Arrangement of Atoms iu Crystals. 185 



complex N0 3 group. In order to complete a stable arrange- 

 ment around the tour nuclei, the N0 3 group has borrowed 

 an electron from the sodium atom, leaving it a positively 

 charged ion. These ions are arranged in the same way in 

 NaN0 3 and NaCl, each ion being surrounded by six o£ the 

 opposite sign. The form of the N0 3 group has, however, 

 distorced the structure so that the crystal is rhombohedral 

 instead of cubic. 



In MgC0 3 the arrangement of the atoms is the same 

 as in NaN0 3 . The magnesium ion has a double positive 

 charge, the C0 3 ion a double negative charge. As a result 

 of the greater electrostatic forces, the dimensions of the 

 structure are reduced, the distance between magnesium and 



o - 9 



oxygen centres being 2'OOA., as compared with 2*38 A. in 

 the case of sodium nitrate. This will make it clear why the 

 divalent element appears to occupy a smaller space in a 

 crystalline structure than the monovalent element preceding- 

 it in the periodic seiies. 



The large diameters assigned to the electropositive elements 

 as compared with the electronegative elements do not imply 

 a corresponding difference in the dimensions of the atomic 

 structure. They are an expression of the fact that the 

 electropositive element does not share electrons with neigh- 

 bouring atoms, it is always surrounded by a complete stable 

 shell. The repulsion between this outer shell and the shells 

 of neighbouring atoms keeps the atom at a distance from its 

 neighbours, so that it appears to occupy a large space in the 

 crystal structure. 



It is interesting to compare the structure of graphite with 

 that of diamond from this point of view. The graphite 

 crystal has been analysed by Debye and Scherrer*. It 

 corresponds to a diamond structure in which, firstly, the 

 dimensions of the whole structures parallel to a trigonal axis 

 have been lengthened in the ratio 0'598 : 1, and, secondly, 

 the carbon atoms in the pairs of (111) planes of the diamond 

 have been so displaced that they lie very nearly in the same 

 plane. The atoms in a (111) plane are therefore very much 

 closer to each other than they are to the atoms in the next 

 planes. This may be explained by supposing that they are 

 sharing electrons with their neighbours in the (111) planes 

 but not with the other atoms, the very ready cleavage 

 parallel to (111) lending support to this view (cp. Debye's 

 paper). In such a case as this, the analogy ot the crystal 

 structure to a set of spheres packed together obviously 



* Debye and Scherrer, Phi/s. Zeit. xviii. (June 1917). 

 Phil Mag. Ser. 6. Vol. 40. No. 230. Aug. 1920. 



