Originally published in Ber. dtsch. chem. Ges. 53, 410 (1920) 



9. THE INTERMOLECULAR EXCHANGE OF ATOMS 



OF THE SAME KIND 



George Hevesy and Laszlo Zechmeisteb 

 From the Chemical Institute of the School of Veterinary Medicine, Budapest 



The present study is intended as a contribution to the answer of tlie 

 question as to whether and when interchange of similar atoms takes 

 place within a molecule and also between neighbouring molecules of 

 like or unlike kinds. In considering a benzene molecule, for example, 

 the question arises as to whether a carbon or hydrogen atom can move by 

 exchanging places with another similar atom from one position to another 

 in the benzene hexagon, or whether a certain hydrogen atom is al- 

 ways bound to the same carbon atom. If two neighbouring benzene 

 molecules are considered there is the further question as to whether 

 carbon or hydrogen atoms which were originally present in the first 

 molecule may or may not be found in the second molecule after a 

 definite time. 



Such an exchange of positions could be produced either directly by 

 the atoms vibrating within a molecule periodically entering into the 

 sphere of attraction of another molecule, or indirectly in the following 

 way : If there is dissociation such that a hydrogen atom splits off from 

 each of two benzene molecules the dynamic nature of the dissociation 

 process in which the atom is recaptured yields a 50 per cent chance 

 that the hydrogen atom which originally was separated from the first 

 molecule will enter the second molecule and thus be subjected to an 

 exchange of position. Because dissociation and recombination processes 

 take place very rapidly^, even the slightest dissociation in the liquid 

 phase, where molecular collisions occur extremely often, will lead to 

 such exchange in a short time. 



Although this question cannot be decided by experiment with benzene, 

 yet this can be done with lead compounds, for example, by means of 

 radioactive methods. It is well known that there are different isotopes 

 of lead which can be distinguished easily and with certainty througli 

 their radioactive properties, although they exhibit the same chemical 

 behaviour. By preparing two different compounds of lead, the one from 



1 M. Le Blanc and K. Schick, Phys. Chem. 46, 213 (1903). 



