PRINCIPLES OF STRUCTURE 



29 



microscopic morphology of organic compounds. It has been shown 

 that the lattice points in Figs. 25 and 26 represent only the centres of 

 gravity of the atoms. The range of their electron orbits, however, 

 extends over such large volumes, that these can be represented by 

 spheres touching each other in the lattice (Figs. 27-29). A crystal 



Fig. 27 Fig. 28 Fig. 29 



Co-ordination numbers (fromMAGNUS, 1922) 



Fig. 27. Number 12; e.g., Au (Au),2 in crystallized gold - Fig. 28. Number 6; e.g., 

 Na(Cl)e in sodium chloride; Fe(CN)6 as ion - Fig. 29. Number 4; e.g., CCI4, C(C)4 in 



diamond. 



lattice, therefore, which is kept together by main valencies is much 

 more closely packed than the common pictures suggest. Unfortunately, 

 the representations in space obtained by drawing continuous spheres ■ 

 instead of lattice points are not very illuminating, whereas in a plane 

 this procedure can be applied with great success (comp. Fig. 3 1, p. 34)- 

 The atomic distances in the lattices of elements correspond, therefore, 

 to the atomic diameters and in binary compounds they represent the 

 sum of the radii of the two partners (Goldschmidt). In this way it 

 has been possible to determine the volume occupied by various atoms 

 and at the same time to find an explanation for the different co-ordina- 

 tion numbers. E.g., four Cl-atoms combined in a tetrahedron to- 

 gether enclose a space which just corresponds to the size of a silicon 

 atom; this accounts for the co-ordination number 4 in the compound 

 SiCl4. Of the smaller fluorine atoms, however, we need 6 spheres to 

 obtain the space occupied by one Si-atom. Hence the co-ordination 

 number 6 (SiFg). 



If the lattice contains homopolar valency bonds, the distances be- 

 tween the atoms, or the diameters of their spheres, show a surprising 



