Arrangement of Atoms in Crystals. 179 



10. In fig. 3 the elements are arranged in the order of 

 their Atomic Numbers. The ordinat.es represent the diameters 

 of the fi Atomic Domain " measured in Angstrom Units. The 

 figure summarizes the empirical relation which has been 

 found to hold, namely, that the distance between neighbour- 

 ing atomic centres in a crystal is the sum of two constants 

 characteristic of the atoms. The crystal may be imagined as 

 an assemblage of spheres packed together, the constants then 

 representing the radii of the spheres. 



The atomic diameters lie on a curve resembling Lothar 

 Meyer's curve of atomic volumes. The alkali metals head 

 each period with the greatest diameter, followed by the 

 alkaline earths. The diameter diminishes steadily as the 

 atomic weight is increased, reaching a minimum for the 

 electronegative elements at the end of the period. In other 

 words, when the atomic arrangement of compounds is taken 

 into account, the periodic relation between the atomic 

 volumes shown by Lothar Meyer's curve can be extended to 

 the compounds of the atoms. 



A list is given below of the " atomic diameters" assigned 

 to the elements, and for convenience the "atomic radii " are 

 added. The second table is a comparison of the observed 

 distance between atoms in crystals with those obtained by 

 adding together the radii of the two atoms concerned. It 

 will be seen that the difference between observed and calcu- 

 lated values is never large, the average difference being 

 0'06xl0- 8 cm. 



It is not intended to assign any physical significance 

 to these " diameters " other than that discussed below. 

 Sodium, for instance, has been given a diameter much 

 larger than that of chlorine, yet it will be seen that there 

 is every reason for supposing that the group of electrons 

 surrounding the sodium nucleus in sodium chloride has 

 smaller dimensions than that surrounding the chlorine nucleus 

 in the same crystal. 



The way of regarding the atoms as spheres packed tightly 

 together is useful in constructing models of crystalline 

 structures. Such models are illustrated in Plate III., the 

 crystalline structures being those of sodium chloride, calcium 

 carbonate, and zinc-blende. 



