254 



BIOLOGICAL EFFECTS OF RADIATION 



Aciivafed 

 state 



different molecules follows a probability curve as shown at A in Fig. 2. 

 This relation is known as the Maxwell-Boltzmann distribution law. In 

 ordinary reactions which proceed slowly, only those molecules which are 

 moving with very high velocities can produce activation sufficient to carry 

 on the chemical reaction. For example, only those molecules which 

 have a velocity greater than Y can take part in a chemical reaction. The 



value of Y will, of course, be 

 different for different chemical 

 reactions. When the temper- 

 ature is raised, the distribution 

 of velocities among the different 

 molecules is changed in a man- 

 ner suggested by the curve B. 

 It will be noted that the forms 

 of these curves are such that 

 at the higher temperatures 

 there is a very great increase in 

 the number of molecules having 

 the high velocities. For exam- 

 ple, the ordinate YT> is much 

 greater than the ordinate FC, 

 but there is not so much differ- 

 activa- gj^cg between the ordinates of 



the slower 



left. This 



number 



c 



-4- 



c 

 o 



c 



Fig. 1. — Relation between energy of 

 tion and heat of reaction. A. Energy absorbed 



in activating reactants. B. Energy evolved when the tWO CUrVCS at 

 activated reactants react to give products. C. Net velocities at the 

 heat evolved in the reaction. 



large mcrease m the 

 of molecules having high velocities at the higher temperature explains 

 the fact that temperature has such a large effect in accelerating a chemical 

 reaction. The number of molecules having sufficiently high velocities to 

 become activated increases rapidly with the temperature. (The broken 

 lines in Fig. 2 will be discussed later.) 



Another concept which is found useful in interpreting chemical reac- 

 tions is that of the variation with distance of the attractive force between 

 the atoms. This relationship shown in Fig. 3 was due originally to 

 Franck. In the case of some atoms (helium, for example) there is no 

 attraction and the atoms repel each other at all distances shown in 

 curve A. In other systems, shown at B, the atoms attract each other 

 and the attraction becomes greater the nearer the atoms approach to 

 each other, as in the case of electrically charged bodies. However, when 

 the atoms come too close to each other, they are repelled. There is then 

 a stable position at a definite distance, below which the atoms will repel 

 each other and beyond which they will attract each other. This position 

 is represented by the point C. When the energy is sufficiently great so 

 that the atoms can no longer attract each other, dissociation occurs as 



