Arrangement of Atoms in Crystals. Ill 



of fluorine to be slightly greater, 1*35 A. From a com- 

 parison of the alkaline halides, the diameters of the halogens 

 are found to be : 



o 



Fluorine 135 A. 



Chlorine 210 A. 



Bromine 238 A. 



Iodine 280 A. 



Sulphur has been taken to have a diameter of 2'05 A. The 

 structure of Galena (PbS) corresponds to that of NaCl. 

 The substitution of Selenium and Tellurium lor Sulphur in 

 the compounds PbSe and PbTe increases the distance between 

 atomic centres by 0*15 A. and 0*31 A. respectively. We 

 therefore oet the values for the diameters : — 



o 



Oxygen 1'30 A. 



Sulphur 2 05 A. 



Selenium 2'35 A. 



Tellurium 2-66 A. 



The diameter of nitrogen is taken to be 1*30 A. So far as 

 the author is aware, no compounds containing phosphorus, 

 arsenic, or antimony have yet been fully worked out, and 

 the diameters of arsenic and antimony in the figure are the 

 distances between atoms in crystals of the element. 



The diameters of carbon and silicon* are taken to be those 

 separating the atoms of the element, 1*54 A. and 2*35 A. 

 Gray tinf has a similar structure, with an inter-atomic 

 distance of 2'80 A. 



Aluminium in its compounds occupies a volume slightly 

 less than trivalent iron. 



The divalent metals Nickel, Zinc, Magnesium, Copper, 

 Cobalt, Iron, Manganese, Cadmium, and Calcium form several 

 series of isomorphous compounds, which have been studied 

 in detail by Tutton. The metals are arranged in the order 

 of the molecular volumes of their compounds, those con- 

 taining nickel having the least volume and those containing 

 calcium the greatest. 



The diameters of the spheres representing the monovalent 

 alkali metals are calculated from the dimensions of the 

 alkaline halides, diameters having been already fixed for 

 fluorine, chlorine, bromine, and iodine. 



* Debye and Scherrer, Phys. Zeit. xvii. (1916). 



+ A. J. Bijl and N. H. Holkmeyer, Proc. Roy. Soc. Ac. Amsterdam, 

 June-Sept. (1918). 



