52 ILLINOIS STATE ACADEMY OF SCIENCE 



filled. With the first element in the next period, then, 

 there must be the beginning of a new shell. Thus sodium 

 has one valence electron forming the beginning of the 

 second shell of eight around the first shells of two and 

 eight, each in the interior of the atom. The first ques- 

 tion, then, that arises, is why does the positive nuckus 

 exert so slight a retaining force upon this valence elec- 

 tron? The answer seems to be this: The vaknce elec- 

 tron is at a considerably greater distance from the 

 nucleus than the inner shells. The electrons in the inner 

 shells tend to neutralize the attractive force of the 

 positive nucleiis for this valence electron. The whole 

 sodium atom, then, exclusive of this valence electron, may 

 be regarded as having a net positive charge of one only, 

 which acts as though it were centered in the nucleus. 

 The force exerted, then, upon the valence electron will 

 be relatively weak. Futhermore, the valence electron of 

 sodium is farther from the nucleus than the valence elec- 

 tron in the lithium atom. Thus, according to Coulombs 

 law, the force exerted upon an electron by a positive 

 charge is inversely proportional to the square of the 

 distance between them. The electron in the sodium atom 

 is then at a. greater distance from the nucleus than in the 

 lithium atom and should be retained with less force. 

 This is the case. Sodium is less electronegative than 

 lithium. This explanation may be applied consistently 

 to the properties of the elements throughout the periodic 

 table. The electronegativety rises to a max:imum at the 

 end of each period, but each maximum is actually less 

 than the preceding as we go from one period to another, 

 because of the increasing distances of the electrons from 

 the positive nucleus. 



This application of the laws of electrostatics to explain 

 the properties of the atom must be recognized as being 

 only qualitative. The unsolved problem is why this ar- 

 rangement of electrons in shells around a positive nucleus 

 is stable at all. Undoubtedly there is something in the 

 geometry of arrangement of electrons which determines 

 how many electrons may be crowded into any shell. 

 When we study the properties of the more complex 



