﻿448 Prof. W. L. Bragg and Messrs. James and Bosanquet : 





(1). 



(2). 





Radius 



(without allowance 

 for thermal 

 agitation). 



Radius 



(with allowance 



for thermal 



agitation). 



odium. — Seven electrons . 



0-31 



0-29 



Three electrons . . . 



0-79 



0-76 



hlorine. — Ten electrons . . . 



0-28 



0-25 



Seven electrons... 



0-81 



0-86 



Three electrons.. 



1-46 



1-46 



A little consideration shows the reason for this. The form 

 of the F curve at large angles is almost entirely decided by 

 the arrangement of the electrons near the centre of the atom. 

 A slight expansion of the grouping in this region causes a 

 large falling off in the intensity of reflexion. This is shown 

 in the analysis by the slight increase (O02 to O03 A) in the 

 radius of the shell which gives the best fit to the uncorrected 

 curve. '1 he effect of the thermal agitation is to make the 

 electron distribution appear more widely diffused ; however, 

 the average displacement of the atom from the reflecting 

 plane owing to its thermal movements is only two or three 

 hundredths of an Angstrom unit at ordinary temperatures, 

 and so we get very little alteration in our estimate of the 

 electron distribution. The uncertainty as to the Debye 

 factor, therefore, does not introduce any appreciable error in 

 our analysis of electron distribution. 



8. It is interesting to see whether any evidence can be 

 obtained as to whether a valency electron has been trans- 

 ferred from one atom to the other or not. This may be put 

 in another way : can we tell from the form of the F curves 

 in fig. 1 whether their maxima are at 10 and 18 or at 

 11 and 17 respectively? It appears impossible to do this ; 

 and, when we come to consider the problem more closely, 

 it seems that crystal analysis must be pushed to a far greater 

 degree of refinement before it can settle the point. If all 

 the electrons were grouped close to the atomic centres, and 

 if the transference of an electron meant that one electron 

 passed from the Na group to the CI group, then a solution 

 along the lines of that attempted by Debye and Scherrer 

 for LiF might be possible. The electron distributions we 

 find extend, on the other hand, right through the volume 

 of the crystal. The distance between Na and CI centres is 

 2*81 A, and we find electron distributions 1 A from the centre 

 in sodium and 1*8 A from the centre in chlorine. If the 



