Atomic Symmetries of Ammonium Chloride. 181 



(6) From O 6 : 



Eight equivalent positions: 



www/ w-f-f,-J— WjW+i; -J— - w,w+i,w+f: ^+ijW+i>i~%- 

 N: At u = u xi 

 CI : At w — w 2 , 



H: At u == w 3 , and 24 other points which can be 

 chosen in different ways. 



(7) An arrangement can be deduced from 7 which is enan- 

 tiomorphic with that from (6). It does not require a more de- 

 tailed treatment. 



Comparison of possible Structures ivitli the Diffraction 

 Data. — The diffraction effects to be expected from each 

 of these crystallographically possible structures 6 can be 

 calculated in the usual fashion 7 and then compared with 

 the observed reflections from various planes. The scat- 

 tering power of the hydrogen atom is negligible compared 

 with the scatterings of the nitrogen and chlorine atoms, so 

 that they may be omitted from these calculations of 

 intensities. Such a comparison leads to the following 

 results. 



Possibility (2) : For planes whose indices are two even 

 and one odd there should be no reflection until the fourth 

 order. Planes such as (100) and (210) give second order 

 reflections, however, so that this structure must be ruled 

 out as impossible. 



Possibility (3): This arrangement must be excluded 

 for the same reason. 



All of the remaining arrangements lead to structures 

 which are chemically highly improbable. Particularly 

 (3) and (4) yield structures that are from the standpoint 

 of the chemist utterly inconceivable in that they group 

 together about one point in the unit all of the ammonium 

 groups and about some other point all of the chlorine 

 atoms. In view of the close agreement that exists 

 between the observed diffraction effects and those calcu- 



6 An arrangement of atoms having holohedral symmetry, but to which an 

 enantiomorphism might be ascribed by assigning to its atoms certain dis- 

 symmetries, could be developed from the space group 3 which would produce 

 the desired diffraction effects. In such a structure, however, though all of 

 the ammonium radicals were alike, four of the chlorine atoms would be dif- 

 ferent from the other four. From the standpoint of its chemistry such a 

 structure is scarcely permissible and may be eliminated upon such grounds. 



7 Ralph W. G. Wyckoff, this Journal, 50, 317, 1920; etc. 



