396 PHENOMENA, ATOMS, AND MOLECULES 



cause electrons in the sheath to be held more firmly and should make it 

 easier for the atom to acquire a negative residual charge. As an example let 

 us consider the electronegative elements of the first two periods. 



As we pass from carbon, through nitrogen and oxygen, to fluorine, the 

 kernel charge increases and the size of the kernel presumably decreases. 

 The residual atomic charge should thus tend to become more negative as 

 we pass towards fluorine and more positive in comparison as we pass 

 towards carbon. In other words in compounds of these elements, we should 

 expect a tendency for fluorine to have a covalence a little less than its 

 negative valence while for nitrogen the covalence should tend to be greater 

 than the negative valence. Since there are only eight electrons in the sheath 

 of these atoms, the covalence of the carbon atom can never exceed four. 

 All these conclusions are in perfect accord with experience. Thus we find 

 the following covalences : * 



Carbon 4 (3) 



Nitrogen 4 3 (2) 



Oxygen (3) 2 i O 



Fluorine i 



Silicon 4 (or electro-positive) 



Phosphorus 4 3 



Sulfur 4 3 2 I O 



Chlorine (4) (3) (2) / 



In this table the numbers in italics give the most common valences, 

 while those in parentheses are only rarely found. It is clear that a large 

 kernel charge favors covalences less than the negative valences while a 

 small kernel charge has the opposite effect. A comparison of the elements 

 of the second period with those of the first, shows a slight tendency for 

 larger covalences among the heavier elements. This is to be explained as 

 the effect of the larger kernel and hence weaker forces. There is also much 

 more scattering among the valences of the heavier elements. This is another 

 result of the weaker forces acting on the electrons for the covalence of such 

 atoms is more dependent upon the electron affinity of the other atoms with 

 which they are combined. 



As an example of these relationships, let us consider the compounds, 

 CH4, NH3, HoO, and HF. In each atom of these compounds the covalence 

 is equal to the negative valopce so that the residual charge is zero and the 

 tendencies of Postulates i and 3 are satisfied. If we mix the NH3 and HF 

 together the larger kernel charge of the fluorine as compared with the 

 nitrogen, gives a tendency for the fluorine atom to become negative at the 

 expense of the nitrogen. Thus the covalence of the fluorine decreases to 

 zero while that of the nitrogen increases to four. This leads to the forma- 



■* See L Langmuir, Jour. Amer. Chem. Soc, 41, 927 (1919). 



