T H E 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



raffiTHiSERIES.] 

 ^tf&UrG US T 1920. 



XVIII. The Arrangement of Atoms in Crystals. By W. 

 Lawrence Bragg, M.A., Langworthy Professor of Physics y 

 The University of Manchester* . 



[Plate III.] 



1. TN many simple crystalline structures the atoms are so 

 JL arranged that their exact positions are determined by 

 the symmetry o£ the crystal. In the diamond, for example, 

 each carbon atom is at the centre of four other carbon 

 atoms arranged at the corners o£ a regular tetrahedron. 

 In the crystal o£ sodium chloride, the sodium atoms may be 

 regarded as arranged at the corners and face-centres of the 

 unit cube, the chlorine atoms being situated at the centres of 

 the cube-edges and at the centre of the cube. Every sodium 

 atom is surrounded symmetrically by six chlorine atoms, 

 every chlorine atom by six sodium atoms. The atoms can- 

 not be displaced from these positions without destroying the 

 symmetry of the structure, and therefore their exact positions 

 are defined by this symmetry. 



In the case of a crystal such as iron pyrites, FeS 2 , the 

 positions of the sulphur atoms are not defined in this manner. 

 The atoms of iron are at centres of symmetry of the struc- 

 ture, and so must be regarded as fixed in position. The 

 sulphur atoms lie on axes of threefold symmetry, but they, 



* Communicated by the Author. 

 Phil. Mag. S. 6. Vol. 40. No. 236. Aug. 1920. N 



