Arrangement of Atoms in Crystals. 187 



and iodine have been assigned diameters of 2*t)6A. and 

 2*80 A. The evidence for the lower limit is imperfect, but 

 it may be estimated as 2*70 A. 



The diameters of the outer electron shells of the inert 

 gases therefore appear to be — 



Neon 1-80 A. 



o 



Argon 2 05 A. 



o 



Krypton 235 A. 



Xenon 2"70 A. 



On Langmuir's theory, the crystal of an electropositive 

 element consists of an assemblage of positively charged ions 

 held together by electrons which are free to move in the 

 structure. The empirical relation between inter-atomic dis- 

 tances in compounds is less accurate when applied to the 

 metals, perhaps as a result of the different nature of the 

 forces in this latter case. For instance, in a number of 

 isomorphous series the substitution of magnesium for iron 

 decreases the molecular volume, vet the distances between 

 atomic centres in metallic magnesium and iron are 3*22 A.* 

 and 247 A, respectively. Silver and sodium form many 

 isomorphous salts of nearly identical molecular volume. The 

 distances between o atomic centres in crystalline silver and 

 sodium are 2*87 A. and 3*72 A. Isomorphous salts of the 

 same molecular volume are formed by rubidium and thallium, 

 by strontium and lead, substances whose atomic volumes differ 

 widely. The relations shown in fig. 3 hold most accurately 

 for compounds and for the electronegative elements. 



The electropositive elements crystallize in the cubic or 

 hexagonal systems. This was pointed out by Barlow and 

 Pope, and used as a basis for the theory of close-packing in 

 crystalline structures, since an assemblage of equal spheres 

 packed together in the closest manner has either cubic or 

 hexagonal symmetry. It is now known that the atoms 

 of some metals are not arranged in a close-packed manner. 

 Nevertheless, the idea of a metal as an assemblage of positive 

 ions held together by electrons indicates a reason for the 

 simple crystalline structure. Each atom has the same 

 relation to its neighbours, it is not bound in any way to one 

 rather than another of them, and the assemblages will take 

 a form like the arrangement of a set of equal spheres. 

 The crystal of an electronegative element, on the other 

 hand, where atoms are linked by holding electrons in 

 common, will have a more complicated structure, as is the 



* A. W. Hull, Phys. Rev., July 1017. 

 02 



