WILLIAM D. HARKINS 



139 



Sodium, magnesium, and aluminium have, respectively, one, two, and three outer 

 electrons. Such atoms lose electrons to form positive ions. If, however, the outer set 

 of electrons is more than half complete in the neutral atom, there is a much greater 

 tendency to pick up electrons than to lose them, so such atoms usually form negative 

 ions. If the three elements mentioned above are in the gaseous form and the sodium 

 atom loses one (Na+), the magnesium two (Mg++), and aluminium three electrons 

 (A1+++), then ten planetary electrons are present in each of these ions (Fig. 5). This 

 is the number of planetary electrons present in the neutral neon (Ne) atom. Since it 

 is supposed that the arrangement of the electrons is largely conditioned by their 

 number, it is supposed that Ne, Na+, Mg++, and A1+++ have the same structure ex- 

 cept that the electrons are bound somewhat more tightly as the charge on the nucleus 

 changes from 10+ for neon to 13+ for aluminium. That the idea of a common struc- 

 ture is justified is indicated by the fact that in so far as they have been investigated 

 the spectra of these ions upon further dissociation is like that of neon. 



O 



o 



o 



o 



o 



o P^ o 



o 



00 GO 



Neutral neon atom Triply positive aluminium ion (A1+ + +) 



Fig. 5. — The composition of Ne, Na+, Mg++, or A1+++. The nuclear charges are 10+, 11+, 

 12+, and 13+, respectively. 



IONS IN SOLID SALTS 



In rock salt the mean distance from the center of a sodium atom to that of any of 

 the six adjacent chlorine atoms is, as has been stated, 2.81 A (2.81X10"* cm.). The 

 fact that the analysis of the structure of such a crystal by X-ray methods has given no 

 evidence that the sodium atom is more firmly attached to any one of these atoms than 

 to the five others has led to the hypothesis that no molecules exist in such a crystal, 

 and that the chlorine atoms are attracted to the sodium atom because the former are 

 charged with a negative (Cl~) and the latter with a positive (Na+) electrical charge. 

 Thus the assumption is that the crystal is built up from individual positive and nega- 

 tive ions held together by the attraction between charges of like sign. The attraction 

 between the ions of unlike sign of charge is assumed to follow Coulomb's law that at- 

 traction or repulsion between electrified particles varies as the inverse of the square of 

 the distance between them: 



F= 



6162 



(i) 



