DIMENSIONS OF ATOMS AND MOLECULES 55 



distance when bromine is substituted for chlorine in a molecule. 

 This comparison is made in the following table : 



In this table o-q is the atomic radius calculated from the 

 viscosity of the inert gas. The quantity o-i is the contribution 

 of the atom to inter-atomic distance when holding electrons in 

 common with another atom. It is, for instance, one-half the 

 distance between two halogen atoms in such molecules as CI2, 

 Bra, etc. In the fourth column the distance S is that between 

 the hydrogen and halogen atomic centres found from the 

 absorption spectrum. It will be seen that there is a general 

 agreement between the estimates of the increase in atomic 

 dimensions (figures in italics) in passing from one atom to that 

 with similar properties in the next period. 



The evidence on which these estimates have been based is 

 still very incomplete. In the case of crystal structures particu- 

 larly, those possessing most interest contain complex molecules 

 or ions, and the corresponding complexity of structure makes 

 analysis difficult. An attempt has been made in this article to 

 summarise what has already been discovered, and to show that 

 some advance has been made in the direction of constructing 

 a model of the molecule, with the atoms in their correct relative 

 positions. 



