186 Prof. W. L. Bragg on the 



o» 



breaks down. The distance between neighbouring carbon 

 atoms in graphite is 1*45 A. 



14. It has been seen that in each period the diameters of 

 the electronegative atoms appear to approach a lower limit. 

 If it is true that these atoms share electrons when combined 

 together in the crystal, the diameters which have been 

 assigned to them should give an estimate o£ the diameters of 

 the outer shells in which the electrons are situated. 



In the first short period the diameters assigned to the 

 atoms of carbon, nitrogen, oxygen, and fluorine are 1*54 A., 

 1-30 A., 1*30 A., 1-35 A. The first three of these have been 

 calculated from compounds in which the atoms share elec- 

 trons, the nitrates, carbonates, and diamond. No compound 

 in which fluorine shares electrons has been analysed, but 

 evidence has been given that it occupies the same volume as 

 oxygen. The outer electron shell which these atoms tend 

 to complete is that of Neon. We may therefore estimate 

 the diameter of the outer neon shell as being 1*30 A. Since 

 two electrons at least are held in common by the elements 

 this estimate may be somewhat too large. 



In the second short period the diameters of silicon, sulphur, 

 and chlorine are 2*35 A., 2*05 A., 2' 10 A. The structure of 

 phosphorus has not yet been analysed. The diameter of the 

 outer Argon shell appears to be 2*05 A. 



In the first long period, the lower limit to which the 

 diameters tend is 2'35 A. The structure of arsenic has not 

 been analysed, but it crystallizes in a form isomorphous with 

 antimony, the structure of which has been recently deter- 

 mined by James and Tunstall. If its structure is that of 

 antimony, the distance between the nearest atoms is 2b2 A. 

 Selenium has been assigned a diameter of 2*35 A., bromine 

 a diameter of 2*38 A. Other elements in the same period 

 tend to approach this limit. When manganese and chromium 

 act as acid-forming elements and so share electrons with 

 other atoms, they enter into compounds isomorphous with 

 the sulphates and selenates, and the molecular volumes of the 

 compounds are very nearly those of the selenates,, so that 

 the atoms appear to have dimensions identical with those of 

 selenium. The distances betvveen atomic centres in iron, 

 nickel, and copper are 2*47 A., 2*39 A., 2*55 A. These 

 figures confirm the estimate of 2*35 A. as the lower limit 

 to which the diameter tends. 



In the second long period, the distance between atomic 

 centres in gray tin is 2*80 A., in antimony 2*80 A. Tellurium 



