ABSORPTION AND EMISSION CENTERS. 



23 



Theory of Stark. 



Stark 1 considers that chemical valency can be explained as due to the 

 presence of negative electrons that hold the positive parts of atoms together. 

 In fig. 1 it is seen how this can take place, the dotted lines representing lines 

 of electric force. 



Fig. 1. 



The conditions represented in fig. 1 are such that there is very little stray 

 electric field beyond the atoms. Under such conditions the valency electrons 

 are saturated and Stark represents this by the symbol < . Under many 

 conditions, however, the valency electrons are not so closely united to the 

 atoms and are more or less unsaturated. Under certain conditions an electron 

 maybe thrown off from the atom, and Stark considers that it is under some con- 

 dition such as this that selective absorption of light takes place. When the elec- 

 tron returns to, the atom it will undergo certain accelerations along its path, and 

 during these accelerations it will emit radiations. Stark believes that under 

 some condition at least similar to this, the fluorescent radiation is emitted, and 

 that the period of this radiation will depend on the amount of energy set free 

 when an electron recombines with the positive part 

 of the molecule. From the heat changes that occur 

 in various chemical reactions, Stark calculates what 

 the approximate period of these radiations should 

 be, and in many cases obtains values which agree 

 with the positions of known bands in the spectrum. 

 Among these bands is the ultra-violet band of 

 benzene. Stark's formula for benzene would be 

 the following : 



The symbol - simply means that one elec- 

 tron is not as closely joined as the other three to 

 the carbon atom. It is possible that even this 

 unsaturated electron may have lines of force run- 

 ning to the other carbon atoms. In this way partial valency can easily 

 be explained. 



H-< ^-C 



Fig. 2. 



1 Phys. Zeit., 9, 85 (1908). 



