184 Prof. W. L. Bragg on the 



In the diamond, eacli carbon has a nuclear charge of six 

 units. The atom has two electrons in its inner shell and four in 

 the outer shell. In order to complete the number of electrons, 

 eight, which would make its outer shell correspond to the 

 stable neon form, it shares its electrons with the four carbon 

 atoms surrounding it in the diamond structure. Each pair 

 of atoms holds two electrons in common. The forces binding 

 the atoms together are of a different type, the atoms are 

 united because they share electrons, not as a result of 

 opposite charges on ions as in KC1. 



A crystal of an electropositive element, such as sodium, 

 consists of an assemblage of the stable " inert gas " shells 

 with an additional electron associated with each in order to 

 neutralize completely the nuclear charge. These electrons 

 have no fixed positions in the structure, they move under 

 the action of an electromotive force and convey a current of 

 electricity through the metal. On the other hand, a crystal 

 in which the atoms are bound together by sharing electrons, 

 so that there are no free electrons, is a non-conductor. This 

 is the case for the typical electronegative element. 



13. The empirical relations of fig. 3 are readily explained 

 by this theory. In each period, the alkali metal which 

 follows one of the inert gases has been assigned a large 

 " diameter." This expresses the fact that it appears to 

 occupy a large space in any crystal structure; the centre of 

 the atom is separated by a considerable distance from the 

 centres of the neighbouring atoms. Successive elements are 

 assigned smaller diameters, and at the end of the period 

 the electronegative elements immediately preceding the next 

 inert gas have diameters which approximate closely to a 

 limiting minimum value for that period. 



The small diameters have been assigned to the electro- 

 negative elements on account of their proximity in a crystal 

 structure. In sodium nitrate, for example, the distance 

 between the oxygen and nitrogen centres is 1*30 A., that 

 between the oxygen and sodium centres 2*38 A. In all these 

 atoms the nuclei are surrounded by the stable neon arrange- 

 ment of electrons, and presumably these electrons are ap- 

 proximately the same distance from the nucleus in sodium, 

 nitrogen, and oxygen. The oxygen and nitrogen atoms have 

 realized the stable arrangement, however, by sharing elec- 

 trons, and their centres are correspondingly close together. 

 The sodium atom is already surrounded by a stable shell and 

 is isolated in the structure. In sodium nitrate there is the 

 same arrangement of positive and negative ions as in sodium 

 chloride, except that the negative ion in this case is the 



