbital movement, believes that the electrons 

 arrange themselves on successive sheaths with 2 on the inmost, 

 8 on the next, then 8 again, followed by 18, 18, and 32. 



Lithium, the first element in Group I, but third in the whole 

 list, is supposed to have one electron in its outer layer, the first 

 two electrons being on an inner layer. The next element, 

 beryllium, is supposed to have two electrons in its outer layer, 

 boron three, carbon four, nitrogen five, oxygen six, fluorine 

 seven, and neon eight. This is the maximum number, and 

 when the next element is reached the additional electron has 

 to begin a new outer sphere, with the result that sodium is like 

 lithium in having one outer free electron. Thus the series 

 starts over again, so that those properties of the elements that 

 depend on the number of electrons in the outer layer of their 

 atoms must recur periodically, and this is just the fact that was 

 expressed more than fifty years ago by Mendeleeff's Periodic 

 Law. 



